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Title: Studies in the Theory of Descent (Volumes 1 and 2)
Author: Weismann, August
Language: English
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Transcriber’s notes


The Fourth Edition of this work originally was published in two volumes;
they have been combined in this eBook. The content of Volume II,
including its Title page, begins after page 400.

Italics is represented by _underscores_; boldface is represented by
=equals signs=.



STUDIES IN THE THEORY OF DESCENT.



  LONDON:
  PRINTED BY GILBERT AND RIVINGTON, LIMITED,
  ST. JOHN’S SQUARE,



  STUDIES IN THE THEORY
  OF DESCENT

  BY

  DR. AUGUST WEISMANN
  PROFESSOR IN THE UNIVERSITY OF FREIBURG

  _WITH NOTES AND ADDITIONS BY THE AUTHOR_

  TRANSLATED AND EDITED, WITH NOTES, BY

  RAPHAEL MELDOLA, F.C.S.
  LATE VICE-PRESIDENT OF THE ENTOMOLOGICAL SOCIETY OF LONDON

  WITH A PREFATORY NOTICE BY

  CHARLES DARWIN, LL.D., F.R.S.
  _Author of “The Origin of Species,” &c._

  IN TWO VOLUMES
  VOL. I.

  WITH EIGHT COLOURED PLATES

  London:

  SAMPSON LOW, MARSTON, SEARLE, & RIVINGTON
  CROWN BUILDINGS, 188, FLEET STREET

  1882

  [_All rights reserved._]



PREFATORY NOTICE.


The present work by Professor Weismann, well known for his profound
embryological investigations on the Diptera, will appear, I believe, to
every naturalist extremely interesting and well deserving of careful
study. Any one looking at the longitudinal and oblique stripes, often
of various and bright colours, on the caterpillars of Sphinx-moths,
would naturally be inclined to doubt whether these could be of the
least use to the insect; in the olden time they would have been called
freaks of Nature. But the present book shows that in most cases the
colouring can hardly fail to be of high importance as a protection.
This indeed was proved experimentally in one of the most curious
instances described, in which the thickened anterior end of the
caterpillar bears two large ocelli or eye-like spots, which give to
the creature so formidable an appearance that birds were frightened
away. But the mere explanation of the colouring of these caterpillars
is but a very small part of the merit of the work. This mainly consists
in the light thrown on the laws of variation and of inheritance by
the facts given and discussed. There is also a valuable discussion
on classification, as founded on characters displayed at different
ages by animals belonging to the same group. Several distinguished
naturalists maintain with much confidence that organic beings tend to
vary and to rise in the scale, independently of the conditions to which
they and their progenitors have been exposed; whilst others maintain
that all variation is due to such exposure, though the manner in which
the environment acts is as yet quite unknown. At the present time
there is hardly any question in biology of more importance than this
of the nature and causes of variability, and the reader will find in
the present work an able discussion on the whole subject, which will
probably lead him to pause before he admits the existence of an innate
tendency to perfectibility. Finally, whoever compares the discussions
in this volume with those published twenty years ago on any branch of
Natural History, will see how wide and rich a field for study has been
opened up through the principle of Evolution; and such fields, without
the light shed on them by this principle, would for long or for ever
have remained barren.

            CHARLES DARWIN.



TRANSLATOR’S PREFACE.


In offering to English readers this translation of Professor Weismann’s
well-known “Studies in the Theory of Descent,” the main part of which
is devoted to entomological subjects, I have been actuated by the
desire of placing in the hands of English naturalists one of the most
complete of recent contributions to the theory of Evolution as applied
to the elucidation of certain interesting groups of facts offered by
the insect world. Although many, if not most, working naturalists
are already familiar with the results of Dr. Weismann’s researches,
of which abstracts have from time to time appeared in English and
American scientific journals, I nevertheless believe that a study
of the complete work, by enabling the reader to follow closely the
detailed lines of reasoning and methods of experiment employed by the
author, will be found to be of considerable value to those biologists
who have not been able to follow the somewhat difficult phraseology of
the original. It is not my intention, nor would it be becoming in me to
discuss here the merits of the results arrived at by the minute and
laborious investigations with which Dr. Weismann has for many years
occupied himself. I may however point out that before the appearance of
the present work the author, in addition to his well-known papers on
the embryology and development of insects, had published two valuable
contributions to the theory of descent, viz. one entitled “Über die
Berechtigung der Darwin’schen Theorie” (1868), and another “Über den
Einfluss der Isolirung auf die Artbildung” (1872). These works, which
are perhaps not so well known in this country as could be desired,
might be advantageously studied in connection with the present volume
wherein they are frequently referred to.

Since every new contribution to science is a fresh starting-point for
future work, I may venture without any great breach of propriety to
dwell briefly upon one or two of the main points which appear to me to
be suggested by Prof. Weismann’s investigations.

Although the causes of Glacial Epochs is a subject which has much
occupied the attention of geologists and physiographers, the question
is one of such great complexity that it cannot yet be regarded as
finally settled. But apart from the question of causes--a most able
discussion of which is given by the author of “Island Life”--there
is not the least doubt that at no very distant geological period
there occurred such an epoch, which, although intermittent, was of
considerable duration. The last great geological event which our globe
experienced was in fact this Ice Age, and the pure naturalist has not
hitherto attributed in my opinion sufficient importance to the _direct_
modifying effects of this prolonged period of cold. It is scarcely
possible that such a vast climatic change as that which came on at the
close of the Pliocene Period should have left no permanent effect upon
our present fauna and flora, all the species of which have survived
from the glacial age. The great principle of Natural Selection leads
us to see how pre-glacial forms may have become adapted to the new
climatic conditions (which came on gradually) by the “survival of the
fittest” or “indirect equilibration.” The influence of the last Glacial
Epoch as a factor in determining the present geographical distribution
of animals and plants has already been amply treated of by many writers
since the broad paths were traced out by Darwin, Lyell, and Wallace.
The last-named author has indeed quite recently discussed this branch
of the subject most exhaustively in his work on “Island Life” above
mentioned. The reference of a particular group of phenomena--the
seasonal dimorphism of butterflies--to the direct action of the Glacial
Period and the subsequent influence of the ameliorating climate, was
however the first step taken in this neglected field by the author
of the present work in 1875. It is possible, and indeed probable,
that future researches will show that other characters among existing
species can be traced to the same causes.

The great generalizations of embryology, which science owes so largely
to the researches of Karl Ernst von Baer, bear to the theory of
descent the same relations that Kepler’s laws bear to the theory of
gravitation. These last-named laws are nothing more than generalized
statements of the motions of the planets, which were devoid of meaning
till the enunciation of the theory of gravitation. Similarly the
generalized facts of embryology are meaningless except in the light
of the theory of descent. It has now become a recognized principle in
biology that animals in the course of their development from the ovum
recapitulate more or less completely the phases through which their
ancestors have passed. The practical application of this principle to
the determination of the line of descent of any species or group of
species is surrounded by difficulties, but attempts have been made of
late years--as by Haeckel in his _Gastrula_ theory--to push the law
to its legitimate consequences. In this country Sir John Lubbock, in
1874, appealed to the embryonic characters of larvæ in support of his
views on the origin of insects. To the author of this work (1876) is
due the first application of the principle of Ontogeny as revealing
the origin of the markings of caterpillars. A most valuable method
of research is thus opened up, and entomologists should not be long
in availing themselves of it. Our knowledge of the subject of larval
development in Lepidoptera is still most imperfect, and it cannot as
yet be foreseen to what extent the existing notions of classification
in this much-studied order may have to be modified when a minute
study of the Comparative Ontogeny of larval characters, worked out as
completely as possible for each family, has enabled a true genealogical
system to be drawn up. The extent to which such a larval genealogy
would coincide with our present classification cannot now be decided,
but he who approaches this fruitful line of inquiry in the true spirit
of an investigator, will derive much instruction from Prof. Weismann’s
remarks on “Phyletic Parallelism in Metamorphic Species.” The
affinities of the larger groups among Lepidoptera would most probably
be made out once and for ever if systematists would devote more time to
observation in this field, and to the co-ordination and working up of
the numerous data scattered throughout the vast number of entomological
publications.

The doctrine of development by no means implies, as has sometimes been
maintained, a continuous _advancement_ in organization. Although the
scale of organic nature has continued to rise as a whole, cases may
occasionally occur where a _lower grade_ of organization is better
adapted to certain conditions of life. This principle of “degeneration”
was recognized by Darwin as early as in the first edition of the
“Origin of Species;” it was soon perceived to be applicable to the
phenomenon of parasitism, and was first definitely formulated by
Dr. Anton Dohrn in 1875. In a lecture delivered before the British
Association at Sheffield in 1879, Prof. E. Ray Lankester ascribed to
“degeneration” a distinct and well-defined function in the theory of
descent. Dr. Weismann’s explanation of the transformation of Axolotl
given in the fourth essay of this work, may be regarded as a special
contribution to this phase of Darwinism. Whilst refuting the idea
held by certain naturalists, that such cases are arguments against
the origin of species by the accumulation of minute variations, and
prove the possibility of development _per saltum_, the theory here
advanced (that _Siredon_ at a former period existed at a higher
stage of development as _Amblystoma_, and that the observed cases of
metamorphosis are but reversions to this lost higher stage) suggests
the question whether there may not still be in existence many other
degenerated forms quite unsuspected by naturalists.

Many of the opponents of Evolution have from time to time denounced
this doctrine as leading to “pure materialism,” a denunciation which
may appear somewhat alarming to the uninitiated, but which may not
seem fraught with any serious consequences to those who have followed
the course of philosophical speculation during the last few years.
Those who attack the doctrine on this ground will however do well to
consider Prof. Weismann’s views set forth in the last essay in this
volume, before hastily assuming that the much dreaded “materialism” is
incompatible with any other conception of Nature.

The small amount of leisure time which I have been able to devote to
the translation of this volume has delayed its completion considerably
beyond the anticipated time, and it was with a view to meeting this
difficulty that I departed from the original form of the German edition
and issued it in parts. Owing to the extremely idiomatic character
of the German text, I have throughout endeavoured to preserve only
the author’s meaning, regardless of literal translation or of the
construction of the original. In some few cases, however, I have
intentionally adopted literal translations of certain technical
expressions which might, I think, be advantageously introduced into
our biological vocabularies. Some alterations have been made in the
original text by the author for the present edition, and many new notes
have been added. For those bearing my initials I am alone responsible.

It gives me much pleasure in conclusion to express my thanks to Dr.
Weismann, not only for the readily given permission to publish an
English translation of his work, but also for much valuable assistance
during the execution of the task. The author has been good enough to
superintend the drawing of the plates for this edition, and he has also
read through the greater part of the manuscript. From Mr. Darwin also
I have received much kindly encouragement, and among entomologists I
am especially indebted to Mr. W. H. Edwards of West Virginia, for his
valuable additions to the first part. To my friends Mr. A. G. Butler,
Mr. Roland Trimen, and Mr. F. Moore, I owe acknowledgments for much
useful information concerning the caterpillars of exotic _Sphingidæ_,
which I have incorporated in the notes and appendices, and Mr. W. S.
Simpson has given me occasional advice in the translation of some of
the more difficult passages.

            R. M.
  _London, November, 1881._



PREFACE TO THE ENGLISH EDITION.


With the appearance of Charles Darwin’s work “On the Origin of
Species,” in the year 1858, there commenced a new era in biology.
Weary of the philosophical speculations which, at the beginning of
this century, had at first been started with moderation but had
afterwards been pushed to excess, biologists had entirely let drop all
general questions and confined themselves to special investigations.
The consideration even of general questions had quite fallen into
disuse, and the investigation of mere details had led to a state of
intellectual shortsightedness, interest being shown only for that which
was immediately in view. Immense numbers of detailed facts were thus
accumulated, but they could not possibly be mastered; the intellectual
bond which should have bound them together was wanting.

But all this was changed in a short time. At first only single
and mostly the younger naturalists fell in with the new theory of
development proclaimed by Darwin, but the conviction soon became
general that this was the only scientifically justifiable hypothesis
of the origin of the organic world.

The materials accumulated in all the provinces of biology now for the
first time acquired a deeper meaning and significance; unexpected
inter-relations revealed themselves as though spontaneously, and
what formerly appeared as unanswerable enigmas now became clear and
comprehensible. Since that time what a vast modification has the
subject of animal embryology undergone; how full of meaning appear the
youngest developmental stages, how important the larvæ; how significant
are rudimentary organs; what department of biology has not in some
measure become affected by the modifying influence of the new ideas!

But the doctrine of development not only enabled us to understand the
facts already existing; it gave at the same time an impetus to the
acquisition of unforeseen new ones. If at the present day we glance
back at the development of the biological sciences within the last
twenty years, we must be astonished both at the enormous array of new
facts which have been evoked by the theory of development, and by the
immense series of special investigations which have been called forth
by this doctrine.

But while the development theory for by far the greater majority of
these investigations served as a light which more and more illuminated
the darkness of ignorance, there appeared at the same time some
other researches in which this doctrine itself became the object of
investigation, and which were undertaken with a view to establish it
more securely.

To this latter class of work belong the “Studies” in the present volume.

It will perhaps be objected that the theory of descent has already
been sufficiently established by Darwin and Wallace. It is true that
their newly-discovered principle of selection is of the very greatest
importance, since it solves the riddle as to how that which is useful
can arise in a purely mechanical way. Nor can the transforming
influence of direct action, as upheld by Lamarck, be called in
question, although its extent cannot as yet be estimated with any
certainty. The _secondary_ modifications which Darwin regards as the
consequence of a change in some other organ must also be conceded. But
are these three factors actually competent to explain the complete
transformation of one species into another? Can they transform more
than mere single characters or groups of characters? Can we consider
them as the sole causes of the regular phenomena of the development
of the races of animals and plants? Is there not perhaps an unknown
force underlying these numberless developmental series as the true
motor power--a “developmental force” urging species to vary in certain
directions and thus calling into existence the chief types and
sub-types of the animal and vegetable kingdoms?

At the time these “Studies” first appeared (1875) they had been
preceded by a whole series of attempts to introduce into science such
an unknown power. The botanists, Nägeli and Askenasy, had designated
it the “perfecting principle” or the “fixed direction of variation;”
Kolliker as the “law of creation;” the philosophers, Von Hartmann and
Huber, as the “law of organic development,” and also “the universal
principle of organic nature.”

It was thus not entirely superfluous to test the capabilities of the
known factors of transformation. We had here before us a question of
the highest importance--a question which entered deeply into all our
general notions, not only of the organic world, but of the universe as
a whole.

This question--does there exist a special “developmental
force”?--obviously cannot be decided by mere speculation; it must also
be attempted to approach it by the inductive method.

The five essays in this volume are attempts to arrive, from various
sides, somewhat nearer at a solution of the problem indicated.

The first essay on the “Seasonal Dimorphism of Butterflies” is
certainly but indirectly connected with the question; it is therein
attempted to discover the causes of this remarkable dimorphism, and
by this means to indicate at the same time the extent of one of the
transforming factors with reference to a definite case. The experiments
upon which I base my views are not as numerous as I could desire,
and if I were now able to repeat them they would be carried out more
exactly than was possible at that time, when an experimental basis had
first to be established. In spite of this, the conclusions to which
I was led appear to be on the whole correct. That admirable and most
conscientious observer of the North American butterflies, Mr. W. H.
Edwards, has for many years experimented with American species in a
manner similar to that which I employed for European species, and his
results, which are published here in Appendix II. to the first essay,
contain nothing as far as I can see which is not in harmony with my
views. Many new questions suggest themselves, however, and it would
be a grateful task if some entomologist would go further into these
investigations.

The second essay directly attacks the main problem above indicated.
It treats of the “Origin of the Markings of Caterpillars,” and is to
some extent a test of the correctness and capabilities of the Darwinian
principles; it attempts to trace the differences in form in a definite
although small group entirely to known factors.

Why the markings of caterpillars have particularly been chosen for this
purpose will appear for two reasons.

The action of Natural Selection, on account of the nature of this
agency, can only be exerted on those characters which are of
biological importance. As it was to be tested whether, besides Natural
Selection and the direct action of external conditions, together
with the correlative results of these two factors, there might not
lie concealed in the organism some other unknown transforming power,
it was desirable to select for the investigation a group of forms
which, if not absolutely excluding, nevertheless appeared possibly to
restrict, the action of one of the two known factors of transformation,
that of Natural Selection; a group of forms consisting essentially
of so-called “purely morphological” characters, and not of those the
utility of which was obvious, and of which the origin by means of
Natural Selection was both possible and probable _ab initio_. Now,
although the _colouring_ can readily be seen to be of value to the life
of its possessors, this is not the case with the quite independent
_markings_ of caterpillars; excepting perhaps those occasional forms
of marking which have been regarded as special cases of protective
resemblance. The markings of caterpillars must in general be considered
as “purely morphological” characters, _i.e._ as characters which we
do not know to be of any importance to the life of the species, and
which cannot therefore be referred to Natural Selection. The most
plausible explanation of these markings might have been that they were
to be regarded as ornaments, but this view precludes the possibility
of referring them either to Natural Selection or to the influence of
direct changes in the environment.

The markings of caterpillars offered also another advantage which
cannot be lightly estimated; they precluded from the first any attempt
at an explanation by means of Sexual Selection. Although I am strongly
convinced of the activity and great importance of this last process
of selection, its effects cannot be estimated in any particular case,
and the origin of a cycle of forms could never be clearly traced to
its various factors, if Sexual Selection had also to be taken into
consideration. Thus, we may fairly suppose that many features in the
markings of butterflies owe their origin to Sexual Selection, but we
are, at least at present, quite in the dark as to how many and which of
these characters can be traced to this factor.

An investigation such as that which has been kept in view in this
second essay would have been impracticable in the case of butterflies,
as well as in the analogous case of the colouring and marking of birds,
because it would have always been doubtful whether a character which
did not appear to be attributable to any of the other transforming
factors, should not be referred to Sexual Selection. It would have been
impossible either to exclude or to infer an unknown developmental
force, since we should have had to deal with two unknowns which could
in no way be kept separate.

We escape this dilemma in the markings of caterpillars, because the
latter do not propagate in this state. If the phenomena are not here
entirely referable to Natural Selection and the direct action of the
environment--if there remains an inexplicable residue, this cannot be
referred to Sexual Selection, but to some as yet unknown power.

But it is not only in this respect that caterpillars offer especial
advantages. If it is to be attempted to trace transformations in
form to the action of the environment, an exact knowledge of this
environment is in the first place necessary, _i.e._ a precise
acquaintance with the conditions of life under the influence of
which the species concerned exist. With respect to caterpillars, our
knowledge of the life conditions is certainly by no means as complete
as might be supposed, when we consider that hundreds of Lepidopterists
have constantly bred and observed them during a most extended period.
Much may have been observed, but it has not been thought worthy of
publication; much has also been published, but so scattered and
disconnected and at the same time of such unequal credibility, that
a lifetime would be required to sift and collect it. A comprehensive
biology of caterpillars, based on a broad ground, is as yet wanting,
although such a labour would be both most interesting and valuable.
Nevertheless, we know considerably more of the life of caterpillars
than of any other larvæ, and as we are also acquainted with an immense
number of species and are able to compare their life and the phenomena
of their development, the subject of the markings of caterpillars must
from this side also appear as the most favourable for the problem set
before us.

To this must be added as a last, though not as the least, valuable
circumstance, that we have here preserved to us in the development of
the individual a fragment of the history of the species, so that we
thus have at hand a means of following the course which the characters
to be traced to their causes--the forms of marking--have taken during
the lapse of thousands of years.

If with reference to the question as to the precise conditions of life
in caterpillars I was frequently driven to my own observations, it was
because I found as good as no previous work bearing upon this subject.
It was well known generally that many caterpillars were differently
marked and coloured when young to what they were when old; in some
very striking cases brief notices of this fact are to be found in the
works,[1] more especially, of the older writers, and principally
in that of the excellent observer Rösel von Rosenhof, the Nuremberg
naturalist and miniature painter. In no single case, however, do the
available materials suffice when we have to draw conclusions respecting
the phyletic development. We distinctly see here how doubtful is the
value of those observations which are made, so to speak, at random,
_i.e._ without some definite object in view. Many of these observations
may be both good and correct, but they are frequently wanting precisely
in that which would make them available for scientific purposes. Thus
everything had to be established _de novo_, and for this reason the
investigations were extended over a considerable number of years,
and had to be restricted to a small and as sharply defined a group
as possible--a group which was easily surveyed, viz. that of the
Hawk-moths or Sphinges.

Since the appearance of the German edition of this work many new
observations respecting the markings of caterpillars have been
published, such, for example, as those of W. H. Edwards and Fritz
Müller. I have, however, made but little use of them here, as I had no
intention of giving anything like a _complete_ ontogeny of the markings
in all caterpillars: larval markings were with me but means to an end,
and I wished only to bring together such a number of facts as were
necessary for drawing certain general conclusions. It would indeed
be most interesting to extend such observations to other groups of
Lepidoptera.

The third essay also, for similar reasons, is based essentially upon
the same materials, viz. the Lepidoptera. It is therein attempted to
approach the general problem--does there or does there not exist an
internal transforming force?--from a quite different and, I may say,
opposite point of view. The form-relationships of Lepidoptera in their
two chief stages of development, imago and larva, are therein analysed,
and by an examination of the respective forms it has been attempted to
discover the nature of the causes which have led thereto.

I may be permitted to say that the fact here disclosed of a _different
morphological_, with the _same genealogical_ relationship, appears
to me to be of decided importance. The agreement of the conclusions
following therefrom with the results of the former investigation has,
at least in my own mind, removed the last doubts as to the correctness
of the latter.

The fourth and shortest essay on the “Transformation of the Axolotl
into Amblystoma,” starts primarily with the intention of showing
that cases of sudden transformation are no proof of _per saltum_
development. When this essay first appeared the view was still widely
entertained that we had here a case proving _per saltum_ development.
That this explanation was erroneous is now generally admitted, but I
believe that those who suppose that we have here to deal with some
quite ordinary phenomenon which requires no explanation, now go too
far towards the other extreme. The term “larval reproduction” is an
_expression_, but no _explanation_; we have therefore to attempt to
find out the true interpretation, but whether the one which I have
given is correct must be judged of by others.

These four essays lead up to a fifth and concluding one “On the
Mechanical Conception of Nature.” Whilst the results obtained are here
summed up, it is attempted to form them into a philosophical conception
of Nature and of the Universe. It will be thought by many that this
should have been left to professed philosophers, and I readily admit
that I made this attempt with some misgiving. Two considerations,
however, induced me to express here my own views. The first was that
the facts of science are frequently misunderstood, or at any rate
not estimated at their true value, by philosophers;[2] the second
consideration was, that even certain naturalists and certainly very
many non-naturalists, turn distrustfully from the results of science,
because they fear that these would infallibly lead to a view of
the Universe which is to them unacceptable, viz. the materialistic
view. With regard to the former I wished to show that the views of
the development of organic Nature inaugurated by Darwin and defended
in this work are certainly correctly designated _mechanical_; with
reference to the latter I wished to prove that such a mechanical
conception of the organic world and of Nature in general, by no means
leads merely to one single philosophical conception of Nature, viz. to
Materialism, but that on the contrary it rather admits of legitimate
development in a quite different manner.

Thus in these last four essays much that appears heterogeneous will
be found in close association, viz. scientific details and general
philosophical ideas. In truth, however, these are most intimately
connected, and the one cannot dispense with the other. As the detailed
investigations of the three essays find their highest value in the
general considerations of the fourth, and were indeed only possible
by constantly keeping this end in view, so the general conclusions
could only grow out of the results of the special investigations as
out of a solid foundation. Had the new materials here brought together
been already known, the reader would certainly have been spared the
trouble of going into the details of special scientific research.
But as matters stood it was indispensable that the facts should be
examined into and established even down to the most trifling details.
The essay “On the Origin of the Markings of Caterpillars” especially,
had obviously to commence with the sifting and compilation of extensive
morphological materials.

            AUGUST WEISMANN.

  _Freiburg in Baden,
      November, 1881._



CONTENTS.


=Part I.=

ON THE SEASONAL DIMORPHISM OF BUTTERFLIES.


I.

_The Origin and Significance of Seasonal Dimorphism_, p. 1.

Historical preliminaries, 1. Does not occur in other orders of insects,
4. Beginning of experimental investigation, 5. Lepidopterous foes, 7.
First experiments with _Araschnia Levana_, 10. Experiments with _Pieris
Napi_, 13. Discussion of results, 17. Origination of _Prorsa_ from
_Levana_, 19. Theoretical considerations, 23. The case of _Papilio
Ajax_, 30. Experiments with _Pieris Napi var. Bryoniæ_, 39. The summer
generations of seasonally dimorphic butterflies the more variable, 42.


II.

_Seasonal Dimorphism and Climatic Variation_, p. 45.

Distinction between climatic and local varieties, 45. The case of
_Euchloe Belia_ and its varieties, 47. The case of _Polyommatus
Phlæas_, 49. The case of _Plebeius Agestis_, 50.


III.

_Nature of the Causes producing Climatic Varieties_, p. 52.

Seasonal dimorphism of the same nature as climatic variation, 52.
How does climatic change influence the markings of a butterfly? 52.
The cause of this to be found in temperature, 54. Part played by the
organism itself, 58. Analogous seasonal dimorphism in _Pierinæ_, 60.
The part played by sexual selection, 62.


IV.

_Why all Polygoneutic Species are not Seasonally Dimorphic_, p. 63.

Homochronic heredity, 63. Caterpillars, pupæ and eggs of summer and
winter generations of seasonally dimorphic butterflies alike, 64. The
law of cyclical heredity, 65. Climatic variation of _Pararga Ægeria_,
68. Continuous as distinguished from alternating heredity, 68. Return
from dimorphism to monomorphism, 70. Seasonally dimorphic species
hibernate as pupæ, 71. Retrogressive disturbance of winter generations,
72. The case of _Plebeius Amyntas_, 75.


V.

_On Alternation of Generations_, p. 80.

Haeckel’s classification of the phenomena, 80. Proposed modification,
81. Derivation of metagenesis from metamorphosis, 82. Primary
and secondary metagenesis, 84. Seasonal dimorphism related to
heterogenesis, 86. Heterogenesis and adaptation, 89. Differences
between seasonal dimorphism and other cases of heterogenesis, 89. The
case of _Leptodora Hyalina_, 93.


VI.

_General Conclusions_, p. 100.

Species produced by direct action of environment, 100. The transforming
influences of climate, 103. The origin of variability, 107. The
influence of isolation, 109. Cyclically acting causes of change produce
cyclically recurring changes, 111. Specific constitution an important
factor, 112. A “fixed direction of variation,” 114.


_Appendix I._, p. 117.

Experiments with _Araschnia Levana_, 117. Experiments with _Pierinæ_,
122.


_Appendix II._, p. 126.

Experiments with _Papilio Ajax_, 126. Additional experiments with _Pap.
Ajax_, 131. Experiments with _Phyciodes Tharos_, 140: with _Grapta
Interrogationis_, 149. Remarks on the latter, 152.


_Explanation of the Plates_, p. 159.


=Part II.=

ON THE FINAL CAUSES OF TRANSFORMATION.


=I.=

THE ORIGIN OF THE MARKINGS OF CATERPILLARS.


_Introduction_, p. 161.


I.

_Ontogeny and Morphology of Sphinx-Markings_, p. 177.

The genus _Chærocampa_, 177; _C. Elpenor_, 177; _C. Porcellus_, 184.
Results of the development of these species and comparison with
other species of the genus, 188. The genus _Deilephila_, 199; _D.
Euphorbiæ_, 201; _D. Nicæa_, 207; _D. Dahlii_, 208; _D. Vespertilio_,
209; _D. Galii_, 211; _D. Livornica_, 215; _D. Zygophylli_, 217; _D.
Hippophaës_, 218. Summary of facts and conclusions from this genus,
223. The genus _Smerinthus_, 232; _S. Tiliæ_, 233; _S. Populi_, 236;
_S. Ocellatus_, 240. Results of the development of these species, 242.
The genus _Macroglossa_, 245; _M. Stellatarum_, 245; comparison of this
with other species, 253. The genus _Pterogon_, 255; _P. Œnotheræ_,
256; comparison with other species, 256. The genus _Sphinx_, 259;
_S. Ligustri_, 259; comparison with other species, 261. The genus
_Anceryx_, 264; _A. Pinastri_, 265; comparison with other species, 268.


II.

_Conclusions from Phylogeny_, p. 270.

The Ontogeny of Caterpillars is a much abbreviated but slightly
falsified repetition of the Phylogeny, 270. Three laws of development,
274. The backward transference of new characters to younger stages is
the result of an innate law of growth, 278. Proof that new characters
always originate at the end of the development; the red spots of _S.
Tiliæ_, 282.


III.

_Biological Value of Marking in general_, p. 285.

Markings of Caterpillars most favourable to inquiry, 285. Are the
Sphinx-markings purely morphological, or have they a biological value?
287.


IV.

_Biological Value of Colour_, p. 289.

General prevalence of protective colouring among caterpillars, 289.
Polymorphic adaptive colouring in _C. Elpenor_, _C. Porcellus_,
_P. Œnotheræ_, _D. Vespertilio_, _D. Galii_, _D. Livornica_, _D.
Hippophaës_, 295. Habit of concealment primary; its causes, 298.
Polymorphism does not here depend upon contemporaneous but upon
successive double adaptation; displacement of the old by a new
adaptation; proof in the cases of _D. Hippophaës_, _D. Galii_, _D.
Vespertilio_, _M. Stellatarum_, _C. Elpenor_, and _S. Convolvuli_, 300.


V.

_Biological Value of special Markings_, p. 308.

Four chief forms of marking among _Sphingidæ_, 309. Complete absence of
marking among small caterpillars and among those living in obscurity,
310. Longitudinal stripes among grass caterpillars, 312. Oblique
striping. Coloured edges are the shadows of leaf ribs, 317. Eye-spots
and ring-spots. Definition, 326: Eye-spots not originally signs of
distastefulness, 328; they are means of alarm, 329; experiments with
birds, 330; possibility of a later change of function in eye-spots,
334. Ring-spots. Are they signs of distastefulness? Are there
caterpillars which are edible and which possess bright colours? 335;
experiments with lizards, 336. In _D. Galii_, _D. Euphorbiæ_, _D.
Dahlii_ and _D. Mauritanica_ the ring-spots are probably signs of
distastefulness, 341. In _D. Nicæa_ they are perhaps also means of
exciting terror, 342. The primary ring-spot in _D. Hippophaës_ is a
means of protection, 344. Subordinate markings. Reticulation, 347. The
dorsal spots of _C. Elpenor_ and _C. Porcellus_, 348. The lateral dots
of _S. Convolvuli_, 348. Origination of subordinate markings by the
blending of inherited but useless markings with new ones, 349.


VI.

_Objections to a Phyletic Vital Force_, p. 352.

Independent origination of ring-spots in species of the genus
_Deilephila_, 352. Possible genealogy of this genus, 358. Independent
origination of red spots in several species of _Smerinthus_, 360.
Functional change in the elements of marking, 365. Colour change in the
course of the ontogeny, 367.


VII.

_Phyletic Development of the Markings of the Sphingidæ. Summary and
Conclusion_, p. 370.

The oldest _Sphingidæ_ were devoid of marking, 370. Longitudinal
stripes the oldest form of marking, 371. Oblique striping, 373. Spot
markings, 375. The first and second elements of marking are mutually
exclusive, but not the first and third, or the second and third, 377.
Results with reference to the origin of markings; picture of their
origin and gradual complication, 380. General results; rejection of a
phyletic vital force, 389.


=II.=

ON PHYLETIC PARALLELISM IN METAMORPHIC SPECIES.

_Introduction_, p. 390.


I.

_Larva and Imago vary in Structure independently of each other_, p. 401.

Dimorphism of one stage only, 402. Independent variability of the
stages (heterochronic variability), 403. Constancy and variability are
not inherent properties of certain forms of marking, 407. Heterochronic
variability is not explained by assuming a phyletic vital force,
410. Rarity of greater variability in pupæ. Greater variability more
common among caterpillars than among the imagines. Causes of this
phenomenon, 412. Apparent independent variability of the single larval
stages. Waves of variability, 416. _Saturnia Carpini_ an instance of
_secondary_ variability, 419. Causes of the exact correlation between
the larval stages and its absence between the larva and imago, 429.


II.

_Does the Form-relationship of the Larva coincide with that of the
Imago?_ p. 432.

Family groups, 432. Families frequently completely congruent, 435.
Exception offered by the _Nymphalidæ_, 435. In transitional families
the larvæ also show intermediate forms, 441. Genera; almost completely
congruent; the Nymphalideous genera can be based on the structure of
the larvæ, 444. So also can certain sub-genera, as _Vanessa_, 445.
Incongruence in _Pterogon_, 450. Species; incongruence very common; _S.
Ocellatus_ and _Populi_, 451. Species of _Deilephila_ show a nearer
form-relationship as imagines than as larvæ, 454. Systemy not only the
expression of morphological relationship, 455. Varieties; incongruence
the rule; seasonal dimorphism; climatic varieties; dimorphism of
caterpillars; local varieties of caterpillars, 456. Result of the
investigation, 458. Causes of incongruence, 460. A phyletic vital force
does not explain the phenomena, 461. This force is superfluous, 464.


III.

_Incongruences in other Orders of Insects_, p. 481.

Hymenoptera. The imagines only possess ordinal characters, 481. Double
incongruence: different distance and different group-formation,
483. Diptera, 488. The larvæ form two types depending on different
modes of life, 489. The similarity of the grub-like larvæ of Diptera
and Hymenoptera depends upon convergence, 494. These data again
furnish strong arguments against a phyletic vital force, 496. The
tribe _Aphaniptera_, 498. Results furnished by the form-relationship
of Diptera and Hymenoptera, 499. Difference between typical and
non-typical parts transient, 501.


IV.

_Summary and Conclusion,_ p. 502.

First form of incongruence, 503. Second form of incongruence, 506.
General conclusion as to the elimination of a phyletic vital force,
511. Parallelism with the transformation of systems of organs, 513.


_Appendix I._, p. 520.

Additional notes on the Ontogeny, Phylogeny, &c., of Caterpillars.
Ontogeny of _Noctua_ larvæ, 520. Additional descriptions of
Sphinx-larvæ, 521. Retention of the subdorsal line by ocellated larvæ,
529. Phytophagic variability, 531. Sexual variation in larvæ, 534.


_Appendix II._, p. 536.

_Acræa_ and the _Maracujà_ butterflies as larvæ, pupæ, and imagines,
536.


_Explanation of the Plates_, p. 546.


=Part III.=

ON THE FINAL CAUSES OF TRANSFORMATION (_continued_).


=III.=

THE TRANSFORMATION OF THE MEXICAN AXOLOTL INTO AMBLYSTOMA.

_Introduction_, p. 555.

_Experiments_, 558. Significance of the facts, 563. The Axolotl rarely
or never undergoes metamorphosis in its native country, 565. North
American Amblystomas, 570. Does the exceptional transformation depend
upon a phyletic advancement of the species? 571. Theoretical bearing
of the case, 574. Differences between Axolotl and Amblystoma, 575.
These are not correlative results of the suppression of the gills,
578. Explanation by reversion, 581. Cases of degeneration to a lower
phyletic stage: Filippi’s sexually mature “_Triton_ larvæ,” 583.
Analogous observations on _Triton_ by Jullien and Schreibers, 591. The
sterility of the artificially produced Amblystomas tells against the
former importance of the transformation, 594. It is not opposed to the
hypothesis of reversion, 596. Attempted explanation of the sterility
from this point of view, 597. Causes which may have induced reversion
in the hypothetical Mexican Amblystomas, 600. Saltness of the water
combined with the drying up of the shores by winds, 604. Consequences
of the reversion hypothesis, 609; Systematic, 609; an addendum to the
“fundamental biogenetic law,” 611; General importance of reversion,
612. _Postscript_; dryness of the air the probable cause of the assumed
reversion of the Amblystoma to the Axolotl, 613. _Addendum_, 622.


=IV.=

ON THE MECHANICAL CONCEPTION OF NATURE.

_Introduction_, p. 634.

Results of the three foregoing essays: denial of a phyletic vital
force, 634. Application of these results to inductive conclusions with
reference to the organic world in general, 636. The assumption of such
a force is opposed to the fundamental laws of natural science, 637. The
“vital force” of the older natural philosopher, 640. Why was the latter
abandoned? Commencement of a mechanical theory of life, 642.


I.

_Are the Principles of the Selection Theory Mechanical?_ p. 645.

Refutation of Von Hartmann’s views, 645. Variability, 646. The
assumption of unlimited variability no postulate of the selection
theory, 647. The acknowledgment of a fixed and directed variability
does not necessitate the assumption of a phyletic vital force, 647.
Heredity, 657. Useful modifications do not occur only singly, 657.
New characters appearing singly may also acquire predominance, 659.
A mechanical theory of heredity is as yet wanting, 665. Haeckel’s
“Perigenesis of the Plastidule,” 667. Correlation, 670. The “specific
type” depends upon the physiological equilibrium of the parts of the
organism, 671. The theoretical principles of the doctrine of selection
are thus mechanical, 675. Importance of the physical constitution
of the organism in determining the quality of variations, 676. All
individual variability depends upon unequal external influences, 677.
Deduction of the limitability of variation, 682. Deduction of local
forms, 686. Parallelism between the ontogenetic and the phyletic vital
force, 687. The two are inseparable, 690.


II.

_Mechanism and Teleology_, p. 694.

Von Baer’s exaction from the theory of selection, 694. Justification of
his claim, but the impossibility of the co-operation of a metaphysical
principle with the mechanism of Nature, 695. _Per saltum_ development
(heterogeneous generation), 698. Weakness of the positive basis of
this hypothesis, 699. The latter refuted by the impossibility of the
co-operation of “heterogeneous generation” with natural selection, 702.
The interruption by a metaphysical principle cannot be reconciled with
gradual transformation, 705. The metaphysical (teleological) principle
can only be conceived of as the ultimate ground of the mechanism of
Nature, 709. Value of this knowledge for the harmonious conception of
the Universe, 711. Explanation of the spiritual by the assumption of
conscious matter, 714. The theory of selection does not necessarily
lead to Materialism, 716.

  INDEX                                                   p. 719.



STUDIES IN THE THEORY OF DESCENT.



Part I.

ON THE SEASONAL DIMORPHISM OF BUTTERFLIES.



I.

THE ORIGIN AND SIGNIFICANCE OF SEASONAL DIMORPHISM.


The phenomena here about to be subjected to a closer investigation have
been known for a long period of time. About the year 1830 it was shown
that the two forms of a butterfly (_Araschnia_) which had till that
time been regarded as distinct, in spite of their different colouring
and marking really belonged to the same species, the two forms of
this dimorphic species not appearing simultaneously but at different
seasons of the year, the one in early spring, the other in summer.
To this phenomenon the term “seasonal dimorphism” was subsequently
applied by Mr. A. R. Wallace, an expression of which the heterogeneous
composition may arouse the horror of the philologist, but, as it is
as concise and intelligible as possible, I propose to retain it in the
present work.

The species of _Araschnia_ through which the discovery of seasonal
dimorphism was made, formerly bore the two specific names _A. Levana_
and _A. Prorsa_. The latter is the summer and the former the winter
form, the difference between the two being, to the uninitiated, so
great that it is difficult to believe in their relationship. _A.
Levana_ (Figs. 1 and 2, Plate I.) is of a golden brown colour with
black spots and dashes, while _A. Prorsa_ (Figs. 5 and 6, Plate I.)
is deep black with a broad white interrupted band across both wings.
Notwithstanding this difference, it is an undoubted fact that both
forms are merely the winter and summer generations of the same species.
I have myself frequently bred the variety _Prorsa_ from the eggs of
_Levana_, and _vice versâ_.

Since the discovery of this last fact a considerable number of
similar cases have been established. Thus P. C. Zeller[3] showed, by
experiments made under confinement, that two butterflies belonging to
the family of the ‘Blues,’ differing greatly in colour and marking,
and especially in size, which had formerly been distinguished as
_Plebeius_ (_Lycæna_) _Polysperchon_ and _P. Amyntas_, were merely
winter and summer generations of the same species; and that excellent
Lepidopterist, Dr. Staudinger, proved the same[4] with species
belonging to the family of the ‘Whites,’ _Euchloe Belia_ Esp. and _E.
Ausonia_ Hüb., which are found in the Mediterranean countries.

The instances are not numerous, however, in which the difference
between the winter and summer forms of a species is so great as to
cause them to be treated of in systematic work as distinct species.
I know of only five of these cases. Lesser differences, having the
systematic value of varieties, occur much more frequently. Thus, for
instance, seasonal dimorphism has been proved to exist among many of
our commonest butterflies belonging to the family of the ‘Whites,’ but
the difference in their colour and marking can only be detected after
some attention; while with other species, as for instance with the
commonest of our small ‘Blues,’ _Plebeius Alexis_ (= _Icarus_, Rott.),
the difference is so slight that even the initiated must examine
closely in order to recognize it. Indeed whole series of species might
easily be grouped so as to show the transition from complete similarity
of both generations, through scarcely perceptible differences, to
divergence to the extent of varieties, and finally to that of species.

Nor are the instances of lesser differences between the two generations
very numerous. Among the European diurnal Lepidoptera I know of about
twelve cases, although closer observation in this direction may
possibly lead to further discoveries.[5] Seasonal dimorphism occurs
also in moths, although I am not in a position to make a more precise
statement on this subject,[6] as my own observations refer only to
butterflies.

That other orders of insects do not present the same phenomenon depends
essentially upon the fact that most of them produce only one generation
in the year; but amongst the remaining orders there occur indeed
changes of form which, although not capable of being regarded as pure
seasonal dimorphism, may well have been produced in part by the same
causes, as the subsequent investigation on the relation of seasonal
dimorphism to alternation of generation and heterogenesis will more
fully prove.

Now what are these causes?

Some years ago, when I imparted to a lepidopterist my intention of
investigating the origin of this enigmatical dimorphism, in the hope
of profiting for my inquiry from his large experience, I received
the half-provoking reply: “But there is nothing to investigate:
it is simply the specific character of this insect to appear in
two forms; these two forms alternate with each other in regular
succession according to a fixed law of Nature, and with this we
must be satisfied.” From his point of view the position was right;
according to the old doctrine of species no question ought to be
asked as to the causes of such phenomena in particular. I would not,
however, allow myself to be thus discouraged, but undertook a series of
investigations, the results of which I here submit to the reader.

The first conjecture was, that the differences in the imago might
perhaps be of a secondary nature, and have their origin in the
differences of the caterpillar, especially with those species which
grow up during the spring or autumn and feed on different plants,
thus assimilating different chemical substances, which might induce
different deposits of colour in the wings of the perfect insect. This
latter hypothesis was readily confuted by the fact, that the most
strongly marked of the dimorphic species, _A. Levana_, fed exclusively
on _Urtica major_. The caterpillar of this species certainly exhibits
a well-defined dimorphism, but it is not seasonal dimorphism: the two
forms do not alternate with each other, but appear mixed in every brood.

I have repeatedly reared the rarer golden-brown variety of the
caterpillar separately, but precisely the same forms of butterfly
were developed as from black caterpillars bred at the same time under
similar external conditions. The same experiment was performed, with a
similar result, in the last century by Rösel, the celebrated miniature
painter and observer of nature, and author of the well-known “Insect
Diversions”--a work in use up to the present day.

The question next arises, as to whether the causes originating the
phenomena are not the same as those to which we ascribe the change of
winter and summer covering in so many mammalia and birds--whether the
change of colour and marking does not depend, in this as in the other
cases, upon the _indirect action_ of external conditions of life, i.e.,
on adaptation through natural selection. We are certainly correct in
ascribing white coloration to adaptation[7]--as with the ptarmigan,
which is white in winter and of a grey-brown in summer, both colours
of the species being evidently of important use.

It might be imagined that analogous phenomena occur in butterflies,
with the difference that the change of colour, instead of taking place
in the same brood, alternates in different broods.[8] The nature of
the difference which occurs in seasonal dimorphism, however, decidedly
excludes this view; and moreover, the environment of butterflies
presents such similar features, whether they emerge in spring or in
summer, that all notions that we may be dealing with adaptational
colours must be entirely abandoned.

I have elsewhere[9] endeavoured to show that butterflies in general are
not coloured protectively during flight, for the double reason that
the colour of the background to which they are exposed continually
changes, and because, even with the best adaptation to the background,
the fluttering motion of the wings would betray them to the eyes of
their enemies.[10] I attempted also to prove at the same time that the
diurnal Lepidoptera of our temperate zone have few enemies which pursue
them when on the wing, but that they are subject to many attacks during
their period of repose.

In support of this last statement I may here adduce an instance. In the
summer of 1869 I placed about seventy specimens of _Araschnia Prorsa_
in a spacious case, plentifully supplied with flowers. Although the
insects found themselves quite at home, and settled about the flowers
in very fine weather (one pair copulated, and the female laid eggs),
yet I found some dead and mangled every morning. This decimation
continued--many disappearing entirely without my being able to find
their remains--until after the ninth day, when they had all, with one
exception, been slain by their nocturnal foes--probably spiders and
_Opilionidæ_.

Diurnal Lepidoptera in a position of rest are especially exposed to
hostile attacks. In this position, as is well known, their wings are
closed upright, and it is evident that the adaptational colours on
the under side are displayed, as is most clearly shown by many of our
native species.[11]

Now, the differences in the most pronounced cases of seasonal
dimorphism--for example, in _Araschnia Levana_--are much less manifest
on the _under_ than on the _upper_ side of the wing. The explanation by
adaptation is therefore untenable; but I will not here pause to confute
this view more completely, as I believe I shall be able to show the
true cause of the phenomenon.

If seasonal dimorphism does not arise from the _indirect_ influence of
varying seasons of the year, it may result from the _direct_ influence
of the varying external conditions of life, which are, without doubt,
different in the winter from those of the summer brood.

There are two prominent factors from which such an influence may be
expected--temperature and duration of development, i.e., duration
of the chrysalis period. The duration of the larval period need not
engage our attention, as it is only very little shorter in the winter
brood--at least, it was so with the species employed in the experiments.

Starting from these two points of view, I carried on experiments for
a number of years, in order to find out whether the dual form of the
species in question could be traced back to the direct action of the
influences mentioned.

The first experiments were made with _Araschnia Levana_. From the eggs
of the winter generation, which had emerged as butterflies in April,
I bred caterpillars, and immediately after pupation placed them in a
refrigerator, the temperature of the air of which was 8°-10° R. It
appeared, however, that the development could not thus be retarded to
any desired period by such a small diminution of temperature, for,
when the box was taken out of the refrigerator after thirty-four days,
all the butterflies, about forty in number, had emerged, many being
dead, and others still living. The experiment was so far successful
that, instead of the _Prorsa_ form which might have been expected
under ordinary circumstances, most of the butterflies emerged as the
so-called _Porima_ (Figs. 3, 4, 7, 8, and 9, Plate I.); that is to say,
in a form intermediate between _Prorsa_ and _Levana_ sometimes found
in nature, and possessing more or less the marking of the former, but
mixed with much of the yellow of _Levana_.

It should be here mentioned, that similar experiments were made
in 1864 by George Dorfmeister, but unfortunately I did not get
this information[12] until my own were nearly completed. In these
well-conceived, but rather too complicated experiments, the author
arrives at the conclusion “that temperature certainly affects the
colouring, and through it the marking, of the future butterfly, and
chiefly so during pupation.” By lowering the temperature of the air
during a portion of the pupal period, the author was enabled to produce
single specimens of _Porima_, but most of the butterflies retained
the _Prorsa_ form. Dorfmeister employed a temperature a little higher
than I did in my first experiments, viz. 10°-11° R., and did not leave
the pupæ long exposed, but after 5½-8 days removed them to a higher
temperature. It was therefore evident that he produced transition forms
in a few instances only, and that he never succeeded in bringing about
a complete transformation of the summer into the winter form.

In my subsequent experiments I always exposed the pupæ to a temperature
of 0°-1° R.; they were placed directly in the refrigerator, and
taken out at the end of four weeks. I started with the idea that it
was perhaps not so much the reduced temperature as the retardation of
development which led to the transformation. But the first experiment
had shown that the butterflies emerged between 8° and 10° R., and
consequently that the development could not be retarded at this
temperature.

A very different result was obtained from the experiment made at a
lower temperature.[13] Of twenty butterflies, fifteen had become
transformed into _Porima_, and of these three appeared very similar to
the winter form (_Levana_), differing only in the absence of the narrow
blue marginal line, which is seldom absent in the true _Levana_. Five
butterflies were uninfluenced by the cold, and remained unchanged,
emerging as the ordinary summer form (_Prorsa_). It thus appeared from
this experiment, that a large proportion of the butterflies inclined
to the _Levana_ form by exposure to a temperature of 0°-1° R. for four
weeks, while in a few specimens the transformation into this form was
nearly perfect.

Should it not be possible to perfect the transformation, so that
each individual should take the _Levana_ form? If the assumption of
the _Prorsa_ or _Levana_ form depends only on the direct influence
of temperature, or on the duration of the period of development, it
should be possible to compel the pupæ to take one or the other form
at pleasure, by the application of the necessary external conditions.
This has never been accomplished with _Araschnia Prorsa_. As in the
experiment already described, and in all subsequent ones, single
specimens appeared as the unchanged summer form, others showed an
appearance of transition, and but very few had changed so completely
as to be possibly taken for the pure _Levana_. In some species of the
sub-family _Pierinæ_, however, at least in the case of the summer
brood, there was, on the contrary, a complete transformation.

Most of the species of our ‘Whites’ (_Pierinæ_) exhibit the phenomenon
of seasonal dimorphism, the winter and summer forms being remarkably
distinct. In _Pieris Napi_ (with which species I chiefly experimented)
the winter form (Figs. 10 and 11, Plate I.) has a sprinkling of deep
black scales at the base of the wings on the upper side, while the tips
are more grey, and have in all cases much less black than in the summer
form; on the underside the difference lies mainly in the frequent
breadth, and dark greenish-black dusting, of the veins of the hind
wings in the winter form, while in the summer form these greenish-black
veins are but faintly present.

I placed numerous specimens of the summer brood, immediately after
their transformation into chrysalides, in the refrigerator (0°-1° R.),
where I left them for three months, transferring them to a hothouse
on September 11th, and there (from September 26th to October 3rd)
sixty butterflies emerged, the whole of which, without exception--and
most of them in an unusually strong degree--bore the characters of the
winter form. I, at least, have never observed in the natural state
such a strong yellow on the underside of the hind wings, and such a
deep blackish-green veining, as prevailed in these specimens (see, for
instance, Figs. 10 and 11). The temperature of the hothouse (12°-24°
R.) did not, however, cause the emergence of the whole of the pupæ; a
portion hibernated, and produced in the following spring butterflies of
the winter form only. I thus succeeded, with this species of _Pieris_,
in completely changing every individual of the summer generation into
the winter form.

It might be expected that the same result could be more readily
obtained with _A. Levana_, and fresh experiments were undertaken, in
order that the pupæ might remain in the refrigerator fully two months
from the period of their transformation (9-10th July). But the result
obtained was the same as before--fifty-seven butterflies emerged in the
hothouse[14] from September 19th to October 4th, nearly all of these
approaching very near to the winter form, without a single specimen
presenting the appearance of a perfect _Levana_, while three were of
the pure summer form (_Prorsa_).

Thus with _Levana_ it was not possible, by refrigeration and
retardation of development, to change the summer completely into the
winter form in all specimens. It may, of course, be objected that
the period of refrigeration had been too short, and that, instead
of leaving the pupæ in the refrigerator for two months, they should
have remained there six months, that is, about as long as the winter
brood remains under natural conditions in the chrysalis state. The
force of this last objection must be recognized, notwithstanding the
improbability that the desired effect would be produced by a longer
period of cold, since the doubling of this period from four to eight
weeks did not produce[15] any decided increase in the strength of the
transformation. I should not have omitted to repeat the experiment in
this modified form, but unfortunately, in spite of all trouble, I was
unable to collect during the summer of 1873 a sufficient number of
caterpillars. But the omission thus caused is of quite minor importance
from a theoretical point of view.

For let us assume that the omitted experiment had been performed--that
pupæ of the summer brood were retarded in their development by cold
until the following spring, and that every specimen then emerged in
the perfect winter form, Levana. Such a result, taken in connexion with
the corresponding experiment upon _Pieris Napi_, would warrant the
conclusion that the direct action of a certain amount of cold (or of
retardation of development) is able to compel all pupæ, from whichever
generation derived, to assume the winter form of the species. From this
the converse would necessarily follow, viz. that a certain amount of
warmth would lead to the production of the summer form, _Prorsa_, it
being immaterial from which brood the pupæ thus exposed to warmth might
be derived. But the latter conclusion was proved experimentally to be
incorrect, and thus the former falls with it, whether the imagined
experiment with _Prorsa_ had succeeded or not.

I have repeatedly attempted by the application of warmth to change the
winter into the summer form, but always with the same negative result.
_It is not possible to compel the winter brood to assume the form of
the summer generation._

_A. Levana_ may produce not only two but three broods in the year, and
may, therefore, be said to be _polygoneutic_.[16] One winter brood
alternates with two summer broods, the first of which appears in July,
and the second in August. The latter furnishes a fourth generation of
pupæ, which, after hibernation, emerge in April, as the first brood of
butterflies in the form _Levana_.

I frequently placed pupæ of this fourth brood in the hothouse
immediately after their transformation, and in some cases even during
the caterpillar stage, the temperature never falling, even at night,
below 12° R., and often rising during the day to 24° R. The result was
always the same: all, or nearly all, the pupæ hibernated, and emerged
the following year in the winter form as perfectly pure _Levana_,
without any trace of transition to the _Prorsa_ form. On one occasion
only was there a _Porima_ among them, a case for which an explanation
will, I believe, be found later on. It often happened, on the other
hand, that some few of the butterflies emerged in the autumn, about
fourteen days after pupation; and these were always _Prorsa_ (the
summer form), excepting once a _Porima_.

From these experiments it appeared that similar causes (heat) affect
different generations of _A. Levana_ in different manners. With
both summer broods a high temperature always caused the appearance
of _Prorsa_, this form arising but seldom from the third brood (and
then only in a few individuals), while the greater number retained
the _Levana_ form unchanged. We may assign as the reason for this
behaviour, that the third brood has no further tendency to be
accelerated in its development by the action of heat, but that by a
longer duration of the pupal stage the _Levana_ form must result. On
one occasion the chrysalis stage was considerably shortened in this
brood by the continued action of a high temperature, many specimens
thus having their period of development reduced from six to three
months. The supposed explanation above given is, however, in reality no
explanation at all, but simply a restatement of the facts. The question
still remains, why the third brood in particular has no tendency to be
accelerated in its development by the action of heat, as is the case
with both the previous broods?

The first answer that can be given to this question is, that the cause
of the different action produced by a similar agency can only lie in
the _constitution_, i.e., in the _physical nature_ of the broods in
question, and not in the external influences by which they are acted
upon. Now, what is the difference in the physical nature of these
respective broods? It is quite evident, as shown by the experiments
already described, that cold and warmth cannot be the _immediate_
causes of a pupa emerging in the _Prorsa_ or _Levana_ form, since the
last brood always gives rise to the _Levana_ form, whether acted on by
cold or warmth. The first and second broods only can be made to partly
assume, more or less completely, the _Levana_ form by the application
of cold. In these broods then, a low temperature is the _mediate_ cause
of the transformation into the _Levana_ form.

The following is my explanation of the facts. The form _Levana_ is
the original type of the species, and _Prorsa_ the secondary form
arising from the gradual operation of summer climate. When we are able
to change many specimens of the summer brood into the winter form by
means of cold, this can only depend upon reversion to the original, or
ancestral, form, which reversion appears to be most readily produced
by cold, that is, by the same external influences as those to which
the original form was exposed during a long period of time, and the
continuance of which has preserved, in the winter generations, the
colour and marking of the original form down to the present time.

I consider the origination of the _Prorsa_ from the _Levana_ form
to have been somewhat as follows:--It is certain that during the
diluvial period in Europe there was a so-called ‘glacial epoch,’
which may have spread a truly polar climate over our temperate zone;
or perhaps a lesser degree of cold may have prevailed with increased
atmospheric precipitation. At all events, the summer was then short
and comparatively cold, and the existing butterflies could have
only produced one generation in the year; in other words, they were
_monogoneutic_. At that time _A. Levana_ existed only in the _Levana_
form.[17] As the climate gradually became warmer, a period must have
arrived when the summer lasted long enough for the interpolation of
a second brood. The pupæ of _Levana_, which had hitherto hibernated
through the long winter to appear as butterflies in the following
summer, were now able to appear on the wing as butterflies during the
same summer as that in which they left their eggs as larvæ, and eggs
deposited by the last brood produced larvæ which fed up and hibernated
as pupæ. A state of things was thus established in which the first
brood was developed under very different climatic conditions from the
second. So considerable a difference in colour and marking between the
two forms as we now witness could not have arisen suddenly, but must
have done so gradually. It is evident from the foregoing experiments
that the _Prorsa_ form did not originate suddenly. Had this been the
case it would simply signify that every individual of this species
possessed the faculty of assuming two different forms according as it
was acted on by warmth or cold, just in the same manner as litmus-paper
becomes red in acids and blue in alkalies. The experiments have shown,
however, that this is not the case, but rather that the last generation
bears an ineradicable tendency to take the _Levana_ form, and is not
susceptible to the influence of warmth, however long continued; while
both summer generations, on the contrary, show a decided tendency to
assume the _Prorsa_ form, although they certainly can be made to assume
the _Levana_ form in different degrees by the prolonged action of cold.

The conclusion seems to me inevitable, that the origination of the
_Prorsa_ form was gradual--that those changes which originated in
the chemistry of the pupal stage, and led finally to the _Prorsa_
type, occurred very gradually, at first perhaps remaining completely
latent throughout a series of generations, then very slight changes
of marking appearing, and finally, after a long period of time, the
complete _Prorsa_ type was produced. It appears to me that the quoted
results of the experiments are not only easily explained on the view
of the _gradual_ action of climate, but that this view is the only one
admissible. The action of climate is best comparable with the so-called
cumulative effect of certain drugs on the human body; the first small
dose produces scarcely any perceptible change, but if often repeated
the effect becomes cumulative, and poisoning occurs.

This view of the action of climate is not at all new, most zoologists
having thus represented it; only the formal proof of this action is
new, and the facts investigated appear to me of special importance as
furnishing this proof. I shall again return to this view in considering
climatic varieties, and it will then appear that also the nature of the
transformation itself confirms the slow operation of climate.

During the transition from the glacial period to the present climate
_A. Levana_ thus gradually changed from a monogoneutic to a digoneutic
species, and at the same time became gradually more distinctly
dimorphic, this character originating only through the alteration of
the summer brood, the primary colouring and marking of the species
being retained unchanged by the winter brood. As the summer became
longer a third generation could be interpolated--the species became
polygoneutic; and in this manner two summer generations alternated with
one winter generation.

We have now to inquire whether facts are in complete accordance with
this theory--whether they are never at variance with it--and whether
they can all be explained by it. I will at once state in anticipation,
that this is the case to the fullest extent.

In the first place, the theory readily explains why the summer but not
the winter generations are capable of being transformed; the latter
cannot possibly revert to the _Prorsa_ form, because this is much the
younger. When, however, it happens that out of a hundred cases there
occurs one in which a chrysalis of the winter generation, having been
forced by warmth, undergoes transformation before the commencement of
winter, and emerges in the summer form,[18] this is not in the least
inexplicable. It cannot be atavism which determines the direction of
the development; but we see from such a case that the changes in the
first two generations have already produced a certain alteration in
the third, which manifests itself in single cases under favourable
conditions (the influence of warmth) by the assumption of the _Prorsa_
form; or, as it might be otherwise expressed, the _alternating_
heredity (of which we shall speak further), which implies the power
of assuming the _Prorsa_ form, remains latent as a rule in the winter
generation, but becomes _continuous_ in single individuals.

It is true that we have as yet no kind of insight into the nature of
heredity, and this at once shows the defectiveness of the foregoing
explanation; but we nevertheless know many of its external phenomena.
We know for certain that one of these consists in the fact that
peculiarities of the father do not appear in the son, but in the
grandson, or still further on, and that they may be thus transmitted
in a latent form. Let us imagine a character so transmitted that it
appears in the first, third, and fifth generations, remaining latent
in the intermediate ones; it would not be improbable, according to
previous experiences, that the peculiarity should exceptionally, i.e.,
from a cause unknown to us, appear in single individuals of the second
or fourth generation. But this completely agrees with those cases in
which “exceptional” individuals of the winter brood took the _Prorsa_
form, with the difference only that a cause (warmth) was here apparent
which occasioned the development of the latent characters, although we
are not in a position to say in what manner heat produces this action.
These exceptions to the rule are therefore no objection to the theory.
On the contrary, they give us a hint that after one _Prorsa_ generation
had been produced, the gradual interpolation of a second _Prorsa_
generation may have been facilitated by the existence of the first.
I do not doubt that even in the natural state single individuals of
_Prorsa_ sometimes emerge in September or October; and if our summer
were lengthened by only one or two months this might give rise to a
third summer brood (just as a second is now an accomplished fact),
under which circumstances they would not only emerge, but would also
have time for copulation and for depositing eggs, the larvæ from which
would have time to grow up.

A sharp distinction must be made between the first establishment
of a new climatic form and the transference of the latter to newly
interpolated generations. The former always takes place very slowly;
the latter may occur in a shorter time.

With regard to the duration of time which is necessary to produce a
new form by the influence of climate, or to transmit to a succeeding
generation a new form already established, great differences occur,
according to the physical nature of the species and of the individual.
The experiments with _Prorsa_ already described show how diverse are
individual proclivities in this respect. In Experiment No. 12 it was
not possible out of seventy individuals to substitute _Prorsa_ for the
_Levana_ form, even in one solitary case, or, in other words, to change
alternating into continuous inheritance; whilst in the corresponding
experiments of former years (Experiment 10, for example), out of an
equal number of pupæ three emerged as _Prorsa_, and one as _Porima_. We
might be inclined to seek for the cause of this different behaviour in
external influences, but we should not thus arrive at an explanation of
the facts. We might suppose, for instance, that a great deal depended
upon the particular period of the pupal stage at which the action of
the elevated temperature began--whether on the first, the thirtieth,
or the hundredth day after pupation--and this conjecture is correct
in so far that in the two last cases warmth can have no further
influence than that of somewhat accelerating the emergence of the
butterflies, but cannot change the _Levana_ into the _Prorsa_ form. I
have repeatedly exposed a large number of _Levana_ pupæ of the third
generation to the temperature of an apartment, or even still higher
(26° R.), during winter, but no _Prorsa_ were obtained.[19]

But it would be erroneous to assume a difference in the action of heat
according as it began on the first or third day after transformation;
whether during or before pupation. This is best proved by Experiment
No. 12, in which caterpillars of the fourth generation were placed in
the hothouse several days before they underwent pupation; still, not
a single butterfly assumed the _Prorsa_ form. I have also frequently
made the reverse experiment, and exposed caterpillars of the first
summer brood to cold during the act of pupation. A regular consequence
was the dying off of the caterpillars, which is little to be wondered
at, as the sensitiveness of insects during ecdysis is well known, and
transformation into the pupal state is attended by much deeper changes.

Dorfmeister thought that he might conclude from his experiments that
temperature exerts the greatest influence in the first place during
the act of pupation, and in the next place immediately after that
period. His experiments were made, however, with such a small number
of specimens that scarcely any safe conclusion can be founded on them;
still, this conclusion may be correct, in so far as everything depends
on whether, from the beginning, the formative processes in the pupa
tended to this or that direction, the final result of which is the
_Prorsa_ or _Levana_ form. If once there is a tendency to one or the
other direction, then temperature might exert an accelerating or a
retarding influence, but the tendency cannot be further changed.

It is also possible--indeed, probable--that a period may be fixed in
which warmth or cold might be able to divert the original direction of
development most easily; and this is the next problem to be attacked,
the answer to which, now that the main points have been determined,
should not be very difficult. I have often contemplated taking the
experiments in hand myself, but have abandoned them, because my
materials did not appear to me sufficiently extensive, and in all such
experiments nothing is to be more avoided than a frittering away of
experimental materials by a too complicated form of problem.

There may indeed be a period most favourable for the action of
temperature during the first days of the pupal stage; it appears from
Experiment No. 12 that individuals tend in different degrees to
respond to such influences, and that the disposition to abandon the
ordinary course of development is different in different individuals.
In no other way can it be explained that, in all the experiments made
with the first and second generations of _Prorsa_, only _a portion_ of
the pupæ were compelled by cold to take the direction of development
of _Levana_, and that even from the former only a few individuals
completely reverted, the majority remaining intermediate.

If it be asked why in the corresponding experiments with _Pieris
Napi_ complete reversion always occurred without exception, it may
be supposed that in this species the summer form has not been so
long in existence, and that it would thus be more easily abandoned;
or, that the difference between the two generations has not become
so distinct, which further signifies that here again the summer form
is of later origin. It might also be finally answered, that the
tendency to reversion in different species may vary just as much as
in different individuals of the same species. But, in any case, the
fact is established that all individuals are impelled by cold to
complete reversion, and that in these experiments it does not depend so
particularly upon the moment of development when cold is applied, but
that differences of individual constitution are much more the cause why
cold brings some pupæ to complete, and others to partial, reversion,
while yet others are quite uninfluenced. In reference to this, the
American _Papilio Ajax_ is particularly interesting.

This butterfly, which is somewhat similar to the European _P.
Podalirius_, appears, wherever it occurs, in three varieties,
designated as var. _Telamonides_, var. _Walshii_, and var. _Marcellus_.
The distinguished American entomologist, W. H. Edwards, has proved
by breeding experiments, that all three forms belong to the same
cycle of development, and in such a manner that the first two appear
only in spring, and always come only from hibernating pupæ, while
the last form, var. _Marcellus_, appears only in summer, and then
in three successive generations. A seasonal dimorphism thus appears
which is combined with ordinary dimorphism, winter and summer forms
alternating with each other; but the first appears itself in two forms
or varieties, vars. _Telamonides_ and _Walshii_. If for the present we
disregard this complication, and consider these two winter forms as
one, we should thus have four generations, of which the first possesses
the winter form, and the three succeeding ones have, on the other hand,
the summer form, var. _Marcellus_.

The peculiarity of this species consists in the fact that in all
three summer generations only a portion of the pupæ emerge after a
short period (fourteen days), whilst another and much smaller portion
remains in the pupal state during the whole summer and succeeding
winter, first emerging in the following spring, and then always in the
winter form. Thus, Edwards states that out of fifty chrysalides of the
second generation, which had pupated at the end of June, forty-five
_Marcellus_ butterflies appeared after fourteen days, whilst five pupæ
emerged in April of the following year, and then as _Telamonides_.

The explanation of these facts is easily afforded by the foregoing
theory. According to this, both the winter forms must be regarded as
primary, and the _Marcellus_ form as secondary. But this last is not
yet so firmly established as _Prorsa_, in which reversion of the summer
generations to the _Levana_ form only occurs through special external
influences; whilst in the case of _Ajax_ some individuals are to be
found in every generation, the tendency of which to revert is still so
strong that even the greatest summer heat is unable to cause them to
diverge from their original inherited direction of development, or to
accelerate their emergence and compel them to assume the _Marcellus_
form. It is here beyond a doubt that it is not different external
influences, but internal causes only, which maintain the old hereditary
tendency, for all the larvæ and pupæ of many different broods were
simultaneously exposed to the same external influences. But, at the
same time, it is evident that these facts are not opposed to the
present theory; on the contrary, they confirm it, inasmuch as they
are readily explained on the basis of the theory, but can scarcely
otherwise be understood.

If it be asked what significance attaches to the duplication of the
winter form, it may be answered that the species was already dimorphic
at the time when it appeared in only one annual generation. Still,
this explanation may be objected to, since a dimorphism of this kind
is not at present known, though indeed some species exhibit a sexual
dimorphism,[20] in which one sex (as, for instance, the case of the
female _Papilio Turnus_) appears in two forms of colouring, but not a
dimorphism, as is here the case, displayed by both sexes.[21] Another
suggestion, therefore, may perhaps be offered.

In _A. Levana_ we saw that reversion occurred in very different degrees
with different individuals, seldom attaining to the true _Levana_
form, and generally only reaching the intermediate form known as
_Porima_. Now it would, at all events, be astonishing if with _P. Ajax_
the reversion were always complete, as it is precisely in this case
that the tendency to individual reversion is so variable. I might,
for this reason, suppose that one of the two winter forms, viz. the
var. _Walshii_, is nothing else than an incomplete reversion-form,
corresponding to _Porima_ in the case of _A. Levana_. Then
_Telamonides_ only would be the original form of the butterfly, and
this would agree with the fact that this variety appears later in the
spring than _Walshii_. Experiments ought to be able to decide this.[22]
The pupæ of the first three generations placed upon ice should give,
for the greater part, the form _Telamonides_, for the lesser portion
_Walshii_, and for only a few, or perhaps no individuals, the form
_Marcellus_. This prediction is based on the view that the tendency
to revert is on the whole great; that even with the first summer
generation, which was the longest exposed to the summer climate, a
portion of the pupæ, without artificial means, always emerged as
_Telamonides_, and another portion as _Marcellus_. The latter will
perhaps now become _Walshii_ by the application of cold.

One would expect that the second and third generations would revert
more easily, and in a larger percentage, than the first, because
this latter first acquired the new _Marcellus_ form; but the present
experiments furnish no safe conclusion on this point. Thus, of the
first summer generation only seven out of sixty-seven pupæ hibernated,
and these gave _Telamonides_; while of the second generation forty
out of seventy-six, and of the third generation twenty-nine out of
forty-two pupæ hibernated. But to establish safer conclusions, a still
larger number of experiments is necessary. According to the experience
thus far gained, one might perhaps still be inclined to imagine
that, with seasonal dimorphism, external influences operating on the
individual might directly compel it to assume one or the other form.
I long held this view myself, but it is, nevertheless, untenable.
That cold does not produce the one kind of marking, and warmth the
other, follows from the before-mentioned facts, viz. that in _Papilio
Ajax_ every generation produces both forms; and, further, in the case
of _A. Levana_ I have frequently reared the fourth (hibernating)
generation entirely in a warm room, and yet I have always obtained the
winter form. Still, one might be inclined not to make the temperature
_directly_ responsible, but rather the retardation or acceleration of
development produced through the action of temperature. I confess that
I for a long time believed that in this action I had found the true
cause of seasonal dimorphism. Both with _A. Levana_ and _P. Napi_ the
difference between the duration of the pupal period in the winter and
summer forms is very great, lasting as a rule, in the summer generation
of _A. Levana_, from seven to twelve days, and in the winter generation
about two hundred days. In this last species the pupal state can
certainly be shortened by keeping them at an elevated temperature;
but I have, nevertheless, only in one case obtained two or three
butterflies at the end of December from caterpillars that had pupated
in September, these generally emerging in the course of February and
March, and are to be seen on the wing in warm weather during the latter
month. The greatest reduction of the pupal period still leaves for this
stage more than 100 days.

From this last observation it follows that it is not the duration of
development which, in individual cases, determines the form of the
butterfly, and which consequently decides whether the winter or summer
form shall emerge, but that, on the contrary, the duration of the pupal
stage is dependent on the tendency which the forthcoming butterfly
had taken in the chrysalis state. This can be well understood when we
consider that the winter form must have had a long, and the summer form
a short pupal period, during innumerable generations. In the former
the habit of slow development must have been just as well established
as that of rapid development in the latter; and we cannot be at all
surprised if we do not see this habit abandoned by the winter form
when the opportunity presents itself. But that it may be occasionally
abandoned the more proves that the duration of the pupal development
less determines the butterfly form than does the temperature directly,
in individual cases.

Thus, for instance, Edwards explicitly states that, whereas the two
winter forms of _P. Ajax_, viz. the vars. _Walshii_ and _Telamonides_,
generally appear only after a pupal period of 150 to 270 days, yet
individual cases occur in which the pupal stage is no longer than in
the summer form, viz. fourteen days.[23] A similar thing occurs with
_A. Levana_, for, as already explained, not only may the development
of the winter form be forced to a certain degree by artificial warmth,
but the summer generation frequently produces reversion-forms without
protraction of development. The intermediate reversion-form _Porima_
was known long before it was thought possible that it could be produced
artificially by the action of cold; it appears occasionally, although
very rarely, at midsummer in the natural state.

If, then, my explanation of the phenomena is correct, the winter form
is primary and the summer the secondary form, and those individuals
which, naturally or artificially, assume the winter form must be
considered as cases of atavism. The suggestion thus arises whether
low temperature alone is competent to bring about this reversion, or
whether other external influences are not also effective. Indeed,
the latter appears to be the case. Besides purely internal causes,
as previously pointed out in _P. Ajax_, warmth and mechanical motion
appear to be able to bring about reversion.

That an unusually high temperature may cause reversion, I conclude
from the following observation. In the summer of 1869 I bred the first
summer brood of _A. Levana_; the caterpillars pupated during the second
half of June, and from that time to their emergence, on 28th June-3rd
July, great heat prevailed. Now, while the intermediate form _Porima_
had hitherto been a great rarity, both in the free state and when bred,
having never obtained it myself, for example, out of many hundreds
of specimens, there were among the sixty or seventy butterflies that
emerged from the above brood, some eight to ten examples of _Porima_.
This is certainly not an exact experiment, but there seems to me a
certain amount of probability that the high summer temperature in this
case brought about reversion.

Neither for the second cause to which I have ascribed the power of
producing reversion can I produce any absolute evidence, since the
experimental solution of all these collateral questions would demand an
endless amount of time. I am in possession of an observation, however,
which makes it appear probable to me that continuous mechanical
movement acts on the development of the pupæ in a similar manner
to cold, that is, retarding them, and at the same time producing
reversion. I had, in Freiburg, a large number of pupæ of the first
summer brood of _Pieris Napi_, bred from eggs. I changed residence
while many caterpillars were in course of transformation and travelled
with the pupæ in this state seven hours by rail. Although this brood of
_P. Napi_, under ordinary circumstances, always emerges in the summer,
generally in July of the same year, as the summer form (var. _Napeæ_),
yet out of these numerous pupæ I did not get a single butterfly
during the year 1872. In winter I kept them in a warm room, and the
first butterflies emerged in January, 1873, the remainder following
in February, March, and April, and two females not until June. All
appeared, however, as exquisite winter forms. The whole course of
development was precisely as though cold had acted on the pupæ; and in
fact, I could find no other cause for this quite exceptional deportment
than the seven hours’ shaking to which the pupæ were exposed by the
railway journey, immediately after or during their transformation.

It is obviously a fact of fundamental importance to the theory of
seasonal dimorphism, that the summer form can be readily changed into
the winter form, whilst the latter cannot be changed into the summer
form. I have thus far only made experiments on this subject with _A.
Levana_, but the same fact appears to me to obtain for _P. Napi_. I
did not, however, operate upon the ordinary winter form of _P. Napi_,
but chose for this experiment the variety _Bryoniæ_, well known to
all entomologists. This is, to a certain extent, the potential winter
form of _P. Napi_; the male (Fig. 14, Plate I.) exactly resembles the
ordinary winter form in the most minute detail, but the female is
distinguished from _Napi_ by a sprinkling of greyish brown scales over
the whole of the upper side of the wings (Fig. 15, Plate I.). This
type, _Bryoniæ_, occurs in Polar regions as the only form of _Napi_,
and is also found in the higher Alps, where it flies in secluded
meadows as the only form, but in other localities, less isolated,
mixed with the ordinary form of the species. In both regions _Bryoniæ_
produces but one generation in the year, and must thus, according to my
theory, be regarded as the parent-form of _Pieris Napi_.

If this hypothesis is correct--if the variety _Bryoniæ_ is really the
original form preserved from the glacial period in certain regions of
the earth, whilst _Napi_ in its winter form is the first secondary form
gradually produced through a warm climate, then it would be impossible
ever to breed the ordinary form _Napi_ from pupæ of _Bryoniæ_ by
the action of warmth, since the form of the species now predominant
must have come into existence only by a cumulative action exerted on
numerous generations, and not _per saltum_.

The experiment was made in the following manner: In the first part
of June I caught a female of _Bryoniæ_ in a secluded Alpine valley,
and placed her in a capacious breeding-cage, where she flew about
among the flowers, and laid more than a hundred eggs on the ordinary
cabbage. Although the caterpillars in the free state feed upon another
plant unknown to me, they readily ate the cabbage, grew rapidly, and
pupated at the end of July. I then brought the pupæ into a hothouse
in which the temperature fluctuated between 12° and 24° R.; but, in
spite of this high temperature, and--what is certainly of more special
importance--notwithstanding the want of cooling at night, only one
butterfly emerged the same summer, and that a male, which, from certain
minute characteristic markings, could be safely identified as var.
_Bryoniæ_. The other pupæ hibernated in the heated room, and produced,
from the end of January to the beginning of June, 28 butterflies, all
of which were exquisite _Bryoniæ_.

Experiment thus confirmed the view that _Bryoniæ_ is the parent-form
of _Napi_, and the description hitherto given by systematists ought
therefore properly to be reversed. _Pieris Bryoniæ_ should be elevated
to the rank of a species, and the ordinary winter and summer forms
should be designated as vars. _Napi_ and _Napeæ_. Still I should not
like to take it upon myself to increase the endless confusion in the
synonomy of butterflies. In a certain sense, it is also quite correct
to describe the form _Bryoniæ_ as a climatic variety, for it is, in
fact, established, if not produced, by climate, by which agency it
is likewise preserved; only it is not a secondary, but the primary,
climatic variety of _Napi_. In this sense most species might probably
be described as climatic varieties, inasmuch as under the influence of
another climate they would gradually acquire new characters, whilst,
under the influence of the climate now prevailing in their habitats,
they have, to a certain extent, acquired and preserved their present
form.

The var. _Bryoniæ_ is, however, of quite special interest, since it
makes clear the relation which exists between climatic variation
and seasonal dimorphism, as will be proved in the next section. The
correctness of the present theory must first here be submitted to
further proof.

It has been shown that the secondary forms of seasonally dimorphic
butterflies do not all possess the tendency to revert in the same
degree, but that this tendency rather varies with each individual. As
the return to the primary form is synonymous with the relinquishing of
the secondary, the greater tendency to revert is thus synonymous with
the greater tendency to relinquish the secondary form, but this again
is equivalent to a lesser stability of the latter; it must consequently
be concluded that the individuals of a species are very differently
influenced by climatic change, so that with some the new form must
become sooner established than with others. From this a variability of
the generation concerned must necessarily ensue, i.e., the individuals
of the summer generation must differ more in colour and marking than
is the case with those of the winter generation. If the theory is
correct, the summer generations should be more variable than the winter
generations--at least, so long as the greatest possible equalization of
individual variations has not occurred through the continued action of
warmth, combined with the constant crossing of individuals which have
become changed in different degrees. Here also the theory is fully in
accord with facts.

In _A. Levana_ the _Levana_ form is decidedly more constant than the
_Prorsa_ form. The first is, to a slight extent, sexually dimorphic,
the female being light and the male dark-coloured. If we take into
consideration this difference between the sexes, which also occurs to
a still smaller extent in the _Prorsa_ form, the foregoing statement
will be found correct, viz. that the _Levana_ form varies but little,
and in all cases considerably less than the _Prorsa_ form, in which
the greatest differences occur in the yellow stripes and in the
disappearance of the black spots on the white band of the hind wing,
these black spots being persistent _Levana_ markings. It is, in fact,
difficult to find two perfectly similar individuals of the _Prorsa_
form. It must, moreover, be considered that the _Levana_ marking,
being the more complicated, would the more readily show variation.
Precisely the same thing occurs in _Pieris Napi_, in which also the
var. _Æstiva_ is considerably more variable than the var. _Vernalis_.
From the behaviour of the var. _Bryoniæ_, on the other hand, which I
regard as the parent-form, one might be tempted to raise an objection
to the theory; for this form is well known to be extraordinarily
variable in colour and marking, both in the Alps and Jura, where it is
met with at the greatest altitudes. According to the theory, _Bryoniæ_
should be less variable than the winter form of the lowlands, because
it is the older, and should therefore be the more constant in its
characters. It must not be forgotten, however, that the variability of
a species may not only originate in the one familiar manner of unequal
response of the individual to the action of varying exciting causes,
but also by the crossing of two varieties separately established in
adjacent districts and subsequently brought into contact. In the
Alps and Jura the ordinary form of _Napi_ swarms everywhere from the
plains towards the habitats of _Bryoniæ_, so that a crossing of the
two forms may occasionally, or even frequently, take place; and it is
not astonishing if in some places (Meiringen, for example) a perfect
series of intermediate forms between _Napi_ and _Bryoniæ_ is met with.
That crossing is the cause of the great variability of _Bryoniæ_ in
the Alpine districts, is proved by the fact that in the Polar regions
this form “is by no means so variable as in the Alps, but, judging
from about forty to fifty Norwegian specimens, is rather constant.”
My friend, Dr. Staudinger, who has twice spent the summer in Lapland,
thus writes in reply to my question. A crossing with _Napi_ cannot
there take place, as this form is never met with, so that the ancient
parent-form _Bryoniæ_ has been able to preserve its original constancy.
In this case also the facts thus accord with the requirements of the
theory.



II.

SEASONAL DIMORPHISM AND CLIMATIC VARIATION.


If, as I have attempted to show, seasonal dimorphism originates
through the slow operation of a changed summer climate, then is this
phenomenon nothing else than the splitting up of a species into two
climatic varieties in the same district, and we may expect to find
various connexions between ordinary simple climatic variation and
seasonal dimorphism. Cases indeed occur in which seasonal dimorphism
and climatic variation pass into each other, and are interwoven in
such a manner that the insight into the origin and nature of seasonal
dimorphism gained experimentally finds confirmation. Before I go more
closely into this subject, however, it is necessary to come to an
understanding as to the conception “climatic variation,” for this term
is often very arbitrarily applied to quite dissimilar phenomena.

According to my view there should be a sharp distinction made between
climatic and local varieties. The former should comprehend only such
cases as originate through the direct action of climatic influences;
while under the general designation of “local forms,” should be
comprised all variations which have their origin in other causes--such,
for example, as in the indirect action of the external conditions of
life, or in circumstances which do not owe their present existence to
climate and external conditions, but rather to those geological changes
which produce isolation. Thus, for instance, ancient species elsewhere
long extinct might be preserved in certain parts of the earth by the
protecting influence of isolation, whilst others which immigrated in
a state of variability might become transformed into local varieties
in such regions through the action of ‘amixia,’[24] i.e. by not being
allowed to cross with their companion forms existing in the other
portions of their habitat. In single cases it may be difficult, or for
the present impossible, to decide whether we have before us a climatic
form, or a local form arising from other causes; but for this very
reason we should be cautious in defining climatic variation.

The statement that climatic forms, in the true sense of the word, do
exist is well known to me, and has been made unhesitatingly by all
zoologists; indeed, a number of authentically observed facts might
be produced, which prove that quite constant changes in a species may
be brought about by the direct action of changed climatic conditions.
With butterflies it is in many cases possible to separate pure climatic
varieties from other local forms, inasmuch as we are dealing with
only unimportant changes and not with those of biological value, so
that natural selection may at the outset be excluded as the cause of
the changes in question. Then again the sharply defined geographical
distribution climatically governed, often furnishes evidence of
transition forms in districts lying between two climatic extremes.

In the following attempt to make clear the relationship between simple
climatic variation and seasonal dimorphism, I shall concern myself only
with such undoubted climatic varieties. A case of this kind, in which
the winter form of a seasonally dimorphic butterfly occurs in other
habitats as the only form, i.e., as a climatic variety, has already
been adduced in a former paragraph. I allude to the case of _Pieris
Napi_, the winter form of which seasonally dimorphic species occurs
in the temperate plains of Europe, whilst in Lapland and the Alps it
is commonly found as a monomorphic climatic variety which is a higher
development of the winter type, viz., the var. _Bryoniæ_.

Very analogous is the case of _Euchloe Belia_, a butterfly likewise
belonging to the _Pierinæ_, which extends from the Mediterranean
countries to the middle of France, and everywhere manifests a very
sharply pronounced seasonal dimorphism. Its summer form was, until
quite recently, described as a distinct species, _E. Ausonia_.
Staudinger was the first to prove by breeding that the supposed two
species were genetically related.[25] This species, in addition to
being found in the countries named, occurs also at a little spot
in the Alps in the neighbourhood of the Simplon Pass. Owing to the
short summer of the Alpine climate the species has in this locality
but one annual brood, which bears the characters of the winter form,
modified in all cases by the coarser thickly scattered hairs of the
body (peculiar to many Alpine butterflies,) and some other slight
differences. The var. _Simplonia_ is thus in the Alps a simple climatic
variety, whilst in the plains of Spain and the South of France it
appears as the winter form of a seasonally dimorphic species.

This _Euchloe_ var. _Simplonia_ obviously corresponds to the var.
_Bryoniæ_ of _Pieris Napi_, and it is highly probable that this form of
_E. Belia_ must likewise be regarded as the parent-form of the species
surviving from the glacial epoch, although it cannot be asserted, as
can be done in the case of _Bryoniæ_, that the type has undergone no
change since that epoch, for _Bryoniæ_ from Lapland is identical with
the Alpine form,[26] whilst _E. Simplonia_ does not appear to occur in
Polar countries.

Very interesting also is the case of _Polyommatus Phlæas_, Linn.,
one of our commonest _Lycænidæ_, which has a very wide distribution,
extending from Lapland to Spain and Sicily.[27] If we compare specimens
of this beautiful copper-coloured butterfly from Lapland with those
from Germany, no constant difference can be detected; the insect has,
however, but one annual generation in Lapland, whilst in Germany it is
double-brooded; but the winter and summer generations resemble each
other completely, and specimens which had been caught in spring on the
Ligurian coast were likewise similarly coloured to those from Sardinia.
(Fig. 21, Plate II.). According to these facts we might believe this
species to be extraordinarily indifferent to climatic influence; but
the South European summer generation differs to a not inconsiderable
extent from the winter generation just mentioned, the brilliant coppery
lustre being nearly covered with a thick sprinkling of black scales.
(Plate II., Fig. 22.) The species has thus become seasonally dimorphic
under the influence of the warm southern climate, although this is
not the case in Germany where it also has two generations in the
year.[28] No one who is acquainted only with the Sardinian summer form,
and not with the winter form of that place, would hesitate to regard
the former as a climatic variety of our _P. Phlæas_; or, conversely,
the north German form as a climatic variety of the southern summer
form--according as he accepts the one or the other as the primary form
of the species.

Still more complex are the conditions in another species of _Lycænidæ_,
_Plebeius Agestis_ (= _Alexis_ Scop.), which presents a double seasonal
dimorphism. This butterfly appears in three forms; in Germany A and B
alternate with each other as winter and summer forms, whilst in Italy
B and C succeed each other as winter and summer forms. The form B
thus occurs in both climates, appearing as the summer form in Germany
and as the winter form in Italy. The German winter variety A, is
entirely absent in Italy (as I know from numerous specimens which I
have caught), whilst the Italian summer form, on the other hand, (var.
_Allous_, Gerh.), does not occur in Germany. The distinctions between
the three forms are sufficiently striking. The form A (Fig. 18, Plate
II.) is blackish-brown on the upper side, and has in the most strongly
marked specimens only a trace of narrow red spots round the borders;
whilst the form B (Fig. 19, Plate II.) is ornamented with vivid red
border spots; and C (Fig. 20, Plate II.) is distinguished from B by the
strong yellowish-brown of the under side. If we had before us only the
German winter and the Italian summer forms, we should, without doubt,
regard them as climatic varieties; but they are connected by the form
B, interpolated in the course of the development of both, and the two
extremes thus maintain the character of mere seasonal forms.



III.

NATURE OF THE CAUSES PRODUCING CLIMATIC VARIETIES.


It has been shown that the phenomenon of seasonal dimorphism has the
same proximate cause as climatic variation, viz. change of climate,
and that it must be regarded as identical in nature with climatic
variation, being distinguished from ordinary, or, as I have designated
it, simple (monomorphic) climatic variation by the fact that, besides
the new form produced by change of climate, the old form continues to
exist in genetic connexion with it, so that old and new forms alternate
with each other according to the season.

Two further questions now present themselves for investigation, viz.
(1) by what means does change of climate induce a change in the marking
and colouring of a butterfly? and (2) to what extent does the climatic
action determine the nature of the change?

With regard to the former question, it must, in the first place, be
decided whether the true effect of climatic change lies in the action
of a high or low temperature on the organism, or whether it may not
perhaps be produced by the accelerated development caused by a high
temperature, and the retarded development caused by a low temperature.
Other factors belonging to the category of external conditions of life
which are included in the term “climate” may be disregarded, as they
are of no importance in these cases. The question under consideration
is difficult to decide, since, on the one hand, warmth and a short
pupal period, and, on the other hand, cold and a long pupal period,
are generally inseparably connected with each other; and without great
caution one may easily be led into fallacies, by attributing to the
influence of causes now acting that which is but the consequence of
long inheritance.

When, in the case of _Araschnia Levana_, even in very cold summers,
_Prorsa_, but never the _Levana_ form, emerges, it would still be
erroneous to conclude that it is only the shorter period of development
of the winter generation, and not the summer warmth, which occasioned
the formation of the _Prorsa_ type. This new form of the species
did not come suddenly into existence, but (as appears sufficiently
from the foregoing experiments) originated in the course of many
generations, during which summer warmth and a short development period
were generally associated together. From the fact that the winter
generation always produces _Levana_, even when the pupæ have not been
exposed to cold but kept in a room, it would be equally erroneous to
infer that the cold of winter had no influence in determining the type.
In this case also the determining causes must have been in operation
during innumerable generations. After the winter form of the species
has become established throughout such a long period, it remains
constant, even when the external influence which produced it (cold) is
occasionally withdrawn.

Experiments cannot further assist us here, since we cannot observe
throughout long periods of time; but there are certain observations,
which to me appear decisive. When, both in Germany and Italy, we see
_Polyommatus Phlæas_ appearing in two generations, of which both the
German ones are alike, whilst in Italy the summer brood is black,
we cannot ascribe this fact to the influence of a shorter period of
development, because this period is the same both in Germany and Italy
(two annual generations), so that it can only be attributed to the
higher temperature of summer.

Many similar cases might be adduced, but the one given suffices for
proof. I am therefore of opinion that it is not the duration of the
period of development which is the cause of change in the formation
of climatic varieties of butterflies, but only the temperature to
which the species is exposed during its pupal existence. In what
manner, then, are we to conceive that warmth acts on the marking
and colouring of a butterfly? This is a question which could only
be completely answered by gaining an insight into the mysterious
chemico-physiological processes by which the butterfly is formed in the
chrysalis; and indeed only by such a complete insight into the most
minute details, which are far beyond our scrutiny, could we arrive
at, or even approximate to, an explanation of the development of any
living organism. Nevertheless an important step can be taken towards
the solution of this problem, by establishing that the change does not
depend essentially upon the action of warmth, but upon the organism
itself, as appears from the nature of the change in one and the same
species.

If we compare the Italian summer form of _Polyommatus Phlæas_ with its
winter form, we shall find that the difference between them consists
only in the brilliant coppery red colour of the latter being largely
suffused in the summer form with black scales. When entomologists speak
of a “black dusting” of the upper side of the wings, this statement
must not of course be understood literally; the number of scales is the
same in both forms, but in the summer variety they are mostly black,
a comparatively small number being red. We might thus be inclined
to infer that, owing to the high temperature, the chemistry of the
material undergoing transformation in _Phlæas_ is changed in such a
manner that less red and more black pigment is produced. But the case
is not so simple, as will appear evident when we consider the fact
that the summer forms have not originated suddenly, but only in the
course of numerous generations; and when we further compare the two
seasonal forms in other species. Thus in _Pieris Napi_ the winter is
distinguished from the summer form, among other characters, by the
strong black dusting of the base of the wings. But we cannot conclude
from this that in the present case more black pigment is produced in
the winter than in the summer form, for in the latter, although the
base of the wings is white, their tips and the black spots on the
fore-wings are larger and of a deeper black than in the winter form.
The quantity of black pigment produced does not distinguish between the
two forms, but the mode of its distribution upon the wings.

Even in the case of species the summer form of which really possesses
far more black than the winter form, as, for instance, _Araschnia
Levana_, one type cannot be derived from the other simply by the
expansion of the black spots present, since on the same place where
in _Levana_ a black band crosses the wings, _Prorsa_, which otherwise
possesses much more black, has a white line. (See Figs. 1-9, Plate
I.) The intermediate forms which have been artificially produced
by the action of cold on the summer generation present a graduated
series, according as reversion is more or less complete; a black spot
first appearing in the middle of the white band of _Prorsa_, and then
becoming enlarged until, finally, in the perfect _Levana_ it unites
with another black triangle proceeding from the front of the band,
and thus becomes fused into a black bar. The white band of _Prorsa_
and the black band of _Levana_ by no means correspond in position; in
_Prorsa_ quite a new pattern appears, which does not originate by a
simple colour replacement of the _Levana_ marking. In the present case,
therefore, there is no doubt that the new form is not produced simply
because a certain pigment (black) is formed in larger quantities, but
because its mode of distribution is at the same time different, white
appearing in some instances where black formerly existed, whilst in
other cases the black remains. Whoever compares _Prorsa_ with _Levana_
will not fail to be struck with the remarkable change of marking
produced by the direct action of external conditions.

The numerous intermediate forms which can be produced artificially
appear to me to furnish a further proof of the gradual character
of the transformation. Ancestral intermediate forms can only occur
where they have once had a former existence in the phyletic series.
Reversion may only take place completely in some particular characters,
whilst in others the new form remains constant--this is in fact the
ordinary form of reversion, and in this manner a mixture of characters
might appear which never existed as a phyletic stage; but particular
characters could certainly never appear unless they were normal to the
species at some stage of phyletic development. Were this possible it
would directly contradict the idea of reversion, according to which
new characters never make their appearance, but only such as have
already existed. If, therefore, the ancestral forms of _A. Levana_
(which we designate as _Porima_) present a great number of transitional
varieties, this leads to the conclusion that the species must have gone
through a long series of stages of phyletic development before the
summer generation had completely changed into _Prorsa_. The view of the
slow cumulative action of climatic influences already submitted, is
thus confirmed.

If warmth is thus without doubt the agency which has gradually changed
the colour and marking of many of our butterflies, it sufficiently
appears from what has just been said concerning the nature of the
change that the chief part in the transmutation is not to be attributed
to the agency in question, but to the organism which is affected by it.
Induced by warmth, there begins a change in the ultimate processes of
the matter undergoing transformation, which increases from generation
to generation, and which not only consists in the appearance of the
colouring matter in one place instead of another, but also in the
replacement of yellow, in one place by white and in another by black,
or in the transformation of black into white on some portions of the
wings, whilst in others black remains. When we consider with what
extreme fidelity the most insignificant details of marking are, in
constant species of butterflies, transmitted from generation to
generation, a total change of the kind under consideration cannot but
appear surprising, and we should not explain it by the nature of the
agency (warmth), but only by the nature of the species affected. The
latter cannot react upon the warmth in the same manner that a solution
of an iron salt reacts upon potassium ferrocyanide or upon sulphuretted
hydrogen; the colouring matter of the butterfly’s wing which was
previously black does not become blue or yellow, nor does that which
was white become changed into black, but a new marking is developed
from the existing one--or, as I may express it in more general terms,
the species takes another course of development; the complicated
chemico-physical processes in the matter composing the pupa become
gradually modified in such a manner that, as the final result, a new
marking and colouring of the butterfly is produced.

Further facts can be adduced in support of the view that in these
processes it is the constitution of the species, and not the external
agency (warmth), which plays the chief part. The latter, as Darwin
has strikingly expressed it, rather performs the function of the
spark which ignites a combustible substance, whilst the character of
the combustion depends upon the nature of the explosive material.
Were this not the case, increased warmth would always change a given
colour[29] in the same manner in all butterflies, and would therefore
always give rise to the production of the same colour. But this does
not occur; _Polyommatus Phlæas_, for example, becoming black in the
south, whilst the red-brown _Vanessa Urticæ_ becomes black in high
northern latitudes, and many other cases well known to entomologists
might be adduced.[30] It indeed appears that species of similar
physical constitution, i.e., nearly allied species, under similar
climatic influences, change in an analogous manner. A beautiful example
of this is furnished by our _Pierinæ_. Most of the species display
seasonal dimorphism; as, for instance, _Pieris Brassicæ_, _Rapæ_,
_Napi_, _Krueperi_, and _Daplidice_, _Euchloe Belia_ and _Belemia_,
and _Leucophasia Sinapis_, in all of which the difference between the
winter and the summer forms is of a precisely similar nature. The
former are characterized by a strong black dusting of the base of the
wings, and by a blackish or green sprinkling of scales on the underside
of the hind wings, while the latter have intensely black tips to the
wings, and frequently also spots on the fore-wings.

Nothing can prove more strikingly, however, that in such cases
everything depends upon the physical constitution, than the fact that
in the same species the males become changed in a different manner to
the females. The parent-form of _Pieris Napi_ (var. _Bryoniæ_) offers
an example. In all the _Pierinæ_ secondary sexual differences are
found, the males being differently marked to the females; the species
are thus sexually dimorphic. Now the male of the Alpine and Polar var.
_Bryoniæ_, which I conceive to be the ancestral form, is scarcely to
be distinguished, as has already been mentioned, from the male of our
German winter form (_P. Napi_, var. _Vernalis_), whilst the female
differs considerably.[31] The gradual climatic change which transformed
the parent-form _Bryoniæ_ into _Napi_ has therefore exerted a much
greater effect on the female than on the male. The external action on
the two sexes was exactly the same, but the response of the organism
was different, and the cause of the difference can only be sought for
in the fine differences of physical constitution which distinguish the
male from the female. If we are unable to define these differences
precisely, we may nevertheless safely conclude from such observations
that they exist.

I have given special prominence to this subject because, in my idea,
Darwin ascribes too much power to sexual selection when he attributes
the formation of secondary sexual characters to the sole action of
this agency. The case of _Bryoniæ_ teaches us that such characters may
arise from purely innate causes; and until experiments have decided
how far the influence of sexual selection extends, we are justified in
believing that the sexual dimorphism of butterflies is due in great
part to the differences of physical constitution between the sexes.
It is quite different with such sexual characters as the stridulating
organs of male Orthoptera which are of undoubted importance to that
sex. These can certainly be attributed with great probability to sexual
selection.

It may perhaps not be superfluous to adduce one more similar case, in
which, however, the male and not the female is the most affected by
climate. In our latitudes, as also in the extreme north, _Polyommatus
Phlæas_, already so often mentioned, is perfectly similar in both
sexes in colour and marking; and the same holds good for the winter
generation of the south. The summer generation of the latter, however,
exhibits a slight sexual dimorphism, the red of the fore wings of the
female being less completely covered with black than in the male.



IV.

WHY ALL POLYGONEUTIC SPECIES ARE NOT SEASONALLY DIMORPHIC.


If we may consider it to be established that seasonal dimorphism is
nothing else than the splitting up of a species into two climatic
varieties in one and the same locality, the further question at once
arises why all polygoneutic species (those which produce more than
_one_ annual generation) are not seasonally dimorphic.

To answer this, it will be necessary to go more deeply into the
development of seasonal dimorphism. This evidently depends upon a
peculiar kind of periodic, alternating heredity, which we might be
tempted to identify with Darwin’s “inheritance at corresponding
periods of life.” It does not, however, in any way completely agree
with this principle, although it presents a great analogy to it and
must depend ultimately upon the same cause. The Darwinian “inheritance
at corresponding periods of life”--or, as it is termed by Haeckel,
“homochronic heredity”--is characterized by the fact that new
characters always appear in the individuals at the same stage of life
as that in which they appeared in their progenitors. The truth of this
principle has been firmly established, instances being known in which
both the first appearance of a new (especially pathological) character
and its transmission through several generations has been observed.
Seasonally dimorphic butterflies also furnish a further valuable proof
of this principle, since they show that not only variations which
arise suddenly (and which are therefore probably due to purely innate
causes) follow this mode of inheritance, but also that characters
gradually called forth by the influence of external conditions and
accumulating from generation to generation, are only inherited at that
period of life in which these conditions were or are effective. In all
seasonally dimorphic butterflies which I have been able to examine
closely, I found the caterpillars of the summer and winter broods to
be perfectly identical. The influences which, by acting on the pupæ,
split up the imagines into two climatic forms, were thus without effect
on the earlier stages of development. I may specially mention that
the caterpillars, as well as the pupæ and eggs of _A. Levana_, are
perfectly alike both in the summer and winter forms; and the same is
the case in the corresponding stages of _P. Napi_ and _P. Bryoniæ_.

I shall not here attempt to enter more deeply into the nature of the
phenomena of inheritance. It is sufficient to have confirmed the law
that influences which act only on certain stages in the development of
the individual, even when the action is cumulative and not sudden, only
affect those particular stages without having any effect on the earlier
or later stages. This law is obviously of the greatest importance to
the comprehension of metamorphosis. Lubbock[32] has briefly shown in a
very clear manner how the existence of metamorphosis in insects can be
explained by the indirect action of varying conditions on the different
life-stages of a species. Thus the mandibles of a caterpillar are,
by adaptation to another mode of nourishment, exchanged at a later
period of life for a suctorial organ. Such adaptation of the various
development-stages of a species to the different conditions of life
would never give rise to metamorphosis, if the law of homochronic, or
periodic, heredity did not cause the characters gradually acquired at
a given stage to be transferred to the same stage of the following
generation.

The origin of seasonal dimorphism depends upon a very similar law, or
rather form, of inheritance, which differs from that above considered
only in the fact that, instead of the ontogenetic stages, a whole
series of generations is influenced. This form of inheritance may
be formulated somewhat as follows:--When dissimilar conditions
alternatingly influence a series of generations, a cycle is produced
in which the changes are transmitted only to those generations
which are acted upon by corresponding conditions, and not to the
intermediate ones. Characters which have arisen by the action of a
summer climate are inherited by the summer generation only, whilst
they remain latent in the winter generation. It is the same as with
the mandibles of a caterpillar which are latent in the butterfly, and
again make their appearance in the corresponding (larval) stage of the
succeeding generation. This is not mere hypothesis, but the legitimate
inference from the facts. If it be admitted that my conception of
seasonal dimorphism as a double climatic variation is correct, the law
of “cyclical heredity,”[33] as I may term it--in contradistinction
to “homochronic heredity,” which relates only to the ontogenetic
stages--immediately follows. All those cases which come under the
designation of ‘alternation of generation,’ can obviously be referred
to cyclical heredity, as will be explained further on. In the one case
the successive generations deport themselves exactly in the same
manner as do the successive stages of development of the individual in
the other; and we may conclude therefrom (as has long been admitted on
other grounds) that a generation is, in fact, nothing else than a stage
of development in the life of a species. This appears to me to furnish
a beautiful confirmation of the theory of descent.

Now if, returning to questions previously solved, the alternating
action of cold in winter and warmth in summer leads to the production
of a winter and summer form, according to the law of cyclical heredity,
the question still remains: why do we not find seasonal dimorphism in
all polygoneutic butterflies?

We might at first suppose that all species are not equally sensitive
to the influence of temperature: indeed, the various amounts
of difference between the winter and summer forms in different
species would certainly show the existence of different degrees of
sensitiveness to the modifying action of temperature. But even this
does not furnish an explanation, since there are butterflies which
produce two perfectly similar[34] generations wherever they occur,
and which, nevertheless, appear in different climates as climatic
varieties. This is the case with _Pararga Ægeria_ (Fig. 23, Plate
II.), the southern variety of which, _Meione_ (Fig. 24, Plate II.),
is connected with it by an intermediate form from the Ligurian coast.
This species possesses, therefore, a decided power of responding to
the influence of temperature, and yet no distinction has taken place
between the summer and the winter form. We can thus only attribute
this different deportment to a different kind of heredity; and we
may therefore plainly state, that changes produced by alternation
of climate are not always inherited _alternatingly_, i.e. by the
corresponding generations, but sometimes _continuously_, appearing
in every generation, and never remaining latent. The causes which
determine why, in a particular case, the one or the other form of
inheritance prevails, can be only innate, i.e. they lie in the organism
itself, and there is as little to be said upon their precise nature as
upon that of any other process of heredity. In a similar manner Darwin
admits a kind of double inheritance with respect to characters produced
by sexual selection; in one form these characters remain limited to the
sex which first acquired them, in the other form they are inherited by
both sexes, without it being apparent why, in any particular case, the
one or the other form of heredity should take place.

The foregoing explanation may obtain in the case of sexual selection,
in which it is not inconceivable that certain characters may not be
so easily produced, or even not produced at all, in one sex, owing
to its differing from the other in physical constitution. In the
class of cases under consideration, however, it is not possible that
the inherited characters can be prevented from being acquired by one
generation owing to its physical constitution, since this constitution
was similar in all the successive generations before the appearance of
dimorphism. The constitution in question first became dissimilar in
the two generations to the extent of producing a change of specific
character, through the action of temperature on the alternating broods
of each year, combined with cyclical heredity. If the law of cyclical
heredity be a general one, it must hold good for all cases, and
characters acquired by the summer generation could never have been also
transmitted to the winter generation from the very first.

I will not deny the possibility that if alternating heredity
should become subsequently entirely suppressed throughout numerous
generations, a period may arrive when the preponderating influence of a
long series of summer generations may ultimately take effect upon the
winter generation. In such a case the summer characters would appear,
instead of remaining latent as formerly. In this manner it may be
imagined that at first but few, and later more numerous individuals,
approximate to the summer form, until finally the dimorphism entirely
disappears, the new form thus gaining ascendency and the species
becoming once more monomorphic. Such a supposition is indeed capable
of being supported by some facts, an observation on _A. Levana_
apparently contradicting the theory having been already interpreted in
this sense. I refer to the fact that whilst some butterflies of the
winter generation emerge in October as _Prorsa_, others hibernate,
and appear the following spring in the _Levana_ form. The winter form
of _Pieris Napi_ also no longer preserves, in the female sex, the
striking coloration of the ancestral form _Bryoniæ_, a fact which may
indicate the influencing of the winter generation by numerous summer
generations. The double form of the spring generation of _Papilio Ajax_
can be similarly explained by the gradual change of alternating into
continuous heredity, as has already been mentioned. All these cases,
however, are perhaps capable of another interpretation; at any rate,
the correctness of this supposition can only be decided by further
facts.

Meanwhile, even if we suppose the above explanation to be correct,
it will not apply to the absence of seasonal dimorphism in cases like
that of _Pararga Ægeria_ and _Meione_, in which only _one_ summer
generation appears, so that a preponderating inheritance of summer
characters cannot be admitted. Another explanation must thus be
sought, and I believe that I have found it in the circumstance that
the butterflies named do not hibernate as pupæ but as caterpillars, so
that the cold of winter does not directly influence those processes of
development by which the perfect insect is formed in the chrysalis.
It is precisely on this point that the origin of those differences of
colour which we designate as the seasonal dimorphism of butterflies
appears to depend. Previous experiments give great probability to this
statement. From these we know that the eggs, caterpillars, and pupæ of
all the seasonally dimorphic species experimented with are perfectly
similar in the summer and winter generations, the imago stage only
showing any difference. We know further from these experiments, that
temperature-influences which affect the caterpillars never entail a
change in the butterflies; and finally, that the artificial production
of the reversion of the summer to the winter form can only be brought
about by operating on the pupæ.

Since many monogoneutic species now hibernate in the caterpillar
stage (e.g. _Satyrus Proserpina_, and _Hermione_, _Epinephele
Eudora_, _Furtina, Ithonus_, _Hyperanthus_, _Ida_, _&c._), we may
admit that during the glacial period such species did not pass the
winter as pupæ. As the climate grew warmer, and in consequence
thereof a second generation became gradually interpolated in many of
these monogoneutic species, there would ensue (though by no means
necessarily) a disturbance of the winter generation, of such a kind
that the pupæ, instead of the caterpillars as formerly, would then
hibernate. It may, indeed, be easily proved _à priori_ that whenever
a disturbance of the winter generation takes place it only does so
retrogressively, that is to say--species which at one time pass the
winter as caterpillars subsequently hibernate in the egg, while those
which formerly hibernate as pupæ afterwards do so as caterpillars.
The interpolation of a summer generation must necessarily delay till
further towards the end of summer, the brood about to hibernate; the
remainder of the summer, which serves for the development of the
eggs and young caterpillars, may possibly under these conditions be
insufficient for pupation, and the species which hibernated in the
pupal state when it was monogoneutic, may perhaps pass the winter in
the larval condition after the introduction of the second brood. A
disturbance of this kind is conceivable; but it is certain that many
species suffer no further alteration in their development than that of
becoming digoneutic from monogoneutic. This follows from the fact that
hibernation takes place in the caterpillar stage in many species of
the sub-family _Satyridæ_ which are now digoneutic, as well as in the
remaining monogoneutic species of the same sub-family. But we cannot
expect seasonal dimorphism to appear in all digoneutic butterflies the
winter generation of which hibernates in the caterpillar form, since
the pupal stage in these species experiences nearly the same influences
of temperature in both generations. We are hence led to the conclusion
that seasonal dimorphism must arise in butterflies whenever the pupæ of
the alternating annual generations are exposed throughout long periods
of time to widely different regularly recurring changes of temperature.

The facts agree with this conclusion, inasmuch as most butterflies
which exhibit seasonal dimorphism hibernate in the pupa stage. Thus,
this is the case with all the _Pierinæ_, with _Papilio Machaon_,
_P. Podalirius_, and _P. Ajax_, as well as with _Araschnia Levana_.
Nevertheless, it cannot be denied that seasonal dimorphism occurs also
in some species which do not hibernate as pupæ but as caterpillars; as,
for instance, in the strongly dimorphic _Plebeius Amyntas_. But such
cases can be explained in a different manner.

Again, the formation of a climatic variety--and as such must we
regard seasonally dimorphic forms--by no means entirely depends
on the magnitude of the difference between the temperature which
acts on the pupæ of the primary and that which acts on those of the
secondary form; it rather depends on the absolute temperature which
the pupæ experience. This follows without doubt from the fact that
many species, such as our common Swallow-tail (_Papilio Machaon_), and
also _P. Podalirius_, in Germany and the rest of temperate Europe,
show no perceptible difference of colour between the first generation,
the pupæ of which hibernate, and the second generation, the pupal
period of which falls in July, whereas the same butterflies in South
Spain and Italy are to a small extent seasonally dimorphic. Those
butterflies which are developed under the influence of a Sicilian
summer heat likewise show climatic variation to a small extent. The
following consideration throws further light on these conditions. The
mean summer and winter temperatures in Germany differ by about 14.9°
R.; this difference being therefore much more pronounced than that
between the German and Sicilian summer, which is only about 3.6° R.
Nevertheless, the winter and summer generations of _P. Podalirius_ are
alike in Germany, whilst the Sicilian summer generation has become a
climatic variety. The cause of this change must therefore lie in the
small difference between the mean summer temperatures of 15.0° R.
(Berlin) and 19.4° R. (Palermo). According to this, a given absolute
temperature appears to give a tendency to variation in a certain
direction, the necessary temperature being different for different
species. The latter statement is supported by the facts that, in the
first place, in different species there are very different degrees of
difference between the summer and winter forms; and secondly, many
digoneutic species are still monomorphic in Germany, first becoming
seasonally dimorphic in Southern Europe. This is the case with _P.
Machaon_ and _P. Podalirius_, as already mentioned, and likewise with
_Polyommatus Phlæas_. Zeller in 1846-47, during his journey in Italy,
recognized as seasonally dimorphic in a small degree a large number of
diurnal Lepidoptera which are not so in our climate.[35]

In a similar manner the appearance of seasonal dimorphism in species
which, like _Plebeius Amyntas_, do not hibernate as pupæ, but as
caterpillars, can be simply explained by supposing that the winter
generation was the primary form, and that the increase in the summer
temperature since the glacial period was sufficient to cause this
particular species to become changed by the gradual interpolation
of a second generation. The dimorphism of _P. Amyntas_ can,
nevertheless, be explained in another manner. Thus, there may have
been a disturbance of the period of development in the manner already
indicated, the species which formerly hibernated in the pupal stage
becoming subsequently disturbed in its course of development by the
interpolation of a summer generation, and hibernating in consequence
in the caterpillar state. Under these circumstances we must regard the
present winter form (var. _Polysperchon_) as having been established
under the influence of a winter climate, this form, since the supposed
disturbance in its development, having had no reason to become changed,
the spring temperature under which its pupation now takes place not
being sufficiently high. The interpolated second generation on the
other hand, the pupal period of which falls in the height of summer,
may easily have become formed into a summer variety.

This latter explanation agrees precisely with the former, both starting
with the assumption that in the present case, as in that of _A. Levana_
and the _Pierinæ_, the winter form is the primary one, so that the
dimorphism proceeds from the said winter form and does not originate
the winter but the summer form, as will be explained. Whether the
winter form has been produced by the action of the winter or spring
temperature is immaterial in judging single cases, inasmuch as we are
not in a position to state what temperature is necessary to cause any
particular species to become transformed.

The reverse case is also theoretically conceivable, viz., that in
certain species the summer form was the primary one, and by spreading
northwards a climate was reached which still permitted the production
of two generations, the pupal stage of one generation being exposed
to the cold of winter, and thus giving rise to the production of a
secondary winter form. In such a case hibernation in the pupal state
would certainly give rise to seasonal dimorphism. Whether these
conditions actually occur, appears to me extremely doubtful; but it may
at least be confidently asserted that the first case is of far more
frequent occurrence. The beautiful researches of Ernst Hoffmann[36]
furnish strong evidence for believing that the great majority of the
European butterflies have immigrated, not from the south, but from
Siberia. Of 281 species, 173 have, according to Hoffmann, come from
Siberia, 39 from southern Asia, and only 8 from Africa, whilst during
the greatest cold of the glacial period, but very few or possibly no
species existed north of the Alps. Most of the butterflies now found
in Europe have thus, since their immigration, experienced a gradually
increasing warmth. Since seasonal dimorphism has been developed in
some of these species, the summer form must in all cases have been the
secondary one, as the experiments upon the reversion of _Pieris Napi_
and _Araschnia Levana_ have also shown.

All the seasonally dimorphic butterflies known to me are found in
Hoffmann’s list of Siberian immigrants, with the exception of two
species, viz., _Euchloe Belemia_, which is cited as an African
immigrant, and _Pieris Krueperi_, which may have come through Asia
Minor, since at the present time it has not advanced farther west
than Greece. No considerable change of climate can be experienced by
migrating from east to west, so that the seasonal dimorphism of _Pieris
Krueperi_ can only depend on a cause similar to that which affected the
Siberian immigrants, that is, the gradual increase of temperature in
the northern hemisphere since the glacial period. In this species also,
the winter form must be the primary one. In the case of _E. Belemia_,
on the other hand, the migration northwards from Africa certainly
indicates removal to a cooler climate, which may have originated a
secondary winter form, even if nothing more certain can be stated. We
know nothing of the period of migration into southern Europe; and even
migration without climatic change is conceivable, if it kept pace with
the gradual increase of warmth in the northern hemisphere since the
glacial epoch. Experiments only would in this case be decisive. If the
summer generation, var. _Glauce_, were the primary form, it would not
be possible by the action of cold on the pupæ of this brood to produce
the winter variety _Belemia_, whilst, on the other hand, the pupæ of
the winter generation by the influence of warmth would be made to
revert more or less completely to the form _Glauce_. It is by no means
to be understood that the species would actually comport itself in this
manner. On the contrary, I am of opinion that in this case also, the
winter form is primary. The northward migration (from Africa to south
Spain) would be quite insufficient, and the winter form is now found in
Africa as well as in Spain.



V.

ON ALTERNATION OF GENERATIONS.


Seasonal dimorphism has already been designated by Wallace as
alternation of generation,[37] a term which cannot be disputed so long
as it is confined to a regular alternation of dissimilar generations.
But little is gained by this definition, however, unless it can be
proved that both phenomena are due to similar causes, and that they
are consequently brought about by analogous processes. The causes of
alternation of generation have, until the present time, been scarcely
investigated, owing to the want of material. Haeckel alone has quite
recently subjected these complicated phenomena generally to a searching
investigation, and has arrived at the conclusion that the various
forms of metagenesis can be arranged in two series. He distinguishes
a progressive and a retrogressive series, comprising under the former
those species “which, to a certain extent, are still in a transition
stage from monogenesis to amphigenesis (asexual to sexual propagation),
and the early progenitors of which, therefore, never exclusively
propagated themselves sexually” (_Trematoda_, _Hydromedusæ_). Under the
other, or retrogressive form of metagenesis, Haeckel includes a “return
from amphigenesis to monogenesis,” this being the case with all those
species which now manifest a regular alternation from amphigenesis to
parthenogenesis (_Aphides_, _Rotatoria_, _Daphniidæ_, _Phyllopoda_,
&c.). Essentially I can but agree entirely with Haeckel. Simply
regarding the phenomena of alternation of generation as at present
known, it appears to me to be readily admissible that these multiform
modes of propagation must have originated in at least two different
ways, which can be aptly formulated in the manner suggested by Haeckel.

I will, however, venture to adopt a somewhat different mode of
conception, and regard the manner of propagation (whether sexual or
asexual) not as the determining, but only as the secondary cause. I
will further hazard the separation of the phenomena of alternating
generations (in their widest sense) into two main groups according to
their origin, designating the cases of one group as true metagenesis
and those of the other as heterogenesis.[38] Metagenesis takes
its origin from a phyletic series of dissimilar forms, whilst
heterogenesis originates from a phyletic series of similar forms--this
series, so far as we can at present judge, always consisting of
similar sexual generations. The former would thus nearly coincide
with Haeckel’s progressive, and the latter with his retrogressive
metagenesis. Metagenesis may further originate in various ways. In the
first place, from metamorphosis, as for example, in the propagation
of the celebrated _Cecidomyia_ with nursing larvæ. The power which
these larvæ possess of propagating themselves asexually has evidently
been acquired as a secondary character, as appears from the fact that
there are many species of the same genus the larvæ of which do not
nurse, these larvæ being themselves undoubted secondary forms produced
by the adaptation of this stage of phyletic development to a mode of
life widely different from that of the later stages. In the form now
possessed by these larvæ they could never have represented the final
stage of their ontogeny, neither could they have formerly possessed
the power of sexual propagation. The conclusion seems inevitable that
metagenesis has here proceeded from metamorphosis; that is to say, one
stage of the ontogeny, by acquiring asexual propagation, has changed
the originally existing metamorphosis into metagenesis.

Lubbock[39] is undoubtedly correct when, for cases like that just
mentioned, he attempts to derive alternation of generations from
metamorphosis. But if we exclude heterogenesis there still remain a
large number of cases of true metagenesis which cannot be explained
from this point of view.

It must be admitted, with Haeckel, that the alternation of generations
in the Hydromedusæ and Trematoda does not depend, as in the case of
_Cecidomyia_, upon the larvæ having acquired the power of nursing,
but that the inferior stages of these species always possessed this
power which they now only preserve. The nursing Trematode larvæ now
existing may possibly have been formerly able to propagate themselves
also sexually, this mode of propagation having at the present time
been transferred to a later phyletic stage. In this case, therefore,
metagenesis was not properly produced by metamorphosis, but arose
therefrom in the course of the phyletic development, the earlier
phyletic stages abandoning the power of sexual reproduction, and
preserving the asexual mode of propagation. A third way in which
metagenesis might originate is through polymorphosis. When the
latter is combined with asexual reproduction, as is especially the
case with the Hydrozoa, metagenesis may be derived therefrom. The
successive stages of transformation of one and the same physiological
individual do not in these cases serve as the point of departure
for alternation of generation, but the different contemporary forms
living gregariously into which the species has become divided through
functional differentiation of the various individuals of the same
stock. Individuals are here produced which alone acquire the power
of sexual reproduction, and metagenesis is thus brought about,
these individuals detaching themselves from the stock on which they
originated, while the rest of the individuals remain in combination,
and retain the asexual mode of propagation. No sharp distinction can be
otherwise drawn between this and the cases previously considered.[40]
The difference consists only in the whole cycle of reproduction being
performed by one stock; both classes have the common character that
the different phyletic stages never appear in the same individual
(metamorphosis), but in the course of further phyletic development
metagenesis at the same time arises, i.e. the division of these stages
among a succession of individuals. We are therefore able to distinguish
this _primary metagenesis_ from the _secondary metagenesis_ arising
from metamorphosis.

It is not here my intention to enter into the ultimate causes of
metagenesis; in this subject we should only be able to advance by
making vague hypotheses. The phenomenon of seasonal dimorphism,
with which this work has mainly to deal, is evidently far removed
from metagenesis, and it was to make this clear that the foregoing
observations were brought forward. The characters common in the origin
of metagenesis are to be found, according to the views previously
set forth, in the facts that here the faculty of asexual and of
sexual reproduction is always distributed among _several_ phyletic
stages of development which succeed each other in an ascending series
(progressive metagenesis of Haeckel), whereas I find differences only
in the fact that the power of asexual propagation may (in metagenesis)
be either newly acquired (larva of _Cecidomyia_) or preserved from
previous ages (_Hydroida_). It seems that in this process sexual
reproduction is without exception lost by the earlier, and remains
confined solely to the most recent stages.

From the investigations on seasonal dimorphism it appears that a
cycle of generations can arise in an entirely different way. In this
case a series of generations originally alike are made dissimilar by
external influences. This appears to me of the greatest importance,
since seasonal dimorphism is without doubt closely related to that
mode of reproduction which has hitherto been exclusively designated
as heterogenesis, and a knowledge of its mode of origination must
therefore throw light on the nature and origin of heterogenesis in
general.

In seasonal dimorphism, as I have attempted to show, it is the
direct action of climate, and indeed chiefly that of temperature,
which brings about the change in some of the generations. Since
these generations have been exposed to the alternating influence
of the summer and winter temperature a periodical dimorphism has
been developed--a regular cycle of dissimilar generations. It has
already been asserted that the consecutive generations of a species
comport themselves with respect to heredity in a manner precisely
similar to that of the ontogenetic stages, and at the same time such
succeeding generations point out the parallelism between metamorphosis
and heterogenesis. If influences capable of directly or indirectly
producing changes operate on any particular stage of development,
these changes are always transmitted to the same stage. Upon this
metamorphosis depends. In a precisely similar manner changes which
operated periodically on certain generations (1, 3, 5, for instance)
are transmitted to these generations only, and not to the intermediate
ones. Upon this depends heterogenesis. We have just been led to the
comprehension of heterogenesis by cyclical heredity, by the fact
that a cycle is produced whenever a series of generations exists
under regularly alternating influences. In this cycle newly acquired
changes, however minute in character at first, are only transmitted to
a later, and not to the succeeding generation, appearing only in the
one corresponding, i.e. in that generation which exists under similar
transforming influences. Nothing can more clearly show the extreme
importance which the conditions of life must have upon the formation
and further development of species than this fact. At the same time
nothing shows better that the action of these conditions is not
suddenly and violently exerted, but that it rather takes place by small
and slow operations. In these cases the long-continued accumulation
of imperceptibly small variations proves to be the magic means by
which the forms of the organic world are so powerfully moulded. By the
application of even the greatest warmth nobody would be able to change
the winter form of _A. Levana_ into the summer form; nevertheless, the
summer warmth, acting regularly on the second and third generations
of the year, has, in the course of a lengthened period, stamped these
two generations with a new form without the first generation being
thereby changed. In the same region two different climatic varieties
have been produced (just as in the majority of cases climatic varieties
occur only in separate regions) which alternate with each other, and
thus give rise to a cycle of which each generation propagates itself
sexually.

But even if seasonal dimorphism is to be ascribed to heterogenesis, it
must by no means be asserted that those cases of cyclical propagation
hitherto designated as heterogenesis are completely identical with
seasonal dimorphism. Their identity extends only to their origin
and manner of development, but not to the mode of operation of the
causes which bring about their transformation. Both phenomena have a
common mode of origination, arising from similar (monomorphic) sexual
generations and course of development, a cycle of generations with
gradually diverging characters coming into existence by the action of
alternating influences. On the other hand, the nature of the changes
by which the secondary differs from the primary generation may be
referred to another mode of action of the exciting causes. In seasonal
dimorphism the differences between the two generations are much less
than in other cases of heterogenesis. These differences are both
quantitatively less, and are likewise qualitative, affecting only
characters of biological insignificance.[41] The variations in question
are mostly restricted to the marking and colouring of the wings and
body, occasionally affecting also the form of the wing, and in a few
cases the size of the body (_Plebeius Amyntas_), whilst the bodily
structure--so far at least as my investigations extend--appears to be
the same in both generations.[42]

The state of affairs is quite different in the remaining cases of
heterogenesis; here the entire structure of the body appears to be
more or less changed, and its size is often very different, nearly all
the internal organs differing in the two generations. According to
Claus,[43] “we can scarcely find any other explanation of the mode of
origination of heterogenesis than the gradual and slow advantageous
adaptation of the organization to important varying conditions of
life”--a judgment in which this author is certainly correct. In all
such cases the change does not affect unimportant characters, as it
does in butterflies, but parts of biological or physiological value;
and we cannot, therefore, consider such changes to have originated
through the _direct_ action of altered conditions of life, but
_indirectly_ through natural selection or adaptation.

Thus, the difference between seasonal dimorphism and the other known
cases of heterogenesis consists in the secondary form in which the
species appears in the former originating through the direct action
of external conditions, whilst in the latter this form most probably
originates through the indirect action of such influences. The first
half of the foregoing proposition is alone capable of provisional
proof, but it is in the highest degree probable that the latter half
is also correct. Naturally we cannot say to what extent the direct
action of external conditions plays also a part in true heterogenesis,
as there have been as yet no experiments made on its origin. That
direct action, working to a certain extent co-operatively, plays only
a secondary part, while the chief cause of the change is to be found
in adaptation, no one can doubt who keeps in view, for instance, the
mode of propagation discovered by Leuckart in _Ascaris nigrovenosa_. In
this worm, the one generation lives free in the water, and the other
generation inhabits the lungs of frogs, the two generations differing
from one another in size of body and structure of internal organs to an
extent only possible with the true Nematoda.

To prevent possible misunderstanding, let it be finally noted--even
if superfluous--that the changes causing the diversity of the two
generations in seasonal dimorphism and heterogenesis are not of such
a nature that the value of different “specific characters” can be
attached to them. Distinctly defined specific characters are well
known not to occur generally, and it would therefore be erroneous to
attach but little value to the differences in seasonal dimorphism
because these chiefly consist in the colouring and marking of the
wings. The question here under consideration is not whether two animal
forms have the value of species or of mere varieties--a question which
can never be decided, since the reply always depends upon individual
opinion of the value of the distinctions in question, and the idea
of both species and varieties is moreover purely conventional. The
question is, rather, whether the distinguishing characters possess
an equal constancy--that is, whether they are transmitted with the
same force and accuracy to all individuals; and whether they occur,
therefore, in such a manner that they can be practically employed
as specific characters. With respect to this, it cannot be doubtful
for a moment that the colouring and marking of a butterfly possess
exactly the same value as the constant characters in any other group
of animals, such as the palate-folds in mice, the structure of the
teeth in mammals, the number and form of the wing and tail feathers in
birds, &c. We have but to remember with what wonderful constancy often
the most minute details of marking are transmitted in butterflies. The
systematist frequently distinguishes between two nearly allied species,
as for instance in the _Lycænidæ_, chiefly by the position of certain
insignificant black spots on the under side of the wing (_P. Alexis_
female, and _P. Agestis_); and this diagnosis proves sufficient, since
_P. Alexis_, which has the spots in a straight row, has a different
caterpillar to _P. Agestis_, in which the central spot is nearer the
base of the hind wing!

For the reasons just given, I maintain that it is neither justifiable
nor useful to designate the di- and polymorphism of butterflies as
di- and polychroism, and thereby to attribute but little importance to
these phenomena.[44] This designation would be only justifiable if the
differences of colour were due to other causes than the differences of
form, using this last word in a narrow sense. But it has been shown
that the same direct action of climate which originates new colours,
produces also in some species differences of form (contour of wing,
size, &c.); whilst, on the other hand, it has long been known that many
protective colours can only be explained by the indirect action of
external conditions.

When I raise a distinction in the nature of the changes between
seasonal dimorphism and the remaining known cases of heterogenesis,
this must be taken as referring only to the biological or physiological
result of the change in the transformed organism itself. In seasonal
dimorphism only insignificant characters become prominently changed,
characters which are without importance for the welfare of the species;
while in true heterogenesis we are compelled to admit that useful
changes, or adaptations, have occurred.

Heterogenesis may thus be defined either in accordance with my proposal
or in the manner hitherto adopted, since it may be regarded as more
morphological than the cyclical succession of differently formed sexual
generations; or, with Claus, as the succession of different sexual
generations, “living under different conditions of existence”--a
definition which applies in all cases to seasonal dimorphism. Varying
conditions of existence, in their widest sense, are the result of the
action of different climates; and a case has been made known recently
in which it is extremely probable that the climatic differences of
the seasons have produced a cycle of generations by influencing the
processes of nutrition. This case is quite analogous to that which
we have observed in the seasonal dimorphism of butterflies, but with
the distinction that the difference between the winter and summer
generations does not, at least entirely, consist in the form of the
reproductive adult, but almost entirely in its ontogeny--in the mode
of its development. A comparison of this case with the analogous
phenomenon in butterflies, may be of interest. In the remarkable
fresh-water Daphnid, _Leptodora hyalina_ Lillejeborg, it was proved
some years ago by P. E. Müller,[45] who studied the ontogeny, that this
last was direct, since the embryo, before leaving the egg, already
possesses the form, members, and internal organs of the adult. This
was, at least, the case with the summer eggs. It was subsequently
shown by Sars[46] that this mode of development only holds good for
the summer brood, the winter eggs producing an embryo in the spring
which possesses only the three first pairs of limbs, and, instead of
compound eyes, only a single frontal eye, thus exhibiting briefly, at
first, the structure of a _Nauplius_, and gradually acquiring that
of _Leptodora_. The mature form derived from the winter eggs is not
distinguishable from the later generations, except by the presence
of the simple larval eye, which appears as a small black spot. The
generations when fully developed are thus distinguished only by this
minute marking, but the summer generation undergoes direct development,
whilst the winter generation, on the contrary, is only developed
by metamorphosis, beginning with the simplest Crustacean type, and
thus fairly representing the phyletic development of the species. We
therefore see, in this case, the combination of a metamorphic and a
direct development taking place to a certain extent under our eyes. It
cannot be proved with certainty what the cause of this phenomenon may
be, but the conjecture is almost unavoidable that it is closely related
to the origin of the seasonal dimorphism of butterflies, since both
depend on the alternating climatic influences of summer and winter:
it is most probable that these influences have directly[47] brought
about a shortening of the period of development in summer. Thus we have
here a case of heterogenesis nearly related to the seasonal dimorphism
of butterflies in a twofold manner--first, because the cycle of
generations is also in this case brought about by the direct action of
the external conditions of life; and secondly, the winter form is here
also the primary, and the summer form the secondary one.

In accordance with the idea first introduced into science by Rudolph
Leuckart, we have hitherto understood heterogenesis to be only the
alternation of dissimilar sexual generations. From this point of
view the reproduction of _Leptodora_ can be as little ascribed to
heterogenesis as can that of _Aphis_ or _Daphnia_, although the
apparent agamic reproduction of the winter and a portion of the summer
generation is undoubtedly parthenogenesis and not propagation by
nursing.[48] As has already been said, however, I would attribute no
fundamental importance to the criterion of agamic reproduction--the
more especially because we are ignorant of the physiological
significance of the two modes of propagation; and further, because this
principle of classification is entirely external, and only valuable
in so far as no better one can be substituted for it. A separation of
the modes of cyclical propagation according to their genesis appears
to me--especially if practicable--not alone to be of greater value,
but the only correct one, and for this the knowledge of the origin of
seasonal dimorphism seems to me to furnish a possible method.

If, as was indicated above, we designate as metagenesis (in the narrow
sense) all those cases in which it must be admitted that a series
of differently aged phyletic stages have furnished the points of
departure, and as heterogenesis those cases in which similar phyletic
stages have been compelled to produce a cycle of generations by the
periodic action of external influences, it is clear that the scope of
heterogenesis is by this means considerably extended, and at the same
time sharply and precisely defined.

Under heterogenesis then is comprised, not only as heretofore the
reproduction of _Ascaris nigrovenosa_, of _Leptodora appendiculata_,
and of the cattle-lice, but also that of the _Aphides_, _Coccidæ_,
_Daphniidæ_, _Rotatoria_, and _Phyllopoda_, and, in short, all those
cases in which we can determine the former identity of the two kinds
of generations from their form, anatomical structure, and mode of
reproduction. This conclusion is essentially supported by a comparison
of the most closely allied species. Thus, for instance, when we see
the genus _Aphis_ and its allies related on all sides to insects which
propagate sexually in all generations, and when we further observe
the great similarity of the whole external and internal structure
in the two kinds of generations of _Aphis_, we are forced to the
conjecture that the apparent asexual reproduction of the _Aphidæ_ is in
reality parthenogenesis, i.e., that it has been developed from sexual
reproduction. Neither can it be any longer disputed that in this case,
as well as in that of _Leptodora_ and other _Daphniidæ_, the same female
alternately propagates parthenogenetically, and produces eggs requiring
fertilization. This was established by Von Heyden[49] some years ago,
in the case of _Lachnus Querci_, and has been since confirmed by
Balbiani.[50]

There can be no doubt that in all these cases the cycle of generations
has been developed from phyletically similar generations. But
instances are certainly conceivable which present themselves with less
clearness and simplicity. In the first place, we do not know whether
parthenogenesis may not finally settle down into complete asexual
reproduction. Should this be the case, it might be possible that from
heterogenesis a mode of propagation would ultimately arise, which
was apparently indistinguishable from pure metagenesis. Such a state
of affairs might result, if the generations settling into asexual
reproduction (as, for instance, the plant-lice), at the same time
by adaptation to varying conditions of life, underwent considerable
change of structure, and entered upon a metamorphosis to some extent
retrogressive. We should then be inclined to regard these generations
as an earlier phyletic stage, whilst, in fact, they would be a later
one, and the idea of metagenesis would thus have been formed after the
manner of heterogenesis.

On the other hand, it is equally conceivable that heterogenesis may
have been developed from true metagenesis in the case of larvæ which,
having acquired the faculty of asexual propagation, are similar in
function to sexually mature insects. This possibility is not at first
sight apparent. If the nursing-larvæ of the _Cecidomyiæ_ were as much
like the sexual insects as are the young Orthoptera to the sexually
mature forms, we should not know whether to regard them as degraded
sexual insects, or as true larvæ which had attained the power of
asexual propagation. Their propagation would be considered to be
parthenogenesis; and as it could not be denied that heterogenesis was
here manifest, the mode of development of their particular kind of
propagation might be proved, i.e., it might be demonstrated, that the
generations now parthenogenetic were formerly mere reproductive larval
stages.

I have only offered these last observations in order to show on what
uncertain ground we are still standing with regard to this subject
whenever we deal with the meaning of any particular case, and how
much still remains to be done. It appears certain that the two forms
of cyclical propagation, heterogenesis and metagenesis, originate in
entirely distinct ways, so that it must be admitted that, under these
circumstances, the idea of the existing conditions respecting the true
genesis may possibly be erroneous. To indicate the manner in which the
cyclical mode of propagation has arisen in any single case, would only
be possible by a searching proof and complete knowledge of existing
facts in addition to experiments.



VI.

GENERAL CONCLUSIONS.


I shall not here give a repetition and summary of the results arrived
at with respect to seasonal dimorphism, but rather the general
conclusions derived from these results; and, at the same time, I may
take the opportunity of raising certain questions which have not
hitherto found expression, or have been but briefly and casually stated.

It must, in the first place, be admitted that differences of specific
value can originate through the direct action of external conditions
of life only. Of the truth of this proposition there can be no doubt,
after what has been above stated concerning the difference between
the two forms of any seasonally dimorphic species. The best proof is
furnished by the older systematists, to whom the genetic relationship
of the two forms was unknown, and who, with unprejudiced taxonomy, in
many cases indicated their distinctness by separate specific names.
This was the case with _Araschnia Levana_ and _Prorsa_, _Euchloe Belia_
and _Ausonia_, _E. Belemia_ and _Glauce_, _Plebeius Polysperchon_ and
_Amyntas_. In the presence of these facts it can scarcely be doubted
that new species can be formed in the manner indicated; and I believe
that this was and is still the case, with butterflies at least, to
a considerable extent; the more so with these insects, because the
striking colours and markings of the wings and body, being in most
cases without biological significance, are useless for the preservation
of the individual or the species, and cannot, therefore, be objects of
natural selection.

Darwin must have obtained a clear insight into this, when he attempted
to attribute the markings of butterflies to sexual and not to natural
selection. According to this view, every new colour or marking first
appears in one sex accidentally,[51] and is there fixed by being
preferred by the other sex to the older coloration. When the new
ornamentation becomes constant (in the male for example), Darwin
supposes that it becomes transferred to the female by inheritance,
either partially or completely, or not at all; so that the species,
therefore, remains more or less sexually dimorphic, or (by complete
transference) becomes again sexually monomorphic.

The admissibility of such different, and, to a certain extent,
arbitrarily limited inheritance, has already been acknowledged.
The question here concerned is, whether Darwin is correct when he
in this manner attributes the entire coloration of butterflies to
sexual selection. The origin of seasonal dimorphism appears to me to
be against this view, howsoever seductive and grand the latter may
seem. If differences as important as those which exist between the
summer and winter forms of many butterflies can be called forth by the
direct action of a changed climate, it would be extremely hazardous to
attribute great importance to sexual selection in this particular case.

The principle of sexual selection appears to me to be incontestible,
and I will not deny that it is also effective in the case of
butterflies; but I believe that as a final explanation of colour this
agency can be dispensed with, inasmuch as we see that considerable
changes of colour can occur without the influence of sexual
selection.[52]

The question now arises, how far does the transforming influence of
climate extend? When a species has become transformed by climatic
change to such an extent that its new form possesses the systematic
value of a new species, does it return to its older form by removal
to the old climatic conditions? or would it under these circumstances
become again transformed in a new manner? This question is not without
importance, inasmuch as in the first case climatic influences would be
of little value in the formation of species, and there would result
at most only a fluctuation between two extremes. In the same manner
as in seasonally dimorphic species the summer and winter forms now
alternate with each other every year, so would the forms produced by
warmth and cold then alternate in the greater periods of the earth’s
history. Other groups of animals are certainly changed by the action of
different climatic influences; but in butterflies, as I believe I have
proved, temperature plays the chief part, and as this only oscillates
between rather narrow limits, it admits of no great differences of
coloration.

The question thus suggests itself, whether species of butterflies
only oscillate between two forms, or whether climatic change, when
sufficiently great to produce variation, does not again originate a new
form. Inasmuch as the reversion experiments with seasonally dimorphic
butterflies appear to correspond with the latter view, I believe that
this must be admitted. I am of opinion that an old form never again
arises through change of climate, but always a new one; so that a
periodically recurring change of climate is alone sufficient, in the
course of a long period of time, to admit of new species arising from
one another. This, at least, may be the case with butterflies.

My views rest essentially upon theoretical considerations. It
has already been insisted upon, as results immediately from the
experiments, that temperature does not act on the physical constitution
of the individual in the same manner as acid or alkali upon litmus
paper, i.e., that one and the same individual does not produce this or
that coloration and marking according as it is exposed to warmth or
cold; but rather that climate, when it influences in a similar manner
many succeeding generations, gradually produces such a change in
the physical constitution of the species that this manifests itself
by other colours and markings. Now when this newly acquired physical
constitution, established, as we may admit, throughout a long series
of generations, is again submitted to a constant change of climate,
this influence, even if precisely similar to that which obtained during
the period of the first form of the species, cannot possibly reproduce
this first form. The nature of the external conditions may be the same,
but not so the physical constitution of the species. Just in the same
manner as a _Pieris_ (as has been already shown), a _Lycæna_, or a
_Satyrus_, produces quite different varieties under the transforming
influence of the same climate, so must the variation originating
from the transformed species of our present case after the beginning
of the primary climate be different from that primary form of the
species, although perhaps in a less degree. In other words, if only
two different climates alternated with each other during the earth’s
geological periods, every species of butterfly submitted to these
changes of climate would give rise to an endless series of different
specific forms. The difference of climate would in reality be greater
than supposed, and for any given species the climatic variation would
not only occur through the periodic shifting of the ecliptic, but also
through geological changes and the migrations of the species itself, so
that a continuous change of species must have gone on from this sole
cause of alternation of climate. When we consider that many species
elsewhere extinct have become locally preserved, and when, further, to
these we add those local forms which have arisen by the prevention of
crossing (amixia), and finally take into consideration the important
effects of sexual selection, we can no longer be astonished at the vast
numbers of species of butterflies which we now meet with on the earth.

Should any one be inclined to conclude, from my reversion experiments
with seasonally dimorphic butterflies, that the secondary species
when exposed to the same climate as that which produced it must
revert to the primary, he forgets that this reversion to the winter
form is nothing but a reversion--i.e., a sudden return to a primary
form through peculiar laws of inheritance--and by no means a gradual
re-acquisition of this primary form under the gradual influence of
the primary climate. Reversion to the winter form occurs also through
other influences, as, for instance, by high temperature. Reversions
of this kind, depending on laws of heredity, certainly happen with
those cases of transmutation which do not alternate with the primary
form, as in seasonal dimorphism, but which occur continuously. They
would, however probably be more quickly suppressed in such cases than
in seasonal dimorphism, where the constant alternation of the primary
and secondary forms must always maintain the tendency of the latter to
produce the former.

That the above conclusion is correct--that a secondary species, when
exposed to the external conditions under the influence of which the
primary form originated, does not again revert to the latter--is proved
by experience with plants. Botanists[53] assure us “that cultivated
races which become wild, and are thus brought back to their former
conditions of life, do not become changed into the original wild form,
but into some new one.”

A second point which appears to me to be elucidated by seasonal
dimorphism, is the origin of variability. It has already been
prominently shown that secondary forms are for the most part
considerably more variable than primary forms. From this it follows
that similar external influences either induce different changes in
the different individuals of a species, or else change all individuals
in the same manner, variability arising only from the unequal time
in which the individuals are exposed to the external influence. The
latter is undoubtedly the case, as appears from the differences which
are shown by the various individuals of a secondary form. These are
always only differences of degree and not of kind, as is perhaps most
distinctly shown by the very variable _A. Prorsa_ (summer form), in
which all the occurring variations differ only by the _Levana_ marking
being more or less absent, and, at the same time, by approximating
more or less to the pure _Prorsa_ marking; but changes in a totally
different direction never occur. It is likewise further evident, as has
been mentioned above, that allied species and genera, and even entire
families (_Pieridæ_), are changed by similar external inducing causes
in the same manner--or, better, in the same direction.

In accordance with these facts the law may be stated, that, in
butterflies at least, all the individuals of a species respond to the
same external influences by similar changes, and that, consequently,
the changes brought about by climatic influences take a fixed
direction, determined by the physical constitution of the species.
When, however, new climatic forms of butterflies, in which natural
selection is completely excluded, and the nature of the species itself
definitely determines the direction of the changes, nevertheless show
variability from the very beginning, we may venture to conclude that
every transformation of a species generally begins with a fluctuation
of its characters. But when we find the primary forms of butterflies
always far more constant, this shows that the continued crossing of the
individuals of a species to a certain extent balances the fluctuations
of form. Both facts taken together confirm the law formerly enunciated
by me,[54] that in every species a period of variability alternates
with one of (relative) constancy--the latter indicating the
culmination, and the former the beginning or end, of its development.
I here call to mind this law, because the facts which I advanced at
that time, viz., Hilgendorf’s history of the phyletic development of
the Steinheim fossil shells, having since become somewhat doubtful, one
might easily be inclined to go too far in mistrusting them and refuse
to give them any weight at all.[55]

In the essay just indicated I traced the origin of a certain class
of local forms to local isolation. I attempted to show that when a
species finds itself in an isolated district in a condition (period)
of variability, it must there necessarily acquire somewhat deviating
characters by being prevented from crossing with the individuals
of other regions, or, what comes to the same thing, a local form
must originate. This production of local forms results because
the different variations which, for the time being, constitute the
variability of the species, would always be in a different numerical
proportion in the isolated district as compared with other regions;
and further, because constancy is produced by the crossing of these
(isolated) varieties among themselves; so that the resultant of
the various components is (local) variation. If the components are
dissimilar the resultant would also be different, and thus, from a
theoretical point of view, there seems to me no obstacle in the way
of the production of such local forms by the process of ‘amixia.’ I
believe that I have further shown that numerous local forms can be
conceived to have arisen through this process of preventive crossing,
whilst they cannot be explained by the action of climatic influences.

That I do not deny the existence of true climatic forms in admitting
this principle of ‘amixia,’ as has been frequently imagined, appears
sufficiently from the treatise in question. The question arises,
however, whether climatic influences may not also originate forms
by ‘amixia’ by making a species variable. It would be difficult at
present to decide finally upon this subject. If, however, in all cases
a variation in a certain fixed direction occurred through climatic
influences, a form could not arise by ‘amixia’ from such a variability,
since the components could then produce resultants different only
in degree and not in kind. But we are not yet able to extend our
researches to such fine distinctions.

As a final, and not unimportant result of these investigations, I may
once more insist that dissimilar influences, when they alternatingly
affect a long series of originally similar generations in regularly
recurring change, only modify the generations concerned, and not
intermediate ones. Or, more briefly, cyclically acting causes of
change produce cyclically recurring changes: under their influence
series of monomorphic generations become formed into a cycle of di- or
polymorphic generations.

There is no occasion to return here to the immediate evidence and
proof of the foregoing law. In the latter, however, is comprised the
question--is not the cycle of generations produced by cyclical heredity
ultimately equivalent to Darwin and Haeckel’s homochronic heredity
which forms the ontogenetic stages into a cycle? It is possible
that from this point, in the future, the nature of the processes of
heredity, which are still so obscure, may be penetrated into, and both
phenomena traced to the same cause, as can now be only surmised but not
clearly perceived.

Finally, the most general, and in so far chief result of these
investigations, appears to me to lie in the conclusion, which may
be thus formulated:--A species is only caused to change through the
influence of changing external conditions of life, this change being
in a fixed direction which entirely depends on the physical nature of
the varying organism, and is different in different species, or even in
the two sexes of the same species.

I am so little disposed to speak in favour of an unknown transforming
power that I may here again insist that the transformation of a
species only partly depends upon external influences, and partly on
the specific constitution of the particular form. I designate this
constitution ‘specific,’ inasmuch as it responds to the same inciting
cause in a manner different to the constitution of another species.
We can generally form a clear conception why this should be the case;
for not only is there in another species a different kind of latent
vital activity, but each species has also a different developmental
history. It must be admitted that, from the earliest period of the
formation of an organism, and throughout all its intermediate stages,
properties which have become established, such as growth, nutrition,
or tendency to development, have been transferred to the species now
existing, each of which bears these tendencies in itself to a certain
extent. It is these innate tendencies which determine the external
and internal appearance of the species at every period of its life,
and which, by their reaction to external factors, represent the life
of the individual as well as that of the species. Since the sum of
these inherited tendencies must vary more or less in every species,
not only is the different external appearance of species as well as
their physiological and biological diversity thus explained, but it
necessarily follows therefrom, that different species must respond
differently to those external causes which tend to produce a change in
their form.

Now, this last conclusion is equivalent to the statement that every
species, through its physical constitution, (in the sense defined) is
impressed with certain fixed powers of variation, which are evidently
extraordinarily numerous in the case of each species, but are not
unlimited; they permit of a wide range for the action of natural
selection, but they also limit its functions, since they certainly
restrain the course of development, however wide the latter may be. I
have elsewhere previously insisted[56] that too little is ascribed to
the part played by the physical constitution of species in the history
of their transformation, when the course of this transformation is
attributed entirely to external conditions. Darwin certainly admits
the importance of this factor, but only so far as it concerns the
individual variation, the nature of which appears to him to depend on
the physical constitution of the species. I believe, however, that in
this directive influence lies the precise reason why, under the most
favourable external circumstances, a bird can never become transformed
into a mammal--or, to express myself generally, why, from a given
starting-point, the development of a particular species cannot now
attain, even under the most favourable external conditions, any desired
goal; and why, from this starting-point, given courses of development,
even when of considerable latitude, must be restricted, just as a ball
rolling down a hill is diverted by a fixed obstacle in a direction
determined by the position of the latter, and depending on the
direction of motion and the velocity at the moment of being diverted.

In this sense I agree with Askenasy’s “fixed” direction of variation;
but not if another new physical force directing variation itself
is thereby intended.[57] The explanation of the phenomena does not
appear to me to require such an admission, and, if unnecessary, it
is certainly not legitimate. According to my view, transmutation by
purely internal causes is not to be entertained. If we could absolutely
suspend the changes of the external conditions of life, existing
species would remain stationary. The action of external inciting
causes, in the widest sense of the word, is alone able to produce
modifications; and even the never-failing “individual variations,”
together with the inherited dissimilarity of constitution, appear
to me to depend upon unlike external influences, the inherited
constitution itself being dissimilar because the individuals have been
at all times exposed to somewhat varying external influences.

A change arising from purely internal causes seems to me above all
quite untenable, because I cannot imagine how the same material
substratum of physical constitution of a species can be transferred to
the succeeding generation as two opposing tendencies. Yet this must
be the case if the direction of development transferred by heredity
is to be regarded as the ultimate ground both of the similarity
and dissimilarity to the ancestors. All changes, from the least to
the greatest, appear to me to depend ultimately only on external
influences; they are the response of the organism to external inciting
causes. It is evident that this response must be different when a
physical constitution of a different nature is affected by the same
inciting cause, and upon this, according to my view, depends the great
importance of these constitutional differences.

If, under “heredity,” we comprise the totality of inheritance--that
is to say, the physical constitution of a species at any time, and
therefore the restricted and, in the foregoing sense, pre-determined
power of variation, whilst under “adaptation” we comprehend the direct
and indirect response of this physical constitution to the changes in
the conditions of life, I can agree with Haeckel’s mode of expression,
and with him trace the transformation of species to the two factors of
heredity and adaptation.



APPENDIX I.

EXPERIMENTS.


EXPERIMENTS WITH ARASCHNIA LEVANA.

1. Bred from eggs laid by a female of the winter form on 12th-15th
May, 1868, in a breeding-cage. The caterpillars emerged on 20th-22nd
May, and pupated on 7th-9th June. The pupæ, kept at the ordinary
temperature, produced:--

  On the 19th of June  4 butterflies.
    ”    20th    ”     5     ”
    ”    21st    ”    10     ”
    ”    22nd    ”     9     ”
    ”    23rd    ”     7     ”
    ”    25th    ”    13     ”
                      --
         Total        48     ”

All these butterflies were of the _Prorsa_ type, 3 females having a
considerable amount of yellow, but none with so much as figs. 3, 4, 7,
8, or 9. Pl. I.

2. August 12th, 1868, found larvæ of the third generation, which
pupated at the beginning of September, and were kept in a room not
warmed. In September three butterflies emerged in the _Prorsa_ form,
the remainder hibernating and producing, after being placed in a heated
room at the end of February, from the 1st to the 17th of March, 1869,
more butterflies, all of the _Levana_ form.

3. Larvæ found on the 17th June, 1869, were sorted according to colour;
the yellow ones, with light brown spines, produced, at the ordinary
temperature, on 8th-12th July, 13 butterflies, 12 of which showed the
ordinary _Prorsa_ type, and one, a male, possessing more yellow than
fig. 3, Pl. I., must be considered as a _Porima_ type.

4. From caterpillars of the second generation, found at the same
time as those of Exp. 3, 30 pupæ were placed in the refrigerator
(temperature 8°-10° R.) on June 25th. When the box was opened on
August 3rd, almost all had emerged, many being dead, and all, without
exception, were of the intermediate form (_Porima_), although nearer
the _Prorsa_ than the _Levana_ type.

5. A large number of caterpillars of the second generation, found at
the same time, pupated, and were kept at a high summer temperature.
After a pupal period of about 19 days, some 70 butterflies emerged from
28th June to 5th July, all of the _Prorsa_ form, with the exception of
5, which were strongly marked with yellow (_Porima_).

6. The 70 butterflies of the foregoing experiment were placed in an
enclosure 6 feet high, and 8 feet long, in which, during warm weather,
they freely swarmed on flowers. Copulation was only once observed, and
but one female laid eggs on nettle on July 4th. At the high summer
temperature prevailing at the time, these eggs produced butterflies
after 30-31 days (third generation). All were _Prorsa_, with more or
less yellow; among 18 none were completely _Porima_.

7. Young larvæ of the fourth generation, found on the 8th of August,
were reared in a hothouse (17°-20° R.). They pupated on 21st-23rd
August. Of these:--

A. 56 pupæ were placed on ice (0°-1° R.) for five weeks, and then
allowed to hibernate in a room not warmed. In April, 1870, they all
gave the _Levana_ form, with the exception of a single _Porima_.

B. About an equal number of pupæ were placed in the hothouse, but
without any result; for, notwithstanding a temperature of 12°-24° R.,
not a single butterfly emerged in the course of October and November.
The pupæ were then allowed to hibernate in an unheated room, and in
April and May gave nothing but _Levana_.

8. Caterpillars of the second generation, found at the beginning of
June, 1870, pupated on 13th-15th June, and gave, at the ordinary
temperature, on June 29th-30th, 7 butterflies of the _Prorsa_ form.

9. Pupæ of the same (second) generation were placed immediately after
pupation on June 18th, 1870, in a refrigerator (0°-1° R.), and after
remaining there four weeks (till July 18th) gave, at the ordinary
summer temperature:--

  On the 22nd of July, 2 _Prorsa_.
     ”   23rd     ”    3    ”
     ”   24th     ”    6 _Porima_, 4 of which were
                           very similar to _Levana_.
     ”   25th     ”    1 _Levana_, without the blue
                           marginal line.
     ”   26th     ”    2 _Levana_, also without the
                           blue marginal line.
     ”   2nd August,   6 _Porima_.
                      --
         Total        20

Of these 20 butterflies only 5 were of the pure _Prorsa_ form.

10. Full grown larvæ of the fourth generation, found on August 20th,
1870, pupated on August 26th to September 5th. The pupæ were divided
into three portions:--

A. Placed in the hothouse (12°-25° R.), immediately after pupation and
left there till October 20th. Of about 40 pupæ only 4 emerged, 3 of
which were _Prorsa_ and 1 _Porima_. The remaining pupæ hibernated and
all changed into _Levana_ the following spring.

B. Kept in a room heated to 6°-15° R. from November. Not a single
specimen emerged the same year. This lot of pupæ were added to C from
November.

C. Placed on ice for a month immediately after pupation; then,
from September 28th to October 19th in the hothouse, where no more
butterflies emerged. The pupæ hibernated, together with those from lot
B, in a room heated by water to 6°-15° R., and gave:--

  On the 6th of February,  1 female _Levana_.
    ”   22nd      ”        1 male _Levana_.
    ”   23rd      ”        1 male _Levana_.
    ”   24th      ”        1 female _Levana_.
    ”   25th      ”        1 male and 1 female _Levana_.
    ”   28th      ”        1 male and 1 female _Levana_.
    ”    1st of March,     1 male _Levana_.
    ”   13th      ”        1 female _Levana_.
    ”   15th      ”        1 female _Levana_.
    ”   19th      ”        1 male _Levana_.
    ”    2nd of April,     2 male and 1 female _Levana_.
    ”    7th      ”        1 female _Levana_.
    ”   21st      ”        1 female _Levana_.
    ”    2nd of May,       1 female _Levana_.
                          --
        Total             18 _Levana_, 10 of which were females.

The exact record of the time of emergence is interesting, because it is
thereby rendered apparent that different individuals respond more in
different degrees to a higher than to the ordinary temperature. Whilst
with many an acceleration of development of 1-2 months occurred, others
emerged in April and May, i.e. at the time of their appearance in the
natural state.

11. Reared the second generation from eggs of the first generation.
Emerged from the eggs on June 6th, 1872, pupated on July 9th. The pupæ
were placed on ice (0°-1° R.) from July 11th till September 11th, and
then transferred to a hothouse, where all emerged:--

  On the 19th of September,   3  male _Prorsa_, 1 male _Porima_.
     ”   21st       ”        13  _Porima_ (12 males, 1 female),
                                   2 female _Levana_.
     ”   22nd       ”        14  _Porima_ (12 males, 2 females)
                                   and 1 female _Levana_.
     ”   23rd       ”        10  female _Levana_, 3 male _Porima_.
     ”   24th       ”         5  female _Levana_.
     ”   25th       ”         1  female _Levana_.
     ”   27th       ”         3  female _Levana_.
     ”    4th of October,     1  male _Porima_.
                             --
             Total           57  butterflies (32 males and 25 females),
   only 3 of which were _Prorsa_, 32 _Porima_, and 22 _Levana_.

It must be pointed out, however, that among those specimens marked
as “_Levana_” there were none which entirely corresponded with the
natural _Levana_, or which indeed approximated so nearly to this form
as did some of the specimens in Exp. 9. All were larger than the
natural _Levana_, and possessed, notwithstanding the large amount of
yellow, more black than any true _Levana_. In all artificially bred
_Levana_ the black band of the basal half of the hind wings is always
interrupted with yellow, which is seldom the case with true _Levana_.
The whole appearance of the artificial _Levana_ is also coarser, and
the contour of the wings somewhat different, the fore-wings being
broader and less pointed. (See figs. 7 to 9, Pl. I.).

12. Larvæ of the fourth generation, found on September 22nd, 1872, were
divided into two portions:--

A. Placed for pupation in an orchid-house at 12°-25° R., and allowed
to remain there till December. In spite of the high temperature not
a single butterfly emerged during this time, whilst pupæ of _Vanessa
C-album_ and _Pyrameis Atalanta_, found at the same time, and placed in
the same hothouse, emerged in the middle of October. From the middle of
December the pupæ were kept in an unheated room, and they emerged very
late in the spring of 1873, all as _Levana_:--

  On the 6th of June, 7 _Levana_.
    ”    8th      ”   2    ”
    ”   11th      ”   2    ”
    ”   12th      ”   1    ”
    ”   15th      ”   6    ”
    ”   16th      ”   1    ”
    ”   19th      ”   2    ”
                     --
       Total         21    ”

B. Kept in an unheated room during the winter. The butterflies emerged
from the 28th of May, all as _Levana_.


EXPERIMENTS WITH PIERINÆ.

13. Females of _Pieris Rapæ_, captured in April, laid eggs on
_Sisymbrium Alliaria_. From these caterpillars were obtained, which
pupated on 1st-3rd June. The pupæ were placed on ice from June 3rd till
September 11th (0°-1° R.), and from September 11th till October 3rd in
the hothouse (12°-24° R.), where there emerged:--

  On the 23rd of October, 1 female.
     ”   24th      ”      1 female.
     ”   25th      ”      2 males, 1 female.
     ”   26th      ”      1 female.
     ”   28th      ”      1 male, 1 female.
                          -------------------
         Total            3 males, 5 females.

All these were sharply impressed with the characters of the winter
form, the females all strongly yellow on the upper side, the males pure
white; on the under side a strong black dusting on the hind wings,
particularly on the discoidal cell. One pupa did not emerge in the
hothouse, but hibernated, and gave in a heated room on January 20th,
1873, a female, also of the winter form.

14. Females of _Pieris Napi_, captured on 27th-28th April, 1872, laid
eggs on _Sisymbrium Alliaria_. The larvæ bred from these pupated on May
28th to June 7th. The pupæ, shortly after transformation, were placed
on ice, where they remained till Sept. 11th (three months). Transferred
to the hothouse on October 3rd, they produced, up to October 20th, 60
butterflies, all with the sharply-defined characters of the winter
form. The remaining pupæ hibernated in a room, and produced:--

  On the 28th of April, 3 males, 6 females.
     ”    4th of May,   1 female.
     ”   12th     ”     4 males.
     ”   15th     ”     1 male, 1 female.
     ”   16th     ”     1 male.
     ”   18th     ”     1 male, 1 female.
     ”   19th     ”     1 female.
     ”   20th     ”     2 males, 1 female.
     ”   23rd     ”     2 males.
     ”   26th     ”     1 male.
     ”   29th     ”     1 female.
     ”    3rd of June,  3 females.
     ”    6th     ”     1 female.
     ”    9th     ”     1 female.
     ”   21st     ”     1 female.
     ”    2nd of July,  1 female.
                       ---------------------
              Total    15 males, 19 females.

15. Several butterflies from Exp. 14, which emerged in May, 1873,
were placed in a capacious breeding-house, where they copulated and
laid eggs on rape. The caterpillars fed on the living plants in the
breeding-house, and after pupation were divided into two portions:--

A. Several pupæ, kept at the ordinary summer temperature, gave
butterflies on July 2nd, having the characters of the summer form.

B. The remainder of the pupæ were placed on ice immediately after
transformation, and remained over three months in the refrigerator
(from July 1st till October 10th). Unfortunately most of them perished
through the penetration of moisture into the box. Only 8 survived, 3
of which emerged on the 20th of October as the winter form; the others
hibernated in an unheated room, and emerged at the beginning of June,
1874. All 5 were females, and all exhibited the characters of the
winter form. Notwithstanding a pupal period of eleven months, they did
not possess these characters to a greater extent than usual, and did
not, therefore, approximate to the parent-form _Bryoniæ_.

16. On June 12th, 1871, specimens of _Pieris Napi_, var. _Bryoniæ_,
were captured on a mountain in the neighbourhood of Oberstorf
(Allgäuer Alpen), and placed in a breeding-house, where they flew
freely about the flowers; but although copulation did not take place,
several females laid eggs on the ordinary garden cabbage. From these
caterpillars were hatched, which at all stages of growth were exactly
like those of the ordinary form of _Napi_. They throve well until
shortly before pupation, when a fungoid epidemic decimated them, so
that from 300 caterpillars only about 40 living pupæ were obtained.
These also completely resembled the ordinary form of _Napi_, and showed
the same polymorphism, some being beautifully green, others (the
majority) straw yellow, and others yellowish grey. Only one butterfly
emerged the same summer, a male, which, by the black dusting of the
veins on the margin of the wings (upper side), could be with certainty
recognized as var. _Bryoniæ_. The remaining pupæ hibernated in a heated
room, and gave, from the end of January to the beginning of June, 10
males and 5 females, all with the characters of the var. _Bryoniæ_.
They emerged:--

  On the 22nd of January,  1 male.
    ”    26th      ”       1 male.
    ”     3rd of February, 1 male.
    ”     4th      ”       1 male.
    ”     5th      ”       1 male.
    ”     7th      ”       1 female.
    ”     9th      ”       1 male.
    ”    24th      ”       1 male.
    ”     4th of March,    1 female.
    ”    11th      ”       1 male, 1 female.
    ”     6th of April,    1 female.
    ”    17th      ”       1 male.
    ”    11th of May,      1 female.
    ”     3rd of June,     1 male.

We here perceive that the tendency to accelerate development through
the action of warmth is, in this case, also very different in different
individuals. Of the 16 butterflies only 1 kept to the normal period
of development from July 27th to June 3rd, fully ten months; all the
others had this period abbreviated, 1 male to eleven days, 8 specimens
to six months, 4 to seven months, 2 to eight months, and 1 to nine
months.



APPENDIX II.


EXPERIMENTS WITH PAPILIO AJAX.[58]

From eggs of var. _Telamonides_ laid on the last of May larvæ were
obtained, which gave on June 22nd-26th, 122 pupæ. These, as fast as
formed, were placed on ice in the refrigerator in small tin boxes, and
when all the larvæ had become transformed the pupæ were transferred
to a cylindrical tin box (4 in. diam. and 6 in. high), and packed in
layers between fine shavings. The tin box was set in a small wooden
one, which was put directly on the ice and kept there till July 20th.
From that date, by an unfortunate accident, the box, instead of being
kept on the surface of the ice in an ice-house, as was intended, was
placed on straw near the ice, so that the action of the cold was
modified, the outside pupæ certainly experiencing its full effects,
but the inside ones were probably at a somewhat higher temperature.
The ice failed on August 20th, so that the pupæ had been subjected
to an equable low temperature in the refrigerator for three to four
weeks, and to a lesser degree of cold in the ice-house for five weeks,
the temperature of the last place rising daily, as the ice had all
thawed by August 20th. On opening the box it was found (probably owing
to the cold not having been sufficiently severe) that the butterflies
had commenced to emerge. Twenty-seven dead and crippled specimens were
removed, together with several dead pupæ. One butterfly that had just
emerged was taken out and placed in a box, and when its wings had fully
expanded it was found to be a “_Telamonides_ of the most pronounced
type.” The experimenter then states:--“Early in the morning I made
search for the dead and rejected butterflies, and recovered a few. It
was not possible to examine them very closely from the wet and decayed
condition they were in, but I was able to discover the broad crimson
band which lies above the inner angle of the hind wings, and which is
usually lined on its anterior side with white, and is characteristic of
either _Walshii_ or _Telamonides_, but is not found in _Marcellus_. And
the tip only of the tail being white in _Walshii_, while both tip and
sides are white in _Telamonides_, enabled me to identify the form as
between these two. There certainly were no _Walshii_, but there seemed
to be a single _Marcellus_, and excepting that all were _Telamonides_.”

The remaining pupæ were kept in a light room where 3 _Telamonides_
emerged the following day, and by September 4th 14 specimens of the
same variety had emerged, but no _Marcellus_ or intermediate forms.
From the 4th to the 20th of September a few more _Telamonides_
appeared, but between the 4th and 15th of the month 12 out of 26
butterflies that had emerged were intermediate between _Telamonides_
and _Marcellus_, some approximating to one form and some to the other
form. The first pure _Marcellus_ appeared on September 4th, and was
followed by one specimen on the 6th, 8th, 13th and 15th respectively.
From this last date to October 3rd, 6 out of 10 were _Marcellus_ and
3 intermediate. On September 3rd, a specimen intermediate between
_Telamonides_ and _Walshii_ emerged, “in which the tails were white
tipped as in _Walshii_, but in size and other characters it was
_Telamonides_, though the crimson band might have belonged to either
form.” Butterflies continued to emerge daily up to September 20th,
after which date single specimens appeared at intervals of from four
to six days, the last emergence being on October 16th. Thus, from
the time the box was removed from the ice-house, the total period of
emerging was fifty-seven days, some specimens having emerged before the
removal of the box. With specimens of _P. Ajax_ which appear on the
wing the first season the natural pupal period is about fourteen days,
individuals rarely emerging after a period of four to six weeks.

Between August 20th and October 16th, the 50 following butterflies
emerged:--

  On the 20th  of August,    1 male _Telamonides_.
    ”    21st       ”        1 male and 2 female _Telamonides_.
    ”    22nd       ”        1 female _Telamonides_.
    ”    24th       ”        1 female _Telamonides_.
    ”    29th       ”        1 male _Telamonides_.
    ”    31st       ”        1 female _Telamonides_.
    ”     1st  of September, 1 female _Telamonides_.
    ”     2nd       ”        1 female _Telamonides_.
    ”     3rd       ”        1 female intermediate between
                               _Telamonides_ and _Walshii_.
    ”      ”        ”        1 male _Telamonides_.
    ”     4th       ”        4 males and 1 female _Telamonides_.
    ”      ”        ”        2 males, medium, nearest _Telamonides_.
    ”      ”        ”        2 males, medium, nearest _Marcellus_.
    ”      ”        ”        2 males, _Marcellus_.
    ”     5th       ”        1 male and 1 female _Telamonides_.
    ”      ”        ”        1 male medium, nearest _Telamonides_.
    ”     6th       ”        1 male _Marcellus_.
    ”     7th       ”        1 male _Telamonides_.
    ”     8th       ”        1 male _Marcellus_ and 1 female
                               _Telamonides_.
    ”     9th       ”        1 male _Marcellus_ and 1 female medium,
                               nearest _Marcellus_.
    ”    13th       ”        1 male medium, nearest _Marcellus_.
    ”      ”        ”        1 male medium, nearest _Telamonides_.
    ”      ”        ”        1 male _Marcellus_.
    ”    14th       ”        1 male _Marcellus_ and 1 female medium,
                               nearest _Marcellus_.
    ”      ”        ”        1 male medium, nearest _Telamonides_.
    ”    15th       ”        1 male _Marcellus_.
    ”    16th       ”        1 female _Marcellus_ and 1 male
                               _Telamonides_.
    ”    18th       ”        1 male medium, nearest _Marcellus_.
    ”    19th       ”        1 female _Marcellus_.
    ”    20th       ”        1 male _Telamonides_.
    ”    24th       ”        1 male _Marcellus_.
    ”    30th       ”        1 female _Marcellus_.
    ”     2nd  of October,   1 female _Marcellus_.
    ”     3rd       ”        1 female medium, nearest _Telamonides_.
    ”     8th       ”        1 female medium, nearest _Telamonides_.
    ”    16th       ”        1 female medium, nearest _Telamonides_.

                        Total.

  _Telamonides_                   22  12 males, 10 females.
  _Telamonides_ partly _Walshii_   1             1 female.
  Medium, nearest _Telamonides_    8   5 males,  3 females.
  Medium, nearest _Marcellus_      6   4 males,  2 females.
  _Marcellus_                     13   9 males,  4 females.

                                  50  30 males, 20 females.

All these butterflies were very uniform in size, being about that of
the ordinary _Telamonides_. The specimens of _Telamonides_ especially
were “strongly marked, the crimson band in a large proportion of them
being as conspicuous as is usual in _Walshii_, and the blue lunules
near the tail were remarkably large and bright coloured. Of the
_Marcellus_, in addition to the somewhat reduced size, the tails were
almost invariably shorter than usual and narrower, and instead of the
characteristic single crimson spot, nearly all had two spots, often
large. In all these particulars they approach _Telamonides_.”

Adding to the _Telamonides_ which emerged after August 20th most of
those specimens which were found dead in the box at that date, the
total number of this form is thus brought up to nearly 50. Of the 122
pupæ with which Mr. Edwards started, 28 remained in a state fit for
hibernation, several having died without emerging. Previous experiments
had shown that 28 out of 122 pupæ is not an unreasonable number to
hibernate, so that the author concludes that the butterflies which
emerged the same season would have done so naturally, and the effect
of the artificial cold was not “to precipitate the emerging of any
which would have slept” till the following spring. Now under ordinary
circumstances all the butterflies which emerged the same season would
have been of the _Marcellus_ form, so that the cold changed a large
part of these into the form _Telamonides_, some (probably from those
pupæ which experienced the lowest temperature) being completely
changed, and others (from those pupæ which were only imperfectly
subjected to the cold) being intermediate, _i.e._, only partly changed.
It appears also that several pupæ experienced sufficient cold to retard
their emergence and stunt their growth, but not enough to change their
form, these being the 13 recorded specimens of _Marcellus_. Had the
degree of cold been equal and constant, the reversion would probably
have been more complete. The application of cold produced great
confusion in the duration of the pupal period, the emergence, instead
of taking place fourteen days after the withdrawal of the cold, as
might have been expected from Dr. Weismann’s corresponding experiment
with _Pieris Napi_ (Appendix I. Exps. 13 and 14), having been extended
over more than two months.

From the results of this experiment it must be concluded that
_Telamonides_ is the primary form of the species.


ADDITIONAL EXPERIMENTS WITH PAPILIO AJAX.

[_Communicated by_ Mr. W. H. EDWARDS, _November 18th, 1879_.]

EXP. 1.--In 1877 chrysalides of _P. Ajax_ and _Grapta Interrogationis_
(the eggs laid by females of the form _Fabricii_) were experimented
upon; but the results were not satisfactory, for the reason that the
author having been absent from home most of the time while the pupæ
were in the ice-box, on his return found the temperature above 5°-6°
R. And so far as could be told, the ice had been put in irregularly,
and there might have been intervals during which no ice at all was in
the box. Six chrysalides of the _Grapta_ so exposed produced unchanged
_Umbrosa_, the co-form with _Fabricii_. But all chrysalides from the
same lot of eggs, and not exposed to cold, also produced _Umbrosa_.
Nothing was learnt, therefore, respecting this species.

But chrysalides of _Ajax_, exposed at same time, did give changed
butterflies to some extent. From a lot of 8, placed in the box when
under twelve hours from pupation, and left for twenty-four days,
there came 5 males and 3 females. Of these was 1 _Telamonides_ in
markings and coloration, and all the rest were between _Marcellus_ and
_Telamonides_. Two other chrysalides on ice for twenty-three days gave
_Telamonides_, but 3 more exposed twenty-six days, and all one hour old
when put on ice, were unchanged, producing _Marcellus_.

During the same season 6 other _Ajax_ chrysalides were placed in the
box, and kept at about 0°-1° R. One was one hour old, and remained
for five days; 1 was one hour old, and remained for two days and
three-quarters; 3 at three hours old for eight days; and 1 (age
omitted), six days. All these gave unchanged butterflies of the form
_Marcellus_.

EXP. 2.--In May, 1878, many chrysalides were placed in the ice-box,
being from eggs laid by _Ajax_, var. _Walshii_. The youngest were
but ten to fifteen minutes from pupation, and were soft; others at
intervals up to twenty-four hours (the chrysalis is hard at about
twelve hours); after that, each day up to eight days after pupation.
All were removed from the box on the same day, 28th May. The exposure
was from nineteen to five days, those chrysalides which were put on ice
latest having the shortest exposure. The author wished to determine if
possible whether, in order to effect any change, it was necessary that
cold should be applied immediately after pupation or if one or several
days might intervene between pupation and refrigeration. Inasmuch as no
colour begins to show itself in the pupæ till a few hours, or at most
a day or two, before the butterfly emerges, it was thought possible
that cold applied shortly before that time would be quite as effective
as if applied earlier and especially very soon after pupation. The
result was, that more than half of the chrysalides exposed before they
had hardened died: 1 exposed at ten minutes, 2 at one hour, 1 at two
hours, 2 at three hours after pupation. On the other hand 1 at fifteen
minutes produced a butterfly, 1 at two hours, another at twelve hours.
The temperature was from 0°-1° R. most of the time, but varied somewhat
each day as the ice melted. The normal chrysalis period is from eleven
to fourteen days, in case the butterfly emerges the same season, but
very rarely an individual will emerge several weeks after pupation.

  On the  14th day after taking the pupæ from the ice, 1 _Telamonides_
          emerged from a chrysalis which had been placed in the ice-box
          three days after pupation, and was on ice sixteen days.

  On 19th day, 1 _Telamonides_ emerged from a pupa put on the ice
          twelve hours after pupation, and kept there eleven days.

  On 19th day, 1 _Walshii_ emerged from a pupa two hours old, and on
          ice eleven days.

All the rest emerged _Marcellus_, unchanged, but at periods prolonged
in a surprising way.

  1 on 43rd day exposed 15 minutes after pupation.
    ”  46th       ”      2 hours           ”
    ”  53rd       ”     24 hours           ”
    ”  62nd       ”      6 days            ”
    ”  63rd       ”      4 days            ”
    ”  66th       ”      7 days            ”
    ”  77th       ”      4 days            ”
    ”  81st       ”     12 hours           ”
    ”  91st       ”      5 days            ”
    ”  96th       ”     19 hours           ”

Five chrysalides lived through the winter, and all gave _Telamonides_
in the spring of 1879.

It appeared, therefore, that the only effect produced by cold in all
chrysalides exposed more than three days after pupation was to retard
the emergence of the butterfly. But even in some of these earliest
exposed, and kept on the ice for full nineteen days, the only effect
seemed to be to retard the butterfly.

EXP. 3.--In June, 1879, eggs of the form _Marcellus_ were obtained, and
in due time gave 104 chrysalides. Of these one-third were placed in the
ice-box at from twelve to twenty-four hours after pupation, and were
divided into 3 lots.

  1st,  9 pupæ, kept on ice 14 days.
  2nd, 12   ”        ”      20 days.
  3rd, 11   ”        ”      25 days.

Temperature 0°-1° R. most of the time, but varying somewhat as the ice
melted. (Both in 1878 and 1879 Mr. Edwards watched the box himself, and
endeavoured to keep a low temperature.)

Of the 69 chrysalides not exposed to cold, 34 gave butterflies at from
eleven to fourteen days after pupation, and 1 additional male emerged
11th August, or twenty-two days at least past the regular period of the
species.

Of the iced chrysalides, from lot No. 1 emerged 4 females at eight days
and a half to nine days and a half after removal from the ice, and 5
are now alive (Nov. 18) and will go over the winter.

From lot No. 2 emerged 1 male and 5 females at eight to nine days;
another male came out at forty days; and 5 will hibernate.

From lot No. 3 emerged 4 females at nine to twelve days; another male
came out at fifty-four days; and 6 were found to be dead.

In this experiment the author wished to see as exactly as
possible--First, in what points changes would occur. Second, if there
would be any change in the shape of the wings, as well as in markings
or coloration--that is, whether the shape might remain as that of
_Marcellus_, while the markings might be of _Telamonides_ or _Walshii_;
a summer form with winter markings. Third, to ascertain more closely
than had yet been done what length of exposure was required to bring
about a decided change, and what would be the effect of prolonging
this period. After the experiments with _Phyciodes Tharos_, which had
resulted in a suffusion of colour, the author hoped that some similar
cases might be seen in _Ajax_. The decided changes in 1878 had been
produced by eleven and sixteen days’ cold. In 1877, an exposure of two
days and three-quarters to eight days had failed to produce an effect.

From these chrysalides 11 perfect butterflies were obtained, 1 male and
10 females. Some emerged crippled, and these were rejected, as it was
not possible to make out the markings satisfactorily.

From lot No. 1, fourteen days, came:--

  1 female between _Marcellus_ and _Telamonides_.
  2 females, _Marcellus_.

These 2 _Marcellus_ were pale coloured, the light parts a dirty white;
the submarginal lunules on hind wings were only two in number and
small; at the anal angle was one large and one small red spot; the
frontal hairs were very short. The first, or intermediate female, was
also pale black, but the light parts were more green and less sordid;
there were 3 large lunules; the anal red spot was double and connected,
as in _Telamonides_; the frontal hairs short, as in _Marcellus_. These
are the most salient points for comparing the several forms of _Ajax_.
In nature, there is much difference in shape between _Marcellus_ and
_Telamonides_, still more between _Marcellus_ and _Walshii_; and the
latter may be distinguished from the other winter forms by the white
tips of the tails. It is also smaller, and the anal spot is larger,
with a broad white edging.

From lot No. 2, twenty days, came:--

  1 female _Marcellus_, with single red spot.

  1 female between _Marcellus_ and _Telamonides_; general coloration
    pale; the lunules all obsolescent; 2 large red anal spots not
    connected; frontal hairs medium length, as in _Telamonides_.

  1 female between _Marcellus_ and _Telamonides_; colour bright and
    clear; 3 lunules; 2 large red spots; frontal hairs short.

  1 female _Telamonides_; colours black and green; 4 lunules; a large
    double and connected red spot; frontal hairs medium.

  2 female _Telamonides_; colours like last; 3 and 4 lunules; 2 large
    red spots; frontal hairs medium.

From lot No. 3, twenty-five days, came:--

  1 male _Telamonides_; clear colours; 4 large lunules; 1 large, 1
    small red spot; frontal hairs long.

  1 female _Telamonides_; medium colours; 4 lunules; large double
    connected red spot; frontal hairs long.

In general shape all were _Marcellus_, the wings produced, the tails
long.

From this it appeared that those exposed twenty-five days were fully
changed; of those exposed twenty days, 3 were fully, 2 partly, 1 not at
all; and of those exposed fourteen days, 1 partly, 2 not at all.

The butterflies from this lot of 104 chrysalides, but which had not
been iced, were put in papers. Taking 6 males and 6 females from the
papers just as they came to hand, Mr. Edwards set them, and compared
them with the iced examples.

Of the 6 males, 4 had 1 red anal spot only, 2 had 1 large 1 small; 4
had 2 green lunules on the hind wings, 2 had 3, and in these last
there was a 4th obsolescent, at outer angle; all had short frontal
hairs.

Of the 6 females, 5 had but 1 red spot, 1 had 1 large 1 small spot; 5
had 2 lunules only, 1 had 3; all had short frontal hairs.

Comparing 6 of the females from the iced chrysalides, being those in
which a change had more or less occurred, with the 6 females not iced:

  1. All the former had the colours more intense, the black deeper,
      the light, green.

  2. In 5 of the former the green lunules on hind wings were decidedly
      larger; 3 of the 6 had 4 distinct lunules, 1 had 3, 1 had 3, and
      a 4th obsolescent. Of the 6 females not iced none had 4, 2 had
      2, and a 3rd, the lowest of the row, obsolescent; 3 had 3, the
      lowest being very small; one had 3, and a 4th, at outer angle,
      obsolescent.

  3. In all the former the subapical spot on fore wing and the stripe
      on same wing which crosses the cell inside the common black band,
      were distinct and green; in all the latter these marks were
      either obscure or obsolescent.

  4. In 4 of the former there was a large double connected red spot,
      and in one of the 4 it was edged with white on its upper side;
      2 had 1 large and 1 small red spot. Of the latter 5 had 1 spot
      only, and the 6th had 1 spot and a red dot.

  5. The former had all the black portions of the wing of deeper
      colour but less diffused, the bands being narrower; on the other
      hand, the green bands were wider as well as deeper coloured.
      Measuring the width of the outermost common green band along the
      middle of the upper medium interspace on fore wing in tenths of a
      millimetre, it was found to be as follows:

  On the iced pupæ    81, 66, 76, 76, 66, 66.
  On the not iced     56, 56, 51, 51, 46, 51.

Measuring the common black discal band across the middle of the lower
medium interspace on fore wing:

  On the iced pupæ    51, 66, 51, 51, 56, 61.
  On the not iced     76, 71, 66, 63, 71, 76.

In other words the natural examples were more melanic than the others.

No difference was found in the length of the tails or in the length and
breadth of wings. In other words, the cold had not altered the shape of
the wings.

Comparing 1 male iced with 6 males not iced:

  1. The former had a large double connected red anal spot, edged with
      white scales at top. Of the 6 not iced, 3 had but 1 red spot, 2
      had 1 large 1 small, 1 had 1 large and a red dot.

  2. The former had 4 green lunules; of the latter 3 had 3, 3 had
      only 2.

  3. The former had the subapical spot and stripe in the cells clear
      green; of the latter 1 had the same, 5 had these obscure or
      obsolescent.

  4. The colours of the iced male were bright; of the others, 2 were
      the same, 4 had the black pale, the light sordid white or
      greenish-white.

Looking over all, male and female, of both lots, the large size of the
green submarginal lunules on the fore wings in the iced examples was
found to be conspicuous as compared with all those not iced, though
this feature is included in the general widening of the green bands
spoken of.

In all the experiments with _Ajax_, if any change at all has been
produced by cold, it is seen in the enlarging or doubling of the red
anal spot, and in the increased number of clear green lunules on the
hind wings. Almost always the frontal hairs are lengthened and the
colour of the wings deepened, and the extent of the black area is also
diminished. All these changes are in the direction of _Telamonides_, or
the winter form.

That the effect of cold is not simply to precipitate the appearance of
the winter form, causing the butterfly to emerge from the chrysalis
in the summer in which it began its larval existence instead of the
succeeding year, is evident from the fact that the butterflies come
forth with the shape of _Marcellus_, although the markings may be of
_Telamonides_ or _Walshii_. And almost always some of the chrysalides,
after having been iced, go over the winter, and then produce
_Telamonides_, as do the hibernating pupæ in their natural state. The
cold appears to have no effect on these individual chrysalides.[59]

With every experiment, however similar the conditions seem to be, and
are intended to be, there is a difference in results; and further
experiments--perhaps many--will be required before the cause of this
is understood. For example, in 1878, the first butterfly emerged on
the fourteenth day after removal from ice, the period being exactly
what it is (at its longest) in the species in nature. Others emerged
at 19-96 days. In 1879, the emergence began on the ninth day, and by
the twelfth day all had come out, except three belated individuals,
which came out at twenty, forty, and fifty-four days. In the last
experiment, either the cold had not fully suspended the changes which
the insect undergoes in the chrysalis, or its action was to hasten them
after the chrysalides were taken from the ice. In the first experiment,
apparently the changes were absolutely suspended as long as the cold
remained.

It might be expected that the application of heat to the hibernating
chrysalides would precipitate the appearance of the summer form, or
change the markings of the butterfly into the summer form, even if the
shape of the wings was not altered; that is, to produce individuals
having the winter shape but the summer markings. But this was not
found to occur. Mr. Edwards has been in the habit for several years of
placing the chrysalides in a warm room, or in the greenhouse, early in
the winter, thus causing the butterflies to emerge in February, instead
of in March and April, as otherwise they would do. The heat in the
house is 19° R. by day, and not less than 3.5° R. by night. But the
winter form of the butterfly invariably emerged, usually _Telamonides_,
occasionally _Walshii_.


EXPERIMENTS WITH PHYCIODES THAROS.

EXP. 1.--In July, 1875, eggs of _P. Tharos_ were obtained on _Aster
Nova-Angliæ_ in the Catskill Mountains, and the young larvæ, when
hatched, taken to Coalburgh, West Virginia. On the journey the larvæ
were fed on various species of _Aster_, which they ate readily. By the
4th of September they had ceased feeding (after having twice moulted),
and slept. Two weeks later part of them were again active, and fed for
a day or two, when they gathered in clusters and moulted for the third
time, then becoming lethargic, each one where it moulted with the cast
skin by its side. The larvæ were then placed in a cellar, where they
remained till February 7th, when those that were alive were transferred
to the leaves of an _Aster_ which had been forced in a greenhouse, and
some commenced to feed the same day. In due time they moulted twice
more, making, in some cases, a total of five moults. On May 5th the
first larva pupated, and its butterfly emerged after thirteen days.
Another emerged on the 30th, after eight days pupal period, this stage
being shortened as the weather became warmer. There emerged altogether
8 butterflies, 5 males and 3 females, all of the form _Marcia_, and
all of the variety designated C, except 1 female, which was var. B.[60]

EXP. 2.--On May 18th the first specimens (3 male _Marcia_) were seen on
the wing at Coalburgh; 1 female was taken on the 19th, 2 on the 23rd,
and 2 on the 24th, these being all that were seen up to that date, but
shortly after both sexes became common. On the 26th, 7 females were
captured and tied up in separate bags on branches of _Aster_. The next
day 6 out of the 7 had laid eggs in clusters containing from 50 to
225 eggs in each. Hundreds of caterpillars were obtained, each brood
being kept separate, and the butterflies began to emerge on June 29th,
the several stages being:--egg six days, larva twenty-two, chrysalis
five. Some of the butterflies did not emerge till the 15th of July.
Just after this date one brood was taken to the Catskills, where they
pupated, and in this state were sent back to Coalburgh. There was no
difference in the length of the different stages of this brood and the
others which had been left at Coalburgh, and none of either lot became
lethargic. The butterflies from these eggs of May were all _Tharos_,
with the exception of 1 female _Marcia_, var. C. Thus the first
generation of _Marcia_ from the hibernating larvæ furnishes a second
generation of _Tharos_.

EXP. 3.--On July 16th, at Coalburgh, eggs were obtained from several
females, all _Tharos_, as no other form was flying. In four days the
eggs hatched; the larval stage was twenty-two, and the pupal stage
seven days; but, as before, many larvæ lingered. The first butterfly
emerged on August 18th. All were _Tharos_, and none of the larvæ had
been lethargic. This was the third generation from the second laying of
eggs.

EXP. 4.--On August 15th, at Coalburgh, eggs were obtained from a female
_Tharos_, and then taken directly to the Catskill Mountains, where they
hatched on the 20th. This was the fourth generation from the third
laying of eggs. In Virginia, and during the journey, the weather had
been exceedingly warm, but on reaching the mountains it was cool, and
at night decidedly cold. September was wet and cold, and the larvæ
were protected in a warm room at night and much of the time by day, as
they will not feed when the temperature is less than about 8° R. The
first pupa was formed September 15th, twenty-six days from the hatching
of the larvæ, and others at different dates up to September 26th, or
thirty-seven days from the egg. Fifty-two larvæ out of 127 became
lethargic after the second moult on September 16th, and on September
26th fully one half of these lethargic larvæ commenced to feed again,
and moulted for the third time, after which they became again lethargic
and remained in this state. The pupæ from this batch were divided into
three portions:--

A. This lot was brought back to Coalburgh on October 15th, the weather
during the journey having been cold with several frosty nights, so
that for a period of thirty days the pupæ had at no time been exposed
to warmth. The butterflies began to emerge on the day of arrival, and
before the end of a week all that were living had come forth, viz., 9
males and 10 females. “Of these 9 males 4 were changed to _Marcia_ var.
C, 3 were var. D, and 2 were not changed at all. Of the 10 females 8
were changed, 5 of them to var. B, 3 to var. C. The other 2 females
were not different from many _Tharos_ of the summer brood, having large
discal patches on under side of hind wing, besides the markings common
to the summer brood.”

B. This lot, consisting of 10 pupæ, was sent from the Catskills to
Albany, New York, where they were kept in a cool place. Between October
21st and Nov. 2nd, 6 butterflies emerged, all females, and all of the
var. B. Of the remaining pupæ 1 died, and 3 were alive on December
27th. According to Mr. Edwards this species never hibernates in the
pupal state in nature. The butterflies of this lot were more completely
changed than were those from the pupæ of lot A.

C. On September 20th 18 of the pupæ were placed in a tin box directly
on the surface of the ice, the temperature of the house being 3°-4° R.
Some were placed in the box within three hours after transformation and
before they had hardened; others within six hours, and others within
nine hours. They were all allowed to remain on the ice for seven days,
that being the longest summer period of the chrysalis. On being removed
they all appeared dead, being still soft, and when they had become
hard they had a shrivelled surface. On being brought to Coalburgh they
showed no signs of life till October 21st, when the weather became
hot (24°-25° R.), and in two days 15 butterflies emerged, “every one
_Marcia_, not a doubtful form among them in either sex.” Of these
butterflies 10 were males and 5 females; of the former 5 were var. C, 4
var. D, and 1 var. B, and of the latter 1 was var. C, and 4 var. D. The
other 3 pupæ died. All the butterflies of this brood were diminutive,
starved by the cold, but those from the ice were sensibly smaller
than the others. All the examples of var. B were more intense in the
colouring of the under surface than any ever seen by Mr. Edwards in
nature, and the single male was as deeply coloured as the females, this
also never occurring in nature.

Mr. Edwards next proceeds to compare the behaviour of the Coalburgh
broods with those of the same species in the Catskills:--

EXP. 5.--On arriving at the Catskills, on June 18th, a few male
_Marcia_, var. D, were seen flying, but no females. This was exactly
one month later than the first males had been seen at Coalburgh. The
first female was taken on June 26th, another on June 27th, and a
third on the 28th, all _Marcia_, var. C. Thus the first female was
thirty-eight days later than the first at Coalburgh. No more females
were seen, and no _Tharos_. The three specimens captured were placed
on _Aster_, where two immediately deposited eggs[61] which were
forwarded to Coalburgh, where they hatched on July 3rd. The first
chrysalis was formed on the 20th, its butterfly emerging on the 29th,
so that the periods were: egg six, larva seventeen, pupa nine days.
Five per cent. of the larvæ became lethargic after the second moult.
This was, therefore, the second generation of the butterfly from the
first laying of eggs. All the butterflies which emerged were _Tharos_,
the dark portions of the wings being intensely black as compared with
the Coalburgh examples, and other differences of colour existed, but
the general peculiarities of the _Tharos_ form were retained. This
second generation was just one month behind the second at Coalburgh,
and since, in 1875, eggs were obtained by Mr. Mead on July 27th and
following days, the larvæ from which all hibernated, this would be
the second laying of eggs, and the resulting butterflies the first
generation of the following season.

Thus in the Catskills the species is digoneutic, the first generation
being _Marcia_ (the winter form), and the second the summer form. A
certain proportion of the larvæ from the first generation hibernate,
and apparently all those from the second.

_Discussion of Results._--There are four generations of this butterfly
at Coalburgh, the first being _Marcia_ and the second and third
_Tharos_. None of the larvæ from these were found to hibernate. The
fourth generation under the exceptional conditions above recorded (Exp.
4) produced some _Tharos_ and more _Marcia_ the same season, a large
proportion of the larvæ also hibernating. Had the larvæ of this brood
been kept at Coalburgh, where the temperature remained high for several
weeks after they had left the egg, the resulting butterflies would have
been all _Tharos_, and the larvæ from their eggs would have hibernated.

The altitude of the Catskills, where Mr. Edwards was, is from 1650
to 2000 feet above high water, and the altitude of Coalburgh is 600
feet. The transference of the larvæ from the Catskills to Virginia
(about 48° lat.) and _vice-versa_, besides the difference of altitude,
had no perceptible influence on the butterflies of the several broods
except the last one, in which the climatic change exerted a direct
influence on part of them both as to form and size. The stages of
the June Catskill brood may have been accelerated to a small extent
by transference to Virginia, but the butterflies reserved their
peculiarities of colour. (See Exp. 5.) So also was the habit of
lethargy retained.[62] The May brood, on the other hand, taken from
Virginia to the Catskills, suffered no retardation of development.
(See Exp. 2.) It might have been expected that all the larvæ of this
last brood taken to the mountains would have become lethargic, but
the majority resisted this change of habit. From all these facts it
may be concluded “that it takes time to naturalize a stranger, and
that habits and tendencies, even in a butterfly, are not to be changed
suddenly.”[63]

It has been shown that _Tharos_ is digoneutic in the Catskills and
polygoneutic in West Virginia, so that at a great altitude, or in a
high latitude, we might expect to find the species monogoneutic and
probably restricted to the winter form _Marcia_. These conditions are
fulfilled in the Island of Anticosti, and on the opposite coast of
Labrador (about lat. 50°), the summer temperature of these districts
being about the same. Mr. Edwards states, on the authority of Mr.
Cooper, who collected in the Island, that _Tharos_ is a rare species
there, but has a wide distribution. No specimens were seen later
than July 29, after which date the weather became cold, and very few
butterflies of any sort were to be seen. It seems probable that none of
the butterflies of Anticosti or Labrador produce a second brood. All
the specimens examined from these districts were of the winter form.

In explanation of the present case Dr. Weismann wrote to Mr.
Edwards:--“_Marcia_ is the old primary form of the species, in the
glacial period the only one. _Tharos_ is the secondary form, having
arisen in the course of many generations through the gradually working
influence of summer heat. In your experiments cold has caused the
summer generation to revert to the primary form. The reversion which
occurred was complete in the females, but not in all the males. This
proves, as it appears to me, that the males are changed or affected
more strongly by the heat of summer than the females. The secondary
form has a stronger constitution in the males than in the females. As
I read your letter, it at once occurred to me whether in the spring
there would not appear some males which were not pure _Marcia_, but
were of the summer form, or nearly resembling it; and when I reached
the conclusion of the letter I found that you especially mentioned that
this was so! And I was reminded that the same thing is observable in
_A. Levana_, though in a less striking degree. If we treated the summer
brood of _Levana_ with ice, many more females than males would revert
to the winter form. This sex is more conservative than the male--slower
to change.”

The extreme variability of _P. Tharos_ was alluded to more than a
century ago by Drury, who stated:--“In short, Nature forms such a
variety of this species that it is difficult to set bounds, or to know
all that belongs to it.” Of the different named varieties, according
to Mr. Edwards, “A appears to be an offset of B, in the direction
most remote from the summer form, just as in _Papilio Ajax_, the var.
_Walshii_ is on the further side of _Telamonides_, remote from the
summer form _Marcellus_.” Var. C leads from B through D directly to the
summer form, whilst A is more unlike this last variety than are several
distinct species of the genus, and would not be suspected to possess
any close relationship were it not for the intermediate forms. The
var. B is regarded as nearest to the primitive type for the following
reasons:--In the first place it is the commonest form, predominating
over all the other varieties in W. Virginia, and moreover appears
constantly in the butterflies from pupæ submitted to refrigeration.
Its distinctive peculiarity of colour occurs in the allied species
_P. Phaon_ (Gulf States) and _P. Vesta_ (Texas), both of which are
seasonally dimorphic, and both apparently restricted in their winter
broods to the form corresponding to B of _Tharos_. In their summer
generation both these species closely resemble the summer form of
_Tharos_, and it is remarkable that these two species, which are the
only ones (with the exception of the doubtful _Batesii_) closely allied
to _Tharos_, should alone be seasonally dimorphic out of the large
number of species in the genus.

Mr. Edwards thus explains the case under consideration:--“When _Phaon_,
_Vesta_, and _Tharos_ were as yet only varieties of one species, the
sole coloration was that now common to the three. As they gradually
became permanent, or in other words, as these varieties became species,
_Tharos_ was giving rise to several sub-varieties, some of them in time
to become distinct and well marked, while the other two, _Phaon_ and
_Vesta_, remained constant. As the climate moderated and the summer
became longer, each species came to have a summer generation; and in
these the resemblance of blood-relationship is still manifest. As the
winter generations of each species had been much alike, so the summer
generations which sprung from them were much alike. And if we consider
the metropolis of the species _Tharos_, or perhaps of its parent
species, at the time when it had but one annual generation, to have
been somewhere between 37° and 40° on the Atlantic slope, and within
which limits all the varieties and sub-varieties of both winter and
summer forms of _Tharos_ are now found in amazing luxuriance, we can
see how it is possible, as the glacial cold receded, that only part of
the varieties of the winter form might spread to the northward, and but
one of them at last reach the sub-boreal regions and hold possession to
this day as the sole representative of the species. And at a very early
period the primary form, together with _Phaon_ and _Vesta_, had made
its way southward, where all three are found now.”


EXPERIMENTS WITH GRAPTA INTERROGATIONIS.[64]

[_Communicated by_ Mr. W. H. EDWARDS, _November 15th, 1879_.]

The experiments with this species were made in June, 1879, on pupæ
from eggs laid by the summer form _Umbrosa_ of the second brood of the
year, and obtained by confining a female in a bag on a stem of hop.
As the pupæ formed, and at intervals of from six to twenty-four hours
after pupation (by which time all the older ones had fully hardened),
they were placed in the ice-box. In making this experiment Mr. Edwards
had three objects in view. 1st. To see whether it was essential that
the exposure should take place immediately after pupation, in order to
effect any change. 2ndly. To see how short a period would suffice to
bring about any change. 3rdly. Whether exposing the summer pupæ would
bring about a change in the form of the resulting butterfly. Inasmuch
as breeding from the egg of _Umbrosa_, in June, in a former year,[64]
gave both _Umbrosa_ (11) and _Fabricii_ (6), the butterflies from the
eggs obtained, if left to nature, might be expected to be of both
forms. The last or fourth brood of the year having been found up to the
present time to be _Fabricii_, and the 1st brood of the spring, raised
from eggs of _Fabricii_ (laid in confinement), having been found to be
wholly _Umbrosa_, the latter is probably the summer and _Fabricii_ the
winter form. The two intervening broods, _i.e._ the 2nd and 3rd, have
yielded both forms. This species hibernates in the imago state.

After the pupæ had been in the ice-box fourteen days they were all
removed but 5, which were left in six days longer. Several were dead at
the end of fourteen days. The temperature most of the time was 1°-2°
R.; but for a few hours each day rose as the ice melted, and was found
to be 3°-6° R.

From the fourteen-day lot 7 butterflies were obtained, 3 males and
4 females. From the twenty-day lot 4 males and 1 female; every one
_Umbrosa_. All had changed in one striking particular. In the normal
_Umbrosa_ of both sexes,[65] the fore wings have on the upper side on
the costal margin next inside the hind marginal border, and separated
from it by a considerable fulvous space, a dark patch which ends a
little below the discoidal nervule; inside the same border at the inner
angle is another dark patch lying on the submedian interspace. Between
these two patches, across all the median interspaces, the ground-colour
is fulvous, very slightly clouded with dark.

In all the 4 females exposed to cold for fourteen days a broad black
band appeared crossing the whole wing, continuous, of uniform shade,
covering the two patches, and almost confluent from end to end with the
marginal border, only a streak of obscure fulvous anywhere separating
the two. In the case of the females from pupæ exposed for twenty days,
the band was present, but while broad, and covering the space between
the patches, it was not so dark as in the other females, and included
against the border a series of obscure fulvous lunules. This is just
like many normal females, and this butterfly was essentially unchanged.

In all the males the patches were diffuse, that at the apex almost
coalescing with the border. In the 3 from chrysalides exposed fourteen
days these patches were connected by a narrow dark band (very different
from the broad band of the females), occupying the same position as
the clouding of the normal male, but blackened and somewhat diffused.
In the 4 examples from the twenty-day pupæ, this connecting band was
scarcely as deeply coloured and continuous as in the other 3. Beyond
this change on the submarginal area, whereby a band is created where
naturally would be only the two patches, and a slight clouding of the
intervening fulvous surfaces, there was no difference of the upper
surface apparent between these examples of both sexes, and a long
series of natural ones placed beside them.

On the under side all the males were of one type, the colours being
very intense. There was considerably more red, both dark and pale, over
the whole surface, than in a series of natural examples in which shades
of brown and a bluish hue predominate. No change was observed in the
females on the under side.

It appears that fourteen days were as effective in producing changes
as a longer period. In fact, the most decided changes were found in
the females exposed the shorter period. It also appears that with
this species cold will produce change if applied after the chrysalis
has hardened. The same experiments were attempted in 1878 with pupæ
of _Grapta Comma_. They were put on ice at from ten minutes to six
hours after forming, and subjected to a temperature of about 0°-1° R.
for eighteen to twenty days, but every pupa was killed. Chrysalides
of _Papilio Ajax_ in the same box, and partly exposed very soon after
pupation, were not injured. It was for this reason that none of the
_Interrogationis_ pupæ were placed in the box till six hours had passed.

It appears further that cold may change the markings on one part of the
wing only, and in cases where it does change dark or dusky markings
melanises them; or it may deepen the colours of the under surface (as
in the females of the present experiment). The females in the above
experiment were apparently most susceptible to the cold, the most
decided changes having been effected in them.

The resulting butterflies were all of one form, although both might
have been expected to appear under natural circumstances.

_Dr. Weismann’s remarks on the foregoing experiments._--The author
of the present work has, at my request, been good enough to furnish
the following remarks upon Mr. Edward’s experiments with _G.
Interrogationis_:--

The interesting experiments of Mr. Edwards are here principally
introduced because they show how many weighty questions in connexion
with seasonal dimorphism still remain to be solved. The present
experiments do not offer a _direct_ but, at most, only an _indirect_
proof of the truth of my theory, since they show that the explanation
opposed to mine is also in this case inadmissible. Thus we have here,
as with _Papilio Ajax_, two out of the four annual generations mixed,
_i.e._, consisting of summer and winter forms, and the conclusion is
inevitable that these forms were not produced by the _gradual_ action
of heat or cold. When, from pupæ of the same generation which are
developed under precisely the same external conditions, both forms of
the butterfly are produced, the cause of their diversity cannot lie
in these conditions. It must rather depend on causes innate in the
organism itself, _i.e._, on inherited duplicating tendencies which
meet in the same generation, and to a certain extent contend with
each other for precedence. The two forms must have had their origin
in earlier generations, and there is nothing against the view that
they have arisen through the gradual augmentation of the influences of
temperature.

In another sense, however, one might perceive, in the facts discovered
by Edwards, an objection to my theory.

By the action of cold the form _Umbrosa_, which flies in June, was
produced. Now we should be inclined to regard the var. _Umbrosa_ as
the summer form, and the var. _Fabricii_, which emerges in the autumn,
hibernates in the imago state, and lays eggs in the spring, as the
winter form. It would then be incomprehensible why the var. _Umbrosa_
(_i.e._, the summer form) should be produced by cold.

But it is quite as possible that the var. _Umbrosa_ as that the var.
_Fabricii_ is the winter form. We must not forget that, in this
species, _not one of the four annual generations is exposed to the cold
of winter in the pupal state_. When, therefore, we have in such cases
seasonal dimorphism, to which complete certainty can only be given by
continued observations of this butterfly, which does not occur very
commonly in Virginia, this must depend on the fact that the species
formerly hibernated in the pupal stage. This question now arises, which
of the existing generations was formerly the hibernating one--the first
or the last?

Either may have done so _à priori_, according as the summer was
formerly shorter or longer than now for this species. If the former
were the case, the var. _Fabricii_ is the older winter form; were the
latter the case, the var. _Umbrosa_ is the original winter form, as
shall now be more closely established.

Should the experiments which Mr. Edwards has performed in the course of
his interesting investigations be repeated in future with always the
same results, I should be inclined to explain the case as follows:--

It is not the var. _Fabricii_, but _Umbrosa_, which is the winter
generation. By the northward migration of the species and the relative
shortening of the summer, this winter generation would be pushed
forward into the summer, and would thereby lose only a portion of the
winter characters which it had till that time possessed. The last of
the four generations which occurs in Virginia is extremely rare, so
that it must be regarded either as one of the generations now supposed
to be originating, or as one now supposed to be disappearing. The
latter may be admitted. Somewhat further north this generation would
be entirely suppressed, and the third brood would hibernate, either
in the imago state or as pupæ or caterpillars. In Virginia this third
generation consists of both forms. We may expect that further north, at
least, where it hibernates as pupæ, it will consist entirely, or almost
entirely, of the var. _Umbrosa_. Still further north in the Catskill
Mountains, as Edwards states from his own observations, the species
has only two generations, and one might expect that the var. _Umbrosa_
would there occur as the winter generation.

Should the foregoing be correct, then the fact that the second
generation assumes the _Umbrosa_ form by the action of cold, as was
the case in Edward’s experiments, becomes explicable. The new marking
peculiar to this form produced by this means must be regarded as a
complete reversion to the true winter form, the characters of which are
becoming partly lost as this generation is no longer exposed to the
influence of winter, but has become advanced to the beginning of summer.

_The foregoing explanation is, of course, purely hypothetical_;
it cannot at present be asserted that it is the correct one. Many
investigations based on a sufficiently large number of facts are still
necessary to be able to attempt to explain this complicated case with
any certainty. Neither should I have ventured to offer any opinion on
the present case, did I not believe that even such a premature and
entirely uncertain explanation may always be of use in serving the
inventive principle, _i.e._, in pointing out the way in which the truth
must be sought.

As far as I know, no attempt has yet been made to prove from a general
point of view the interpolation of new generations, or the omission
of single generations from the annual cycle, with respect to causes
and effects. An investigation of this kind would be of the greatest
importance, not only for seasonal dimorphism, but also for the
elucidation of questions of a much more general nature, and would be a
most satisfactory problem for the scientific entomologist. I may here
be permitted to develope in a purely theoretical manner the principles
in accordance with which such an investigation should be made:--

_On the change in the number of generations of the annual cycle._--A
change in the number of generations which a species produces annually
must be sought chiefly in changes of climate, and therefore in a
lengthening or shortening of the period of warmth, or in an increase or
diminution of warmth within this period; or, finally, in both changes
conjointly. The last case would be of the most frequent occurrence,
since a lengthening of the period of warmth is, as a rule, correlated
with an elevation of the mean temperature of this period, and _vice
versâ_. Of other complications I can here perceive the following:--

Climatic changes may be _active_ or _passive_, _i.e._, they occur by a
change of climate or by a migration and extension of the species over
new districts having another climate.

By a lengthening of the summer, as I shall designate the shorter
portion of the whole annual period of warmth, the last generation of
the year would be advanced further in its development than before;
if, for instance, it formerly hibernated in the pupal state, it would
now pass the winter in the imago stage. Should a further lengthening
of the summer occur, the butterflies might emerge soon enough to lay
eggs in the autumn, and by a still greater lengthening the eggs also
might hatch, the larvæ grow up and hibernate as pupæ. In this manner
we should have a new generation interpolated, owing to the generation
which formerly hibernated being made to recede step by step towards the
autumn and the summer. _By a lengthening of the summer there occurs
therefore a retrogressive interruption of generations._

The exact opposite occurs if the summer should become shortened. In
this case the last generation would no longer be so far developed
as formerly; for instance, it might not reach the pupal stage, as
formerly, at the beginning of winter, and would thus hibernate in
a younger stage, either as egg or larvæ. Finally, by a continual
shortening of the summer it would no longer appear at the end of
this period but in the following spring; in other words, it would be
eliminated. _By a shortening of the summer accordingly the interruption
of generations occurs by advancement._

The following considerations, which submit themselves with reference to
seasonal dimorphism, are readily conceivable, at least, in so far as
they can be arrived at by purely theoretical methods. Were the summer
to become shorter the generation which formerly hibernated in the pupal
stage would be advanced further into the spring. It would not thereby
necessarily immediately lose the winter characters which it formerly
possessed. Whether this would happen, and to what extent, would rather
depend upon the intensity of the action of the summer climate on the
generation in question, and on the number of generations which have
been submitted to this action. Hitherto no attempts have been made to
expose a monomorphic species to an elevated temperature throughout
several generations, so as to obtain an approximate measure of the
rapidity with which such climatic influences can bring about changes.
For this reason we must for the present refrain from all hypothesis
relating to this subject.

The disturbance of generations by the shortening of summer might also
occur to a species in such a manner that the generation which formerly
hibernated advances into the spring, the last of the summer generations
at the same time reaching the beginning of winter. The latter would
then hibernate in the pupal state, and would sooner or later also
assume the winter form through the action of the cold of winter. We
should, in this case, have two generations possessing more or less
completely the winter form, the ancient winter generation now gradually
losing the winter characters, and the new winter generation gradually
acquiring these characters.

In the reverse case, _i.e._, by a lengthening of the summer, we
should have the same possibilities only with the difference that the
disturbance of generations would occur in a reverse direction. In this
case it might happen that the former winter generation would become the
autumnal brood, and more or less preserve its characters for a long
period. Here also a new winter generation would be produced as soon as
the former spring brood had so far retrograded that its pupæ hibernated.

I am only too conscious how entirely theoretical are these conjectures.
It is very possible that observation of nature will render numerous
corrections necessary. For instance, I have assumed that every species
is able, when necessary, to adapt any one of its developmental stages
to hibernation. Whether this is actually the case must be learnt from
further researches; at present we only know that many species hibernate
in the egg stage, others in the larval state, others as pupæ, and yet
others in the perfect state. We know also that many species hibernate
in several stages at the same time, but we do not know whether each
stage of every species has an equal power of accommodation to cold.
Should this not be the case the above conjectures would have to be
considerably modified. To take up this subject, so as to completely
master all the facts connected therewith, naturalists would have to
devote their whole time and energy to the order Lepidoptera, which I
have been unable to do.

From the considerations offered, it thus appears that the phenomena
of seasonal dimorphism may depend on extremely complex processes, so
that one need not be surprised if only a few cases now admit of certain
analysis. We must also admit, however, that it is more advantageous to
science to be able in the first place to analyze the simplest cases by
means of breeding experiments, than to concern oneself in guessing at
cases which are so complicated as to make it impossible at present to
procure all the materials necessary for their solution.

[Illustration:

  Plate I.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

[Illustration:

  Plate II.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]



EXPLANATION OF THE PLATES.


PLATE I.

Fig. 1. Male _Araschnia Levana_, winter form.

Fig. 2. Female _A. Levana_, winter form.

Fig. 3. Male _A. Levana_, artificially bred intermediate form
(so-called _Porima_).

Fig. 4. Female _A. Levana_, intermediate form (_Porima_), artificially
bred from the summer generation, agreeing perfectly in marking with
the winter form, and only to be distinguished from it by the somewhat
darker ground colour.

Fig. 5. Male _A. Levana_, summer form (_Prorsa_).

Fig. 6. Female _A. Levana_, summer form (_Prorsa_).

Figs. 7 to 9. Intermediate forms (_Porima_), artificially bred from the
first summer generation.

Figs. 10 and 11. Male and female _Pieris Napi_, winter form,
artificially bred from the summer generation; the yellow ground-colour
of the underside of the hind wings brighter than in the natural winter
form.

Figs. 12 and 13. Male and female _Pieris Napi_, summer form.

Figs. 14 and 15. _Pieris Napi_, var. _Bryoniæ_, male and female reared
from eggs.


PLATE II.

Fig. 16. _Papilio Ajax_, var. _Telamonides_, winter form.

Fig. 17. _P. Ajax_, var. _Marcellus_, summer form.

Fig. 18. _Plebeius Agestis_ (_Alexis_, Scop.), German winter form.

Fig. 19. _P. Agestis_ (_Alexis_, Scop.), German summer form.

Fig. 20. _P. Agestis_ (_Alexis_, Scop.), Italian summer form. (The
chief difference between figs. 19 and 20 lies on the under-side, which
could not be here represented.)

Fig. 21. _Polyommatus Phlæas_, winter form, from Sardinia; the German
winter and summer generations are perfectly similar.

Fig. 22. _P. Phlæas_, summer form, from Genoa.

Fig. 23. _Pararga Ægeria_, from Freiburg, Baden.

Fig. 24. _P. Meione_, southern climatic form of _Ægeria_ from Sardinia.


END OF PART I.



STUDIES IN THE THEORY OF DESCENT.



Part II.

ON THE FINAL CAUSES OF TRANSFORMATION.



=I.=

THE ORIGIN OF THE MARKINGS OF CATERPILLARS.



INTRODUCTION.


The general idea which has instigated the researches described in the
present essay has already been expressed in the Preface, where it has
also been explained why the markings of caterpillars, and especially
those of the Sphinx-larvæ, were chosen for testing this idea.

The task presented itself in the following form:--In order to test
the idea referred to, it must be investigated whether all the forms
of marking which occur in the Sphinx-larvæ can or cannot be traced to
known transforming factors.

That natural selection produces a large number of characters can
be as little doubted as that many varying external influences can
bring about changes in an organism by direct action. That these two
transforming factors, together with their correlatively induced
changes, are competent to produce _all_ characters, howsoever
insignificant, has indeed been truly asserted, but has never yet
been proved. The solution of the problem, however, appeared to me to
depend particularly on this point. We are now no longer concerned in
proving that a changing environment reacts upon the organism--this
has already been shown--but we have to deal with the question whether
_every change_ is the result of the action of the environment upon the
organism. Were it possible to trace all the forms of markings which
occur, to one of the known factors of species transformation, it could
be thus shown that here at least an “innate power of development” was
of no effect; were this not possible, _i.e._ did there remain residual
markings which could not be explained, then the notion of an “innate
principle of development” could not be at once entirely discountenanced.

The attempt to solve this problem should commence by the acquisition
of a morphological groundwork, so that the phyletic development of
the markings might by this means be represented as far as possible.
It cannot be stated with certainty, _primâ facie_, whether some form
of development conformable to law is here to be found, but it soon
becomes manifest that such is certainly the case in a great measure.
In all species the young caterpillars are differently marked to
the adults, and in many the markings change with each of the five
stages of growth indicated by the four ecdyses, this gradational
transformation of the markings being a “development” in the true sense
of the word, _i.e._, an origination of the complex from the simple,
the development of characters from those previously in existence,
and never an inconstant, unconnected series of _per saltum_ changes.
This development of the markings in individuals very well reveals
their phyletic development, since there can be no doubt but that we
have here preserved to us in the ontogeny, as I shall establish more
fully further on, a very slightly altered picture of the phyletic
development. The latter can have been but slightly “falsified” in these
cases, although it is indeed considerably abbreviated, and that in very
different degrees; to the greatest extent in those species which are
most advanced in their phyletic development, and to the least extent
in those which are less advanced. From this the value of being able to
compare a large number of species with respect to their ontogeny will
appear. Unfortunately, however, this has only been possible to a very
limited extent.

The youngest larval stages are those which are of the most importance
for revealing the phyletic development, because they make us
acquainted with the markings of the progenitors of the existing
species. For these investigations it is therefore in the first place
necessary to obtain fertile eggs. Female _Sphingidæ_, however, do not
generally lay eggs in confinement,[66] or at most only a very small
number. In the case of many species (_Deilephila Galii_, _D. Lineata_,
_D. Vespertilio_, _D. Hippophaës_) I have for this reason unfortunately
been unable to observe the entire development, and such observations
would in all probability have given especially valuable information.

I was certainly successful in finding the young larvæ of some of the
above as well as of other species on their food-plants, but even in
the most favourable instances only individuals of the second stage
and generally older. When, however, notwithstanding this imperfection
of the materials, and in spite of the important gaps thus inevitably
caused in these series of observations, it has nevertheless been
possible to form a picture, on the whole tolerably complete, of the
phyletic development of the Sphinx-markings, this well indicates what
a fertile field is offered by the investigation of this subject, and
will, I trust, furnish an inducement to others, not only to fill up the
various gaps in the small family of the _Sphingidæ_, but also to treat
other Lepidopterous families in a similar manner. Such an investigation
of the _Papilionidæ_ appears to me to be especially desirable; not only
of the few European but also of the American and Indian species. We
know practically nothing, of the youngest stages of the _Papilio_ larvæ
from this point of view. No entomological work gives any description of
the form and marking of the newly hatched larvæ, even in the case of
our commonest species (_Papilio Machaon_ and _P. Podalirius_), and I
believe that I do not go too far when I assert that up to the present
time nobody has observed them at this early stage.[67] When, however,
we consider that in these young caterpillars we have preserved to us
the parent-form, extinct for centuries, of the existing species of
_Papilio_, it must assuredly be of the greatest interest to become
accurately acquainted with them, to compare them with the earliest
stages of allied species, and to follow the gradual divergence of the
succeeding stages in different directions, thus forming a picture of
the phyletic development of an evolving group. In the course of such
observations numerous collateral results would doubtless come out.
Investigations of this kind, whether conducted on this or on any other
group, would, above all, show the true systematic affinities of the
forms, _i.e._, their _genealogical_ affinities, and that in a better
way than could be shown by the morphology of the perfect insects or
the adult caterpillars alone. If I am diffident in founding these
conclusions upon the development of the Sphinx-markings treated of
in the present essay, this arises entirely from a knowledge of the
imperfections in the basis of facts. If however, through the united
labours of many investigators, the individual development of all the
species of _Sphingidæ_ now existing should at some future period be
clearly laid before us, we should then not only have arrived at a
knowledge of the relative ages of the different species, genera and
families, but we should also arrive at an explanation of the nature of
their affinities.

It is erroneous to assert that Classification has only to take
form-relationship into consideration; that it should and can be
nothing else than the expression of form-relationship. The latter
is certainly our only measure of blood-relationship, but those who
maintain the assertion that form- and blood-relationship are by no
means always synonymous, are undoubtedly correct. I shall in a future
essay adduce facts which leave no doubt on this point, and which prove
at the same time that modern systematists--especially in the order
Lepidoptera--have always endeavoured--although quite unconsciously--to
make the blood-relationship the basis of their classification. For
this reason alone, larvæ and pupæ would have an important bearing upon
the establishment of systematic groups, although certainly in a manner
frequently irregular.

It must be admitted that so long as we are able to compare the species
of one group with those of another _in one form only_, we are often
unable to ascertain the blood-relationship.[68] In such cases we can
only determine the latter from the form-relationship, and as these
are not always parallel, any conclusion based on a single form must
be very unsound. If, for instance, butterflies emerged from the egg
directly, without passing through any larval stage, a comparison of
their resemblances of form would alone be of systematic value; we
should unite them into groups on the ground of these resemblances
only, and the formation of these groups would then much depend upon
the weight assigned to this or that character. We might thus fall into
error, not only through a different valuation of characters but still
more because two species of near blood-relationship frequently differ
from one another in form to a greater extent than from other species.
We should have no warrant that our conception of the form-relationship
expressed the genealogical connection of the species. But it would
be quite different if every species presented itself in two or three
different forms. If in two species or genera the butterflies as well
as the larvæ and pupæ exhibited the same degree of form-relationship,
the probability that this expressed also the blood-relationship would
then be exceedingly great. Now this agreement certainly does not always
occur, and when these different stages are related in form in unequal
degrees, the problem then is to decide which of these relationships
expresses the genealogy. This decision may be difficult to arrive at
in single cases, since the caterpillar may diverge in form from the
next blood-related species to a greater extent than the butterfly, or,
conversely, the butterfly may diverge more widely from its nearest
blood-related species than the caterpillar.

For such cases there remains the developmental history of the
caterpillar, which will almost always furnish us to a certain extent
with information respecting the true genealogical relationship of the
forms, because it always reveals a portion of the phyletic (ancestral)
development of the species. If we see two species of butterflies
quite dissimilar in form of wing and other characters, we should be
inclined, in spite of many points of agreement between them, to place
them in entirely different genera. But should we then find that not
only did their adult larvæ agree in every detail of marking, but also
that the entire phyletic development of these markings, as revealed
by the ontogeny of the larvæ, had taken precisely the same course in
both species, we should certainly conclude that they possessed a near
blood-relationship, and should place them close together in the same
genus. Such an instance is afforded by the two Hawk-moths, _Chærocampa
Elpenor_ and _C. Porcellus_, as will appear in the course of these
investigations. These two species were placed by Walker in different
genera, the form-relationship of the imagines being thus correctly
represented, since _Porcellus_ (imago), is indeed more nearly related
in form to the species of the genus _Pergesa_, Walker, than to those
of the genus _Chærocampa_.[69] Nevertheless, these species must remain
in the same genus, as no other arrangement expresses their degree of
blood-relationship.

An intimate knowledge of the development-stages of caterpillars thus
offers, even from a systematic point of view, an invaluable means of
judging the degree of blood-relationship, and from this standpoint we
must regard the study of the caterpillar as of more importance than
that of the perfect insect. Certainly all groups would not be so rich
in information as the _Sphingidæ_, or, as I am inclined to believe, the
_Papilionidæ_, since all families of caterpillars do not possess such a
marked and diversified pattern, nor do they present such a varied and
characteristic bodily form. The representation of the true, _i.e._, the
blood-relationship, and through this the formation of natural groups
with any completeness, can certainly only be looked for when we are
intimately acquainted with the different stages of development of the
larvæ of numerous species in every group, from their emergence from
the egg to their period of pupation. The genealogical relationship of
many forms at present of doubtful systematic position would then be
made clear. This investigation, however, could not be the work of a
single individual; not only because the materials for observation are
too great, but, above all, because they are spread over too wide a
field. It is not sufficient to study the European types only--we should
endeavour to learn as much as possible of the Lepidoptera of the whole
world. But such observations can only be made on the spot. Why should
it not be possible to trace the development from the egg, even under
a tropical sky, and to devote to breeding and observing, a portion of
that time which is generally spent in mere collecting? I may perhaps be
able to convince some of the many excellent and careful observers among
entomologists, that beyond the necessary and valuable search for new
forms, there is another field which may be successfully worked, viz.,
the precise investigation of the development of known species.

The first portion of the present essay consists of the determination
of this development in those species of _Sphingidæ_ which have been
accessible to me. Seven genera are successively treated of, some
completely, and others only in some of their stages; and thus I have
sought to present a picture of the course of development of the
markings in each genus, by comparing the species with each other, and
with allied forms in cases where the young stages were unknown. In
this portion, as far as possible, the facts only have been given, the
working up of the latter into general conclusions upon the development
of marking being reserved for the second portion. A complete separation
of facts from generalizations could not, however, be carried out; it
appeared convenient to close the account of each genus with a summary
of the results obtained from the various species.

After having established that the markings of the Sphinx-caterpillars
had undergone an extremely gradual phyletic development, conformable to
law, in certain fixed directions, it appeared desirable to investigate
the causes of the first appearance of these markings, as well as
of their subsequent development. The question as to the biological
significance of marking here presented itself in the first place
for solution, and the third section is devoted to this subject. If
it is maintained that marking is of no importance to the life of
the insect, or that it is so only exceptionally, and that it is in
reality, as it appears to be, a character of purely morphological,
_i.e._, physiological, insignificance, then its striking phylogenetic
development conformable to law cannot be explained by any of the
known factors of species transformation, and we should have to
assume the action of an innate transforming power. In the present
investigations, this subject in particular has been extensively treated
of, and not only the markings of Sphinx-caterpillars, but also those
of caterpillars in general, have been taken into consideration. The
results arrived at are indeed quite opposed to this assumption--marking
is shown to be a character of extreme importance to the life of the
species, and the admission of a phyletic vital force must, at least
from the present point of view, be excluded. This leads to the fifth
section, in which I have attempted to test certain objections to the
admission of a “phyletic vital force.” The sixth section finally gives
a summary of the results obtained.

I may now add a few explanations which are necessary for understanding
the subsequent descriptions. It was found impossible to avoid the
introduction of some new technicalities for describing the various
elements of larval markings, especially as the latter had to be
treated of scientifically. I have therefore chosen the simplest and
most obvious designations, all of which have already been employed by
various authors, but not in any rigorously defined sense. I understand
by the “dorsal line” that which runs down the middle of the back; the
lines above and below the spiracles will be respectively distinguished
as the “supra-” and “infra-spiracular” lines, and the line between the
dorsal and spiracular as the “subdorsal line.” The distinction between
“ring-spots” and “eye-spots” will be made manifest in the course of the
investigation. A glance at any of the existing descriptions of larvæ
will show how necessary it was to introduce a precise terminology.
Even when the latter is exact as far as it goes, the want of precise
expressions not only makes the descriptions unnecessarily long, but it
also considerably increases the difficulty of comparing one species
with another, since we can never be sure whether the same designation
applies to the same homologous character. For instance, when the
larva of _Chærocampa Elpenor_ is said to have “a light longitudinal
line on the sides of the thoracic segments,” this statement is indeed
correct; but it is not apparent whether the line is above or below,
and consequently it does not appear whether it is the equivalent
of the longitudinal line “on the sides” of the segments in other
species. If, however, it is said that this line is “_subdorsal_ on
the thoracic segments, and on the eleventh abdominal segment,” it is
thereby indicated that we have here a residue of the same marking
which is found completely developed in many other Sphinx-larvæ,
and indeed in the young stages of this same species. The mode of
describing caterpillars hitherto in vogue is in fact unscientific;
the descriptions have not been made with a view to determining the
_morphology_ of the larvæ, but simply to meet the practical want of
being able to readily identify any species that may be found: even for
this purpose, however, it would have been better to have employed a
more precise mode of description.



I.

ONTOGENY AND MORPHOLOGY OF SPHINX-MARKINGS.


THE GENUS CHÆROCAMPA, DUPONCHEL.

Although by no means in favour of the excessive subdivision of genera,
I am of opinion that Ochsenheimer’s genus _Deilephila_ has been
correctly separated by Duponchel into the two genera _Chærocampa_ and
_Deilephila_, _sensû strictiori_. Such a division may appear but little
necessary if we examine the perfect insects only; but the developmental
history of the caterpillars shows that there is a wide division between
the two groups of species, these groups however being branches of one
stem.


CHÆROCAMPA ELPENOR, LINN.

Some captured females laid single eggs sparsely on grass, wood,
and especially on the tarlatan with which the breeding-cage was
covered. The eggs are nearly spherical, but somewhat compressed, of a
grass-green colour, a little lighter, and somewhat larger (1.2 millim.)
than those of _Deilephila Euphorbiæ_. During the development of the
embryo the eggs first became yellowish-green, and finally yellowish.


_First Stage._

The young caterpillars are four millimeters in length, and immediately
after hatching are not green, but of a yellowish-white opalescent
colour, the large and somewhat curved caudal horn being black. The
caterpillars were so transparent that under a low magnifying power the
nervous, tracheal, and alimentary systems could be beautifully seen.
As soon as the larvæ began to feed (on _Epilobium parviflorum_) they
became green in consequence of the food appearing through the skin, but
the latter also gradually acquired a dark green colour (Pl. IV., Fig.
17). All the specimens (some twenty in number) were exactly alike, and
showed _no trace of marking_.


_Second Stage._

The first ecdysis occurred after 5-6 days, the length of the
caterpillars being from nine to ten millimeters. After this first moult
they appeared of a shining green, the horn, which was black during the
first stage, becoming a little red at the base, while a fine white
subdorsal line extended from the horn to the head (Fig. 18). The head
and legs were green; the divisions between the segments appeared as
fine light rings, and the entire upper surface of the segments was also
crossed by fine transverse rings, as was also the case in the first
stage.

At the beginning of the present stage no trace of the eye-spots could
be detected; but a few days after the first moult it was observed that
the white subdorsal line was no longer straight on the fourth and fifth
segments, but had become curved upwards into two small crescents. The
latter soon stood out more strongly, owing to the filling up of their
concavities with darker green. These are the first rudiments of the
eye-spots (Figs. 19 and 30). A very fine white line now connected the
spiracles (infra-spiracular line), and could be traced from the last
segment to the head. This line takes no further part in the subsequent
development of the markings, but disappears in the following stage. The
blood-red colour of the base of the black caudal horn is retained till
the fifth stage, and then also disappears.

Before the second moult, which occurs after another period of 5-6
days, the caterpillars, which were about 1.3 centimeters in length,
had assumed their characteristic tapering, slug-like form. I did not
notice that the larvæ at this stage possessed the power of withdrawing
the three foremost segments into the two succeeding ones, as is so
frequently to be observed in the adults; neither were these two
segments so strikingly enlarged as they are at an earlier period.


_Third Stage._

After the second ecdysis the marking and colouring only undergo change
with respect to the eye-spots. The concavities of the crescent-shaped
portions of the subdorsal line become black,[70] the remainder of this
line at the same time losing much of its whiteness, and thus becoming
less distinct, whilst the crescents assume the appearance of small
eye-spots (Fig. 20). During this stage the curved, crescent-formed
portions become prepared for complete separation from the remainder
of the subdorsal line; and just before the third moult the eye-spots
become sharply defined both in front and behind, whilst the black
ground-colour curves upwards, and the white spots gradually become
lenticular and commence to enlarge (Fig. 21).


_Fourth Stage._

The third moult takes place after another interval of 5-6 days, the
eye-spots then becoming very prominent. The white nucleus of the front
spot is kidney-shaped, and that of the hind spot egg-shaped; whilst
the black ground-colour extends as a slender border upwards along the
sides of the spots, but does not completely surround them till towards
the end of the present stage (Fig. 21). The central portion of the
white spots at the same time becomes of a peculiar violet-brown colour
inclining to yellow above, the peripheral region alone remaining pure
white.

Of the subdorsal line only traces are now to be recognized, and these
are retained, with almost unchanged intensity, sometimes into the last
stage, remaining with the greatest persistence on the three front and
on the penultimate segments, whilst on those containing the eye-spots,
_i.e._, the fourth and fifth, not a trace remains. At the present stage
the peculiar mingling of colours becomes apparent over the whole of
the upper surface; the green is no longer uniform, but a mixture of
short and gently sinuous, dark green striations on a lighter ground
now appear. On the sides of the caterpillar these stripes, which are
at first indistinct, but become more strongly pronounced in the next
stage, are arranged obliquely on the spiracles, with the lower portions
directed forwards.


_Fifth Stage._

The fourth moult occurs 7-8 days after the third, the caterpillar being
4-5 centimeters in length. Whilst all the specimens hitherto observed
were with one exception light green, they now mostly changed their
colour and became dark brown. In one case only did the brown colour
appear in the previous (fourth) stage. The striations previously
mentioned appear as dull and interrupted dirty yellow streaks, the same
dirty yellow colour showing itself continuously on the sides of the
four front segments. Of the subdorsal line only a distinct trace is
now to be seen on the eleventh and on the three front segments, whilst
on the third segment the formation of another eye-spot commences to
be plainly perceptible by a local deposition of black (Fig. 23). This
third spot does not, however, become completely developed, either in
this or in the last stage, but the subdorsal line remains continuous
on the three front segments. Among other changes at this stage, there
occurs a considerable shortening of the caudal horn, which at the same
time loses its beautiful black and red colours and becomes brownish.

The two large eye-spots have now nearly attained complete development.
The kidney-shaped white spot has become entirely surrounded by black;
and on the brown, red, and yellow tints present in this spot during the
last stage, a nearly black spot has been developed--the pupil of the
eye (Fig. 33). In order to establish a definite terminology for the
different portions of the eye-spot, I shall designate the pupil as the
“nucleus,” the light ground on which the pupil stands as the “mirror,”
and the black ground which surrounds the mirror as the “ground-area.”

In this fifth stage the larva attains a length of six centimeters,
after which the fifth moult takes place, the caterpillar becoming ready
for pupation in the sixth stage. No striking changes of colouring or
marking occur after the present stage, but only certain unimportant
alterations, which are, however, of the greatest theoretical interest.


_Sixth Stage._

In this stage the eye-like appearance of the spots on the front
segments becomes still more distinct than in the fifth stage; at the
same time these spots repeat themselves on all the other segments from
the fifth to the eleventh, although certainly without pupils, and
appearing only as diffused, deep black spots, of the morphological
significance of which, however, there cannot be the least doubt. They
are situated in precisely the same positions on the 5-11 segments as
those on the third and fourth--near the front, and above and below
the subdorsal line. A feeble indication of the latter can often be
recognized (Fig. 23).

In all dark brown specimens the repeated spots can only be detected in
a favourable light, and after acquiring an intimate knowledge of the
caterpillar; but in light brown and green specimens they appear very
sharply defined.

There is one other new character which I have never observed at an
earlier period than the sixth stage, viz. the small dots which
appear in pairs near the posterior edge of segments 5-11. These dots
cannot have been developed from the subdorsal line, as they are
situated higher than the latter. Their colour varies according to the
ground-colour of the caterpillar, but it is always lighter, being light
green in green specimens, dull yellow in those that are light brown,
and grey in the blackish-brown caterpillars. These “dorsal spots,” as
I shall term them, are chiefly of interest because they are present in
_Chærocampa Porcellus_, in which species they appear one stage earlier
than in _C. Elpenor_.


CHÆROCAMPA PORCELLUS, LINN.

Females captured on the wing, laid in the breeding-cage single eggs of
a light green colour, spheroidal in form, and very similar to those of
_C. Elpenor_.


_First Stage._

The caterpillars on first hatching measure 3.5 millimeters in length,
and are of a uniform light green colour, with a fine white transverse
line on the posterior edge of each segment, precisely similar to that
which appears in the second stage of _C. Elpenor_. They resemble the
latter species still further in showing a fine white subdorsal line,
which can easily be recognized by the naked eye (Fig. 24). Although
the adult larva is distinguished from all the other known species of
_Chærocampa_ by the absence of a caudal horn, a distinct but very small
one is nevertheless present at this first stage, and is indeed retained
throughout the entire course of development, but does not increase
further in size, and thus gradually becomes so small in proportion to
the size of the caterpillar that it may be entirely overlooked.

The first moult takes place after 4-5 days.


_Second Stage._

The blue-green coloration remains unchanged; but a somewhat darker
green dorsal line becomes apparent down the middle of the back (the
dorsal vessel?), and the subdorsal line now becomes very broad and pure
white, being much more conspicuous than in any stage of _C. Elpenor_
(Fig. 25). The tapering of the three front segments occurs at this
stage, and oblique, dark green striations on a lighter ground stand out
distinctly on the spiracles. As with _C. Elpenor_, the first traces of
the future eye-spots appear during the second stage; not in the present
case as a curvature of the subdorsal line, but as a spot-like widening
of the latter, of a brighter white than the somewhat greenish colour of
the remainder of the line.


_Third Stage._

After the second moult, the formation of the dark “ground-area” of the
eye-spots commences by the appearance of a little brown on the under
edge of the foremost of the white spots, this coloration gradually
increasing in extent and in depth. At the same time both spots become
more sharply distinguishable from the subdorsal line, which becomes
constantly greener (Fig. 27). The brown colour soon grows round the
white of the front eye-spot, which becomes so far perfected; whilst
the completion of the hind spot is effected slowly afterwards. The
formation of the eye-spots does not therefore proceed any more rapidly
in this species than in _C. Elpenor_.

At the end of the present stage the length of the caterpillar is about
four centimeters; the ground colour is still sea-green; the subdorsal
line is much diminished, completely fading away at its lower edge, but
remaining sharply defined above, against the green ground-colour (Fig.
26).


_Fourth Stage._

After the third moult all the caterpillars (5) became brown, this
change occurring therefore one stage earlier than is generally the case
with _C. Elpenor_. In single instances the brown colour appeared in the
third stage. The subdorsal line had disappeared from all the segments
but the three first and the last. The eye-spots now rapidly attained
complete development; they contained a black pupil, and gave the insect
a truly repulsive appearance when, on being threatened by danger, it
drew in the front segments, and expanded the fourth (Fig. 28). The
eye-spots of the fifth segment are much less developed than in _C.
Elpenor_; they remain small, and are not readily detected. On the other
hand, there now appear on all the segments with the exception of the
last, just as in the sixth stage of _C. Elpenor_, distinct rudiments of
eye-spots, which present the appearance of irregular, roundish, black
spots on the front borders of the segments, at the height of the former
subdorsal line. In this latter region the black pigment is disposed as
a longitudinal streak, and to this a median line is added, the whole
forming a marking which perhaps makes the caterpillar appear still more
alarming to its foes. This marking is, however, only to be distinctly
recognized on the three first segments. The “dorsal spots” mentioned in
the case of _C. Elpenor_ then appear very distinctly on segments 5-11.

The caterpillars continued to feed for eleven days after the third
moult, at the end of which period the fourth moult took place, but
without the occurrence of any change of marking. The larvæ then buried
themselves, the complete development having taken 28-29 days.

The development of the _Porcellus_ caterpillar was twice followed;
in 1869 in twelve, and in 1874 in five specimens. In no case did I
obtain caterpillars which remained green throughout the entire course
of development, although this colour is stated in the books to occur
occasionally in these larvæ; neither have I been able to find any
figure of an adult green specimen, so that it must in the meantime be
admitted that such specimens, if they occur at all, are exceptional
instances.[71] The theoretical bearing of this admission will appear
later on.


RESULTS OF THE DEVELOPMENT OF CHÆROCAMPA ELPENOR AND C. PORCELLUS;
COMPARISON OF THESE WITH THE OTHER KNOWN SPECIES OF CHÆROCAMPA.

The first stage of _Elpenor_ shows that the most remote ancestor of
the genus possessed no kind of marking, but was uniformly green. At a
later period, the white longitudinal stripe which I have designated
the “subdorsal line” made its appearance, and at a still later period
this line vanished, with the exception of a few more or less distinct
remnants, whilst, at the same time, from certain portions of it, the
eye-spots of the fourth and fifth segments became developed. After the
perfecting of the eye-spots, weak repetitions of the latter appeared as
black spots on all the segments except the last.

In _Porcellus_ the caterpillar emerges from the egg with the subdorsal
line, the first stage of _Elpenor_ being omitted. From this fact we may
venture to conclude that _Porcellus_ is the younger species, or, what
comes to the same thing, that it has further advanced in development.
The whole subsequent history of _Porcellus_ agrees with this view,
its course of development being essentially but a repetition of the
phenomena displayed by _Elpenor_, and differing only in one point, viz.
that all new characters make their appearance one stage earlier than
in the latter species. This is the case with the transformation of the
green into a brown ground-colour; with the repetition of the eye-spots
on the remaining segments in the form of suffused black spots; and
with the appearance of the light “dorsal spots.” Only the eye-spots
themselves appear, and the snout-like tapering of the front segments
occurs in the same stage as in _Elpenor_, _i.e._ the second.

From these data alone, we may venture to infer the occurrence of four
chief stages in the phyletic development of the genus. The first stage
was simply green, without any marking; the second showed a subdorsal
line; the third, eye-spots on the third and fourth segments; and
the fourth stage showed a repetition of the eye-spots, although but
rudimentary, on all the remaining segments with the exception of the
twelfth.

Now if we compare the other known species of _Chærocampa_ larvæ with
the above, we shall arrive at the interesting conclusion that all these
species can be arranged in three groups, which correspond exactly with
the three last phyletic stages as just deduced from the ontogeny of _C.
Elpenor_ and _Porcellus_.

Of the genus _Chærocampa_,[72] over fifty species have been
described,[73] of which the larvæ of only fifteen are known in the
form which they possess at the last ontogenetic stage.

GROUP 1.--I can furnish but little information with respect to this
group. The first species with which I became acquainted was _Chærocampa
Syriaca_,[74] of which I saw two blown caterpillars in Staudinger’s
collection, and which I have figured in Pl. IV., Fig. 29. The larva
is green, and has the short oblique stripes over the legs common to
so many species of _Chærocampa_, the only marking besides these being
a simple white subdorsal line, without any trace of eye-spots. This
species exactly corresponds therefore with the second ontogenetic
stage of _C. Elpenor_ and _Porcellus_. The account of the species,
both in the larval and perfect state, is unfortunately so imperfect,
that we cannot with certainty infer the age of the two caterpillars
from their size. If the moth were of the same size as _Elpenor_, then
the caterpillar figured, having a length of 5.3 centimeters, would not
be in the last but in the penultimate stage, and it remains doubtful
whether it may not acquire eye-spots in the last stage.

That species exist, however, which in their last stage correspond to
the second stage of _Elpenor_, is shown by two of the forms belonging
to Walker’s genus _Darapsa_, which was founded on the characters
of the imagines only. Ten species of this genus are given in Gray’s
catalogue, the adult larva of two of these being known through the
excellent figures of Abbot and Smith.[75] These two caterpillars
possess the characteristic tapering form in a very marked degree;
one is figured in the attitude so often assumed by our species of
_Chærocampa_ on the approach of danger, the three front segments
being withdrawn into the fourth. (Fig. 34, Pl. IV., is copied from
this Plate). There are no eye-spots either in _D. Myron_ or _D.
Chœrilus_,[76] but only a broad white subdorsal line; underneath which,
and to a certain extent proceeding from it, there are oblique white
stripes, precisely similar to those which meet the subdorsal line in
the third stage of _C. Porcellus_.[77]

GROUP 2.--This group contains numerous species which, like our native
_C. Elpenor_ and _Porcellus_, show eye-spots on the fourth and fifth
segments, whilst these markings are absent, or at most only present in
traces, on the remainder. To this section there belong, besides the
two species mentioned, five others, viz. in Europe, _C. Celerio_ and
_Alecto_ (not certainly known?);[78] in India, _C. Nessus_, Drury, and
_Lucasii_, Boisduval;[79] and an unnamed species from Port Natal.

In the species belonging to this group the subdorsal line may be more
or less retained. Thus, _C. Celerio_, according to Hübner’s figure,
has a broad yellow line extending from the horn to the sixth segment,
whilst it is completely absent on the three front segments. In the
unnamed species from Port Natal[80] the subdorsal line extends to the
front edge of the fifth segment, and on the fourth segment only is
there a perfect eye-spot, whilst on the succeeding segments traces of
such markings can be recognized as dark spots similar to those in
_Elpenor_ and _Porcellus_. The transition to the third group is through
another unnamed species from Mozambique,[81] in which rather large
eye-spots have become developed on the fourth and fifth segments and
these are followed by a subdorsal line, which only appears distinctly
at certain places. On this broken subdorsal line, and not completely
separated from it, there are small, roundish eye-spots, situated near
the front edge of each segment; these being, therefore, a somewhat more
perfect repetition of the front eye-spots.[82]

GROUP 3.--In the species of this group the eye-spots are repeated on
all the segments. I am acquainted with seven such _Chærocampa_ larvæ,
of which _C. Bisecta_, Horsfield,[83] shows some affinity to the
foregoing group, since the eye-spots on segments 6-11 have not yet
attained full perfection. In _C. Odenlandiæ_, Fabr.,[84] and in _C.
Alecto_ from India,[85] the eye-spots appear to be perfectly alike on
all the segments; whilst in _C. Acteus_, Cram.,[86] and in the North
American _C. Tersa_[87] (Pl. IV., Fig. 35) they are smaller on the
other segments than on the fourth; and in _C. Celerio_, Linn., from
India,[88] the size of the spots diminishes from the head to the tail.

In this group also the subdorsal line is retained in a very variable
degree. In some species it appears to have completely vanished (_C.
Acteus_, _Celerio_); in others it is present as a light stripe
extending along all the segments (_C. Alecto_); whilst in others it
is retained as a broad white stripe, which extends only to the fourth
segment (_C. Tersa_, Fig. 35). In species possessing eye-spots, the
subdorsal line is thus a very variable character. It is, however, an
interesting fact that even in the present group, which has made the
greatest step forward, the subdorsal line is of general occurrence,
because the eye-spots in all these species may have almost a similar
development to those of _Elpenor_ and _Porcellus_. The ontogeny of the
tropical species would alone give a definite reply on this point, but
unfortunately we are not acquainted with any of the young forms, so
that we can but presume that some of them at least would show only in
the first stage the simple subdorsal line without eye-spots; that in
the second stage the primary pairs of eye-spots would be formed on the
fourth and fifth segments, whilst the transference of these spots to
the remaining segments would take place in the last stage.

The foregoing assumption is based immediately on the ontogeny of
_Elpenor_ and _Porcellus_; it is supported by the considerable size
attained by the eye-spots in many species of the third group, and
would receive additional confirmation by observations on the Indian
_C. Celerio_, supposing that Horsfield’s statements do not arise from
a confusion of species. This skilful observer, who was the first to
breed systematically a large number of tropical larvæ, has given a
figure of the Indian caterpillar of _C. Celerio_, according to which
this species possesses eye-spots on all the segments from the fourth to
the tenth. The European form of this same species has eye-spots only on
segments four and five, a fact which does not appear to have been known
to Horsfield, as no mention of it is made in his notice of the Indian
species. If the caterpillar figured is really that of _Celerio_, which
I consider to be by no means improbable, not only is it thus shown that
in the species of the third group the ocelli on the hind segments have
a secondary origin through a repetition of the primary ones of the
front segments, but we can also establish that the same species in two
different regions may arrive at two different phyletic stages.

If, finally, we sum up the facts taught by the ontogeny of the two
German species, and the adult forms of the other species, we can form
therefrom a tolerably complete picture of the course of development of
the genus _Chærocampa_. Of the four phyletic stages indicated by the
ontogeny of _Elpenor_ and _Porcellus_, three still form the terminus
of the development of existing species. The great differences among
the caterpillars of this genus can be very simply explained on the
view that they stand at different levels of phyletic development; some
species having remained far behind (Group 1), others having advanced
further (Group 2), and others having reached the highest point of
development (Group 3). The fact that the species of the third group
are only tropical accords well with this view, since many facts prove
that phyletic development proceeds more rapidly in the tropics than in
temperate climates.

The striking markings of the _Chærocampa_ larvæ may, in brief, be
stated to originate from a local transformation of two portions of the
subdorsal line into eye-spots, and the subsequent transference of
these two primary ocelli to the other segments. The eye-spots always
originate on segments four and five, and from these the transference
mostly occurs backwards, although in certain cases it takes place at
the same time forwards. Herein, _i.e._ in the origin of the eye-spots,
there lies a great distinction between the genus _Chærocampa_ and the
genus _Deilephila_, with which it was formerly associated, and in which
the origin of a very similar kind of marking can be traced to quite
another source.


THE GENUS DEILEPHILA, OCHSENHEIMER.

I am acquainted with the caterpillars of nine European and one North
American species, these differing in marking to such a wonderful extent
that they appear to offer at first sight but little hope of being able
to trace them to a common form. These ten species can be separated,
according to their markings, into five groups, which I will briefly
define before entering upon their ontogeny.

The first group consists of three species, and comprises the commonest
and most widely-ranging of all the European species, _Deilephila
Euphorbiæ_, as well as _D. Dahlii_ from Sardinia and Corsica, and _D.
Nicæa_, a species of very restricted range, which appears to occur only
in one small district on the French coast of the Mediterranean. These
three species agree in marking to the extent of their possessing in the
adult form two rows of ring-spots on each side, whilst the subdorsal
line is completely absent.

The second group, consisting also of three species, shows a great
resemblance to _Euphorbiæ_, but has only one row of ring-spots.
It contains _D. Vespertilio_, _D. Galii_, and the Algerian _D.
Mauritanica_.

For the third group I only know one representative, _D. Livornica_,
Esp., which possesses a single row of ring-spots connected by a
subdorsal line.

Another group is composed of _D. Zygophylli_, which occurs on the
shores of the Caspian Sea, and the North American _D. Lineata_; these
species possessing a strongly marked subdorsal line, associated with
more or less distinct ring-spots, which I shall designate as “open
rings,” because their black border does not intersect the subdorsal
line, but has the form of an arch above and below it.

In the last group, represented by _D. Hippophaës_, which occurs at the
foot of the Alps (Wallis), and southward as far as Andalusia, there is
only a broad subdorsal line, generally without any trace of a row of
spots.

The important differences of marking displayed by these five groups
are not in any way accidental, but they represent different stages
of phyletic development; or, in other words, the five groups are of
different ages, the first (_Euphorbiæ_, &c.) being the youngest, and
the last (_Hippophaës_) the oldest of the genus.

According to their phyletic age, the groups follow each other in
inverse order, the first being _Hippophaës_, the second that of
_Zygophylli_, the third that of _Livornica_, the fourth that of
_Galii_, and the fifth and youngest that of _Euphorbiæ_. Only in this
last am I acquainted with the _complete_ development of one species,
for which reason I commence with this group, thus proceeding from the
youngest to the oldest forms, instead of taking the more natural course
from the simplest and oldest to the youngest and most complicated.


DEILEPHILA EUPHORBIÆ, LINN.

Some captured females were at once placed in an enclosure about the
size of a small sitting-room. It was evident that they did not feel
quite at home under these conditions, frequently beating their heads
and wings against the tarlatan, but some of them nevertheless laid eggs
at the base of the leaves of _Euphorbia Cyparissias_. The eggs much
resemble those of _Chærocampa Elpenor_, being spheroidal in form, but
rather smaller, and of a somewhat darker green. They were laid in small
clusters composed sometimes of as many as seven, the single eggs being
placed near together, but never touching, and seldom at the point of
the leaf, but generally near the end of a twig, where young shoots are
in close proximity. During the embryonic development the eggs become
coloured, first yellow and partly blackish, and finally completely
black.


_First Stage._

The young caterpillars (Fig. 37, Pl. V.) immediately after hatching
measure four millimeters in length; they are at first rather light,
but in the course of half-an-hour they are seen by the naked eye to
become of a deep velvety black; later, on increasing in size, they
again become paler, appearing of a greenish-black, and subsequently
blackish-green. On further increasing in size (Fig. 38), they are
blackish-green, with the horn, head, legs, and a crescent-shaped
chitinous plate on the back of the prothorax black. There are also on
the last segment a double and two single black chitinous plates. Of the
later marking of the caterpillar there is scarcely anything present.
The spiracles appear as white spots, and on each segment there are
a number (mostly ten) of small warts, each of which emits a single
bristle.

When the young larvæ have attained a length of seven millimeters they
are olive-green, and do not contrast so brilliantly with the green of
the _Euphorbia_ leaves as before; neither do they as yet possess any
markings.


_Second Stage._

The first ecdysis occurs after five days, and with this there appears
quite suddenly a very complicated pattern. The ground-colour is now a
light yellowish-green (Fig. 39), and on each of the twelve segments,
near the front border, there is a pure white round spot in the
middle of a large black transverse spot. I shall designate these, in
accordance with the nomenclature employed for _Chærocampa_, as the
white “mirrors” on black “ground-areas,” both together constituting
“ring-spots,” as distinguished from “eye-spots” proper, in which a
“nucleus,” the pupil of the eye, is also added. In many, but not in all
specimens, very distinct traces of a subdorsal line can be seen as a
light whitish stripe connecting the white spots. The horn, the thoracic
and prolegs, and some spots on the head, are black.

The caterpillars remain unaltered till after four days, when, having a
length of 17 millimeters, the second moult takes place, bringing with
it changes quite as great as those which occurred with the first.


_Third Stage._

The caterpillar now assumes the shagreened appearance which it
possesses in the adult state. Small white warts are arranged in rows
from the dorsal to the spiracular line, and again underneath this line
on the abdominal legs. These dots are not only of value as a character
for differentiating the genera _Deilephila_ and _Chærocampa_, but they
also play a part in the peculiar spot-marking which will be shown later
on. The ground-colour of the caterpillar is now light green (Fig. 40),
replaced by black on certain parts. From the black “ground-area” of the
ring-spots, two black triangles extend towards the posterior borders of
the segments, but usually without reaching them.

The ring-spots are not essentially changed, although it may be observed
that in most specimens the shagreen-dots under each ring-spot are
somewhat larger, and stand closer together than in other places. In
the following stage they become fused into a second white “mirror,” so
that two ring-spots stand one above the other, their black ground-areas
meeting. The formation of the second ring-spot sometimes takes place in
the present stage (Fig. 42).

The subdorsal line has now completely vanished, whilst the spiracular
line[89] appears as a broad stripe above the legs. The horn is yellow
with a black point, and the black spots on the head have increased in
size.


_Fourth Stage._

The third moult, which again occurs after four days, is not accompanied
by such important changes. The green ground-colour has now completely
disappeared, and is replaced by a dull black. The caterpillars are now,
as also in the previous stage, extremely variable. Thus, for example,
a triangular patch of the green ground-colour may be retained on the
posterior edge of the segments (Fig. 41), those specimens which possess
this character generally having their markings retarded in development,
as shown by the absence of the second “mirror” of the ring-spots.

In Fig. 41 the shagreen-dots from which this second “mirror” is
subsequently formed, are distinctly larger than the others, and on the
eleventh segment two of them have already coalesced.


_Fifth Stage._

After another period of four days, the fourth moult takes place. The
marking remains the same, but the colours become more vivid; the
brick-red of the head, horn, dorsal line and legs, changing into a
fiery red. The spiracular line, formerly green alternating with yellow,
generally becomes resolved into a row of reddish-yellow spots. Ten days
later the caterpillar (8.5 centimeters in length), ceases to feed, and
prepares for pupation.

In this last stage also there is great variability of colour, but
although each particular character is subject to fluctuation, the
individuals of the same brood show but little variation among
themselves.[90] Thus, the dorsal line is sometimes black, and sometimes
red, or again, this colour interrupted with black, so that only small
red spots mark its course. The head may be entirely red, or this
colour mixed with black. On the under side of the caterpillar, red
generally predominates, but in some specimens this is replaced by
black. The ground-colour is also variable, being generally a shining
brownish-black, but sometimes dull coaly black. The shagreen-dots
are sometimes white and sometimes yellow, and the “mirrors” of the
ring-spots are also often yellowish.

The most interesting variation, however, appears to me to be the
following:--In many specimens from Kaiserstuhl (Breisgau), the red
was unusually vivid, and was not limited to the ordinary places, but
occupied also the triangles on the posterior edges of the segments
(Fig. 44), which are green in the third and fourth stages (Fig. 42).
This variety has also been figured by Hübner. In one individual (Fig.
43), the under ring-spots were wanting, whilst the upper ones possessed
a beautiful red nucleus fading away anteriorly, and showing the first
step in the formation of a complete eye-spot.

I cannot positively assert that a fifth moult occurs in the last ten
days, although I am very doubtful whether this is the case. It is
certain, however, that some time before pupation, and whilst the larva
is still feeding, the striking colours fade out, and become replaced
chiefly by black.

The ontogeny of this species is obviously but a very incomplete
representation of its phyletic development. This is at once apparent
from the large gap between the first and second stages. It is not
possible that a row of ring-spots can have arisen suddenly; in all
probability they have been developed from a subdorsal line, which in
_Euphorbiæ_ is now only indicated in the second stage by a faint line.
This conjecture is raised to a certainty when we call in the aid of the
remaining species of _Deilephila_.


DEILEPHILA NICÆA, DE PRUNNER.

I only know this species from blown larvæ in Staudinger’s collection,
and Duponchel’s figure, of which Fig. 51, Pl. VI. is a copy. The adult
insect possesses two perfectly separated rows of ring-spots. Duponchel
figures also two younger stages, of which the youngest is probably the
third stage. The larva is 18 millimeters in length, of a leaf-green
colour, and shows no trace of a subdorsal line, but possesses the two
rows of ring-spots, which only differ from those of the succeeding
stages in the green colour of the “mirror.”


DEILEPHILA DAHLII, TREITSCHKE.

I am familiar with numerous specimens in various stages, collected in
Sardinia by Dr. Staudinger, and preserved by inflation.

The first stage is blackish, and shows no kind of marking; thus
agreeing with the corresponding stage of _Euphorbiæ_. The second stage
is unfortunately not represented in Staudinger’s collection.

The third stage shows a row of ring-spots, which are, however,
connected by a very distinct and sharply defined subdorsal line. In the
fourth stage a second row of (under) ring-spots is added, whilst the
subdorsal line generally at the same time disappears.

The caterpillar remains unchanged during the fifth stage, when it shows
a great resemblance in marking to _Euphorbiæ_; neither does it appear
to differ essentially from this species in colour, so far as can be
judged from preserved specimens and single figures (in Duponchel and
Hübner). I have, moreover, seen several larvæ in the last stage, and
the subdorsal could be distinctly recognized as a broad light stripe.

Of the four groups, the second (that of _Galii_), appears to me to be
of but very little importance, as I shall now proceed to show from the
development of _D. Vespertilio_.


DEILEPHILA VESPERTILIO, FABRICIUS.

Hitherto I have unfortunately been unable to obtain fertile eggs of
this species, so that I can say nothing about the first stage. The
latter would have been of interest, not only because of the marking,
but also because of the presence of a residual caudal horn.

I am likewise only acquainted with the end of the second stage, having
found, at the end of June 1873, a single caterpillar on _Epilobium
Rosmarinifolium_, just previous to its second ecdysis. In the case of
such young caterpillars, however, the new characters which appear in
the succeeding stage are generally perceptible through the transparent
chitinous skin at the end of the preceding stage, so that the markings
of the insect are thus caused to change. The caterpillar found was
about 16 millimeters long, and of a beautiful smooth and shining
grass-green (Fig. 13). A broad white subdorsal line extended from the
first to the penultimate segment, from which the horn was completely
absent. On close inspection the first traces of the ring-spots could
be detected near the anterior edge of each segment as feeble, round,
yellow, ill-defined spots, situated on the subdorsal line itself (Fig.
13). On the first segment only there is no spot, and here no ring-spot
is afterwards formed. Besides these markings, there was only to be seen
a yellowish-white spiracular line.

This solitary specimen unfortunately buried itself before the moult
for which it had prepared itself had occurred; but this ecdysis is
associated with a very important transformation. This statement is
founded on a blown specimen in Staudinger’s collection; it is only
18 millimeters in length, but already shows the later grey colouring
in place of the beautiful green. In this, the third stage, the broad
white subdorsal line bears on each segment a red spot enclosed between
black crescents above and below (Fig. 49 A). In the fourth stage,
during which I have seen many living caterpillars, the subdorsal line
is still distinctly present in some individuals (Fig. 14), but the
spots (“mirrors”) are now completely surrounded by a narrow black
ring (“ground-area”), which sharply separates them from the subdorsal
line (Fig. 49 B). In the fifth stage this ring becomes a somewhat
irregularly formed black “ground-area,” whilst the subdorsal line
completely vanishes (Figs. 51 and 49 C). The mirrors are white, but
generally have a reddish nucleus, which obviously corresponds to the
primary yellow spots from which the whole development of the ring-spots
originates. This character is, however, sometimes absent; and many
other variations also occur in the earlier stages, all of which can be
easily explained as cases of arrested, or retarded development. Thus,
the subdorsal line often disappears earlier, and is only present in the
fourth stage as a feeble light stripe.


DEILEPHILA GALII, FABRICIUS.

The markings of this species appear to be developed in a precisely
similar manner to those of _D. Vespertilio_. The adult larva, as in
the last species, shows no trace of a subdorsal line. A row of large
black spots, each having an irregular round, yellowish-white nucleus,
is situated on an olive-green, blackish-brown, brown, or dirty yellow
ground. I have, unfortunately, also in this case been unable to procure
fertile eggs. There is, however, one figure of a caterpillar, 2.5
centimeters long, by Hübner, which is of a light green colour, and has
five longitudinal lines; one dorsal, two subdorsal, and a spiracular
line. The subdorsal is white, and bears in the place of the ring-spots
small red dots, whilst the line itself is bordered with black where the
red spots are situated. Hübner has probably figured the third stage,
so that we may venture to conclude that in the second stage there is
a subdorsal line either quite free from spots, or only showing such
feeble rudiments as are to be seen in the second stage of _Vespertilio_.

I found two specimens in the fourth stage in the Upper Engadine.
One of these (Fig. 45) was already of a dark, blackish-green
ground-colour[91] with a broad, greenish-white subdorsal line sharply
defined throughout its entire length, and containing ring-spots of a
sulphur-yellow with an orange-red nucleus; the black “ground-area” did
not encroach upon the subdorsal line, but was confined to two faint
crescents situated above and below the “mirror.” Only the two foremost
“mirrors” (on the second and third segments) were without nuclei.

The remaining peculiarities of coloration are shown in the figure. I
may here only point out the shagreening present on the sides and a
portion of the under surface.

The specimen figured was 3.3 centimeters long; a second example
measured 2.8 centimeters in length, and was essentially similar, but
showed that a considerable amount of variability must prevail at this
stage of development. It was pitchy black, with a very indistinct
subdorsal line and a few ring-spots, the “mirrors” of which were also
sulphur-yellow, with the orange-red nucleus. The shagreening was quite
as strong as in the first specimen, the dots being yellow instead
of white. It is specially to be observed, because of its important
theoretical bearing, that in this larva the ring-spots were absent on
the three front segments, and on the fourth only, a faint indication
of one could be perceived. In the caterpillar figured the ring-spots
increase also in distinctness from the tail to the head.


_Fifth Stage._

The two specimens just mentioned, after moulting, acquired the
well-known markings of the adult caterpillar already briefly described
above. The fifth is the last stage.

The larva is known to occur in several variations, Rösel having figured
it in three forms; light green, olive-green, and dirty yellow. It has
not been since considered worth the trouble to attend to the subject of
caterpillar coloration. Thus, Wilde,[92] in his well-known work, takes
no notice of Rösel’s observation, but simply describes the caterpillar
of _Galii_ as “blackish olive-green.”

Having had an opportunity of observing twenty-five adult specimens of
this somewhat scarce species at one time, I am able to state that it
is not in this instance di- or polymorphism, but a case presenting a
great degree of variability, with which we have to deal. There are
not several sharply-defined types of coloration; but the extremes are
connected by numerous intermediate forms. The extreme forms, however,
certainly preponderate.

I have never met with Rösel’s light green form; neither was there
a dark green specimen among the twenty-five mentioned, and I
only know this variety from single individuals, found at a former
period. Among the twenty-five caterpillars; all gradations of colour
occurred, from pitchy black to light clay-yellow, and even to an
almost whitish-yellow; some were brownish-black, others of a beautiful
chestnut-brown, and others yellowish brown, dark clay-yellow, or
brownish-red. Out of twenty-one specimens of which the ground-colours
were noted, there were nine black, nine clay-yellow, and three brown;
each of the three groups again showing various minor modifications of
colour. The other colours also varied somewhat. Thus, the “mirrors”
were sometimes white, sometimes strong yellow, and occasionally they
also contained a reddish nucleus.

The variations in the shagreening were especially interesting, inasmuch
as these appeared to have a striking connection with the general
colouring of the caterpillar. Black specimens seldom show such sparse
shagreening as that represented in Pl. V., Fig. 46, but are generally
thickly scattered with large shagreen-dots right up to the dorsal line
(Fig. 47, Pl. VI.), then strikingly resembling the adult larva of _D.
Euphorbiæ_. The light ochreous-yellow individuals, on the other hand,
were sometimes entirely without shagreening (Fig. 48, Pl. VI.), being
smooth, and much resembling the light ochreous-yellow or yellowish-red
caterpillar of _D. Nicæa_ (Fig. 51, Pl. VI.). I have never seen a
caterpillar of _Galii_ which showed traces of the subdorsal line in
the last stage, nor have I ever met with one which possessed a second
row of “mirror” spots; so that retrogression or a sudden advance in
development does not appear to occur.

Of the North African _D. Mauritanica_, which likewise belongs to the
_Galii_ group, I have not been able to obtain specimens or figures
of the younger stages. The adult caterpillar is very similar to that
of _Euphorbiæ_, but differs in the absence of the second row of
ring-spots. For this reason it must be regarded as a retarded form at
an older stage of phyletic development.

I now proceed to the _Livornica_ group.


DEILEPHILA LIVORNICA, ESPER.

This, the only European species here to be considered, possesses
almost the same markings as _Galii_ in its fourth stage, _i.e._, a
subdorsal line with interpolated ring-spots. The species is known to
be rare, and I have not been able to obtain living specimens, but I
have examined several blown larvæ, all of which agree in having the
ring-spots sharply distinct from the whitish subdorsal line, so that
the latter is thereby interrupted. Figures of the adult larva are given
in the works of Hübner, Boisduval, and Duponchel. In most specimens
the ground-colour is brown, although Boisduval[93] also figures a
light green specimen; from which it may be inferred, from analogy with
_Galii_ and _Vespertilio_, that the first stages are green. In Dr.
Staudinger’s collection there is a young larva, probably in the fourth
stage, the ground-colour of which is light ash-grey. The dorsal and
subdorsal lines are white, the latter showing in the positions where
the ring-spots subsequently appear, small white “mirrors” with red
nuclei, exactly corresponding to the stage of _Vespertilio_ represented
in Fig. 49 A, Pl. VI. The “mirrors” are nothing more than dilatations
of the subdorsal line, which is not therefore interrupted by them. The
black “ground-area” does not surround the “mirrors” completely, but
borders them only above and below, and is much more strongly developed
above, extending in this direction to the dorsal line.

The fourth group comprises the two species _D. Lineata_, Fabr., and _D.
Zygophylli_, Ochs., the former being the North American representative
of our _D. Livornica_, but differing in remaining permanently at the
fourth stage of this last species. I am acquainted with _D. Lineata_
only through the figure of the adult larva given by Abbot and Smith,
which figure, judging from the position and form of the spots, I
am compelled to believe is not quite correct, notwithstanding the
excellence of the other illustrations. The ground-colour of the
caterpillar is green; the subdorsal yellow, bordered with black,
slightly curved, arched lines, which nowhere interrupt its continuity.
This North American species appears therefore to be an older form than
our _Livornica_.


DEILEPHILA ZYGOPHYLLI, OCHSENHEIMER.

This species, which is the next allied form to _D. Lineata_, is
an inhabitant of Southern Russia. I have seen four specimens of
the caterpillar in Dr. Staudinger’s collection, three of which are
certainly in the last ontogenetic stage. The ground-colour appears
ash-grey, ash-brown, or blackish with whitish granulations. A broad
white subdorsal line extends to the base of the black caudal horn, this
line in one specimen appearing at first sight not to possess a trace of
spot rudiments (Fig. 50). On closer investigation, however, there could
be observed, in the same position where the ring-spots stand in the
other species of _Deilephila_, small black crescents above and below
the subdorsal line. In other specimens the white subdorsal line had
also become expanded in these positions into distinct spots; indeed, in
one individual light white mirror-spots, bordered above and below by
black crescents, stood on the subdorsal line (Fig. 50 A).

It is thus in this distinguishing character that the caterpillar is
extremely variable, and we may suppose either that this species is now
in a state of transition to a higher stage of phyletic development, or
else that the ring-spots were formerly more strongly developed, and
are now degenerating. The developmental history of the larva could
alone decide which of these two views is correct. There would be no
difficulty in procuring materials for this purpose if one of the
numerous and zealous Russian naturalists would take up the subject.


DEILEPHILA HIPPOPHAËS, ESPER.

This is the only representative of the fifth and oldest group known to
me. The moth resembles _D. Euphorbiæ_ to the extent of being sometimes
confounded with it, a circumstance which is made the more remarkable by
the fact that the caterpillars are so completely different.

The adult larva of this local moth has been made known by the figures,
more or less exact, in the works of Hübner, Boisduval, and Duponchel.
Wilde also gives a description of it, although from a foreign source.
I will not here delay myself by criticizing the different descriptions
and figures; they are partly correct, partly inexact, and sometimes
altogether erroneous; they were of no avail for the question which here
primarily concerns us, and new observation had to be undertaken.

I have been able to compare altogether about forty caterpillars,
thirty-five of which were living. All these specimens possessed nearly
the same greyish-green ground-colour, and most of them had exactly
the simple marking as represented, for instance, in Hübner’s figure,
_i.e._, a rather broad greenish-white subdorsal line, somewhat faded
at the edges, and without a trace of spots on any of the segments
with the exception of the eleventh, on which there was a yellowish,
black-bordered mirror-spot, with a broad, diffused, vivid orange-red
nucleus. Specimens also occur, and by no means uncommonly, in which no
other markings are to be seen than those mentioned; there were nine
among twenty-eight examples compared from this point of view.

In many other individuals of this species small red spots appear on
the subdorsal line, exactly in the positions where the ring-spots are
situated in the other species of the genus (Fig. 60), so that these
spots are thus repetitions of the single ring-spot--a fact which must
appear of the greatest interest in connection with the development
of the markings throughout the whole genus. But this is not all, for
again in other specimens, these red spots stand on a large yellow
“mirror,” and in one individual (Fig. 59), they had become developed
into well-formed ring-spots through the addition of a black border. We
have thus presented to us in one and the same stage of a species, the
complete development of ring-spots from a subdorsal line.

These facts acquire a still greater interest, as showing how new
elements of marking are produced. The spots on the subdorsal line
decrease from the posterior to the anterior segments, so that they
must undoubtedly be regarded as a repetition or transference of the
ring-spot previously developed on the eleventh segment. I will now
proceed to furnish proofs in support of this statement.

I have never met with any specimens having ring-spots on all the
segments--in the most prominent instances these spots were present
on segments 10-5. This was the case in three out of the twenty-eight
caterpillars minutely examined. On all these segments, however, the
ring-spots were not equally developed, but increased in perfection
from the posterior towards the anterior segments. In the larva
represented in Fig. 59 for example, there is a completely developed
ring-spot on segment 10, which, although possessing but a feeble black
“ground-area,” is still distinctly bordered; on segment 9 this border
is less sharp, and not so dark, and it is still less sharp and much
lighter on segments 8 and 7, whilst it has completely disappeared from
segment 6, the yellow “mirror” having at the same time lost in size. On
segment 5, only two small contiguous reddish spots, the first rudiments
of the nucleus,[94] can be recognized on close inspection.

Specimens in which the spots extend from the eleventh to the seventh
segment are of more frequent occurrence, five having been found
among the twenty-eight. In these the spots diminish anteriorly in
size, perfection, and intensity of colour. Still more frequently (in
eleven specimens) are the ring-spots or their rudiments restricted to
the tenth and ninth segments, the spot on the latter being without
exception less developed than that on the former segment.

An anteriorly progressing formation of ring-spots thus undoubtedly
occurs, the spots generally diminishing in perfection very suddenly
towards the front segments; and specimens, such as that represented in
Fig. 60, Pl. VII., in which traces of ring-spots are to be seen on all
the segments from the tenth to the fifth, are of rare occurrence.

From what elements of marking are these _secondary_ ring-spots
resulting from transference developed? They do not, as in the case
of the _primary_ eye-spots of the _Chærocampinæ_, originate in the
separation of one portion of the subdorsal line, and the subsequent
formation of this detached spot into a “mirror;” but they arise from
the formation of a nucleus, first one and then two of the shagreen-dots
on the subdorsal line acquiring a yellowish or reddish colour (Fig.
61, Pl. VII., segments 6 and 7). The ground on which these two spots
are situated then becomes yellow (Fig. 61, Pl. VII., segment 8), and
a more or less distinct black border, having the form of two small
crescents, is afterwards formed. At a later period these two crescents
and also the two primary nuclei coalesce, producing a ring-spot which,
as in Fig. 61, Pl. VII., segment 9, can be distinctly resolved into two
portions.

It certainly cannot be denied that these facts may also be
theoretically interpreted in a reverse sense. We might interpret
the phenomena in this case, as also in that of _D. Zygophylli_, as
a gradual disappearance from the front towards the hind segments of
ring-spots formerly present, a view which could only be refuted by the
ontogeny of the species. I have not been fortunate enough to procure
eggs of _D. Hippophaës_, so that the younger stages are unknown to me.
Among my caterpillars, however, there were two in the fourth stage of
development, but these did not show ring-spots on all the segments, as
we should expect on the above view; on the contrary, no trace of such
spots could be seen on any of the segments with the exception of the
eleventh, on which there was a ring-spot less perfectly developed than
in the last stage.

In this fourth stage the larva of _D. Hippophaës_ is of a lighter green
(Fig. 58), the subdorsal yellowish with sharp boundaries, and the
infra-spiracular line pure white, as in the next stage. The shagreening
is present, but none of the shagreen-dots are red or reddish, and no
trace of a ring-spot can be detected on the subdorsal line with the
exception of that on the eleventh segment. In this last position this
line is somewhat widened, and a long, diffused, rose-red spot can there
be recognized upon it (Fig. 58 A). The black “ground-area” present
in the fifth stage is as yet absent, and the spot is not so sharply
separated anteriorly from the subdorsal line as it becomes later.

From these observations we might venture to expect that in the third
stage of _Hippophaës_, the subdorsal line would also be free from this
spot on the eleventh segment, and it is possible that in the second
stage this line is itself absent.


THE GENUS DEILEPHILA: SUMMARY OF FACTS AND CONCLUSIONS.

Regarding only the _adult_ larvæ of the species of _Deilephila_,
these represent in their five groups, five stages in the phyletic
development of the genus; but if we also take into consideration the
developmental history, two more stages must be added, viz., that in
which the caterpillar possesses no particular marking, as was found to
be the case in the first stage of the development of _D. Euphorbiæ_
and _D. Dahlii_; and a second stage with a subdorsal line, but without
any ring-spot formations. Seven stages of phyletic development must
therefore be distinguished.

_Stage 1._--No species with entire absence of marking in the adult form
now occurs.

_Stage 2._--A subdorsal, accompanied by a spiracular line, extends from
the caudal horn to the first segment. This also no longer forms the
final stage of the ontogeny, but is, however, undoubtedly retained in
the second stage of several species (_D. Vespertilio_, _Livornica_,
_Lineata_, and perhaps also _Galii_).

_Stage 3._--The subdorsal line bears a ring-spot on the penultimate
segment; the other markings as in the last stage. _D. Hippophaës_ only
belongs to this stage, a small number of specimens, however, showing
a transition to the following stage by the transference of ring-spots
from the posterior to the anterior segments.

_Stage 4._--Open ring-spots appear on the subdorsal line on all the
segments from the eleventh to the first. _D. Zygophylli_ and the North
American _D. Lineata_ belong here.

_Stage 5._--Closed ring-spots are situated on the subdorsal line. Of
the known species, only _D. Livornica_ concludes its development at
this phyletic stage.

_Stage 6._--A single row of ring-spots replaces the subdorsal line. _D.
Galii_, _Vespertilio_, and _Mauritanica_ represent this stage at the
conclusion of their ontogeny.[95]

_Stage 7._--A double row of ring-spots. Only _D. Dahlii_, _Euphorbiæ_,
and _Nicæa_ attain to this highest stage of _Deilephila_ marking, the
two first species in the fourth stage, and _Nicæa_ in the third stage
of its ontogeny.

Although our knowledge of the history of the development of the
individual species is still so fragmentary, we may conclude with
certainty that the development of the markings has been uniform
throughout--that it has proceeded in the same manner in all species.
All the species appear to be making for the same goal, and the
question thus arises whether there may not be an innate force
urging their phyletic development. The rigorous examination of this
conception must be reserved for a later section. Here, as we are
only occupied essentially in establishing facts, it must be remarked
that retrogression has never been observed. The young larval forms
of a species never show the markings of a later phyletic stage than
the older larval forms; the development takes the same course in all
species, only making a greater advance in the same direction in some
than in others.

Thus, _Nicæa_ and _Euphorbiæ_ have advanced to the seventh phyletic
stage, _Zygophylli_ and _Hippophaës_ only to the third, and some
specimens of _Zygophylli_ to the fourth. But at whatever phyletic
stage the ontogeny of a species may terminate, the young larval
stages always display the older phyletic stages. Thus, _Galii_ in its
last ontogenetic stage reaches the _sixth_ phyletic stage; in its
penultimate stage it reaches the _fifth_ phyletic stage; and in its
third stage; the _fourth_ phyletic stage is represented, so that little
imagination is required to anticipate that in the second stage the
_third_ or _second_ phyletic stage would be pictured.

If we tabulate the development of the various species, indicating the
ontogenetic stages by Arabic numerals, and the stages of the phylogeny
which are reached in each stage of the ontogeny by Roman numerals, we
obtain a useful synopsis of the series of developments, and, at the
same time, it shows how many gaps still remain to be filled up in order
to complete our knowledge even of this small group of species.


TABLE OF DEVELOPMENT OF THE SPECIES OF DEILEPHILA.

  +-----------------+---------+---------+---------+---------+---------+
  |   Deilephila.   | Ontogeny| Ontogeny| Ontogeny| Ontogeny| Ontogeny|
  |                 | Stage 1.| Stage 2.| Stage 3.| Stage 4.| Stage 5.|
  +-----------------+---------+---------+---------+---------+---------+
  |  1. Hippophaës  |    ?    |    ?    |    ?    |   III.  | III.-IV.|
  |  2. Zygophylli  |    ?    |    ?    |    ?    |    ?    | III.-IV.|
  |  3. Lineata     |    ?    |    ?    |    ?    |    ?    |   IV.   |
  |  4. Livornica   |    ?    |    ?    |    ?    |   IV.   |    V.   |
  |  5. Galii       |    ?    |    ?    |   IV.   |    V.   |   VI.   |
  |  6. Vespertilio |    ?    | II. (?) |   IV.   |    V.   |   VI.   |
  |  7. Mauritanica |    ?    |    ?    |    ?    |    ?    |   VI.   |
  |  8. Dahlii      |    I.   |    ?    |   VI.   |   VII.  |  VII.   |
  |  9. Euphorbiæ   |    I.   |    V.   |   VI.   |   VII.  |  VII.   |
  | 10. Nicæa       |    ?    |    ?    |  VII.   |   VII.  |  VII.   |
  +-----------------+---------+---------+---------+---------+---------+

From this very incomplete table we perceive that, in certain instances,
the stages can be represented as a continuous series of phyletic
steps, as in the case of _D. Galii_; that in others certain steps
may be omitted, as with _D. Euphorbiæ_, in which grade I. of stage 1
is immediately followed by grade V. in stage 2. In reality the gap
caused by this omission is still greater than would appear, as grade
V. is only indicated, and not actually reached, the subdorsal not
being present as a sharply-defined line, but only as a faint stripe.
The suppression of phyletic steps increases with the advancement in
phyletic development. The higher the step to which a species finally
attains, the greater is the tendency of the initial stages to be
compressed, or omitted altogether.

From what has thus far been seen with respect to the development of
_D. Hippophaës_, there may be drawn what to me appears to be a very
important conclusion, viz. that the ring-spots of _Deilephila_ first
originated on the segment bearing the caudal horn, and were then
gradually transferred as secondary spots to the preceding segments.
Complete certainty would be given to this conclusion by a knowledge
of the young forms of other phyletically retarded species, especially
those of the American _D. Lineata_, and perhaps also those of
_Zygophylli_ and _Livornica_. The few observations on the development
of _D. Galii_ already recorded give support to this view, since
the absence of ring-spots on the three front segments in the young
caterpillar (one instance), or their less perfect formation on these
segments (second instance), indicates a forward transference of the
spots.

If the foregoing view be accepted, there follows from it a fundamental
difference between the development of the genera _Chærocampa_ and
_Deilephila_. In the former the formation of the eye-spots proceeds
from a subdorsal line, but they first appear on two of the front
segments, and are then transferred to the _posterior_ segments. In
_Deilephila_, on the other hand, a single ring-spot is formed on the
penultimate segment bearing the caudal horn, and this is repeated on
the _anterior_ segments by secondary transference. With respect to
the origination of the ring-spot also, there is a distinction between
this genus and _Chærocampa_, inasmuch as the first step towards the
eye-formation in the latter consists in the separation of a curved
portion of the subdorsal line, whilst in _Deilephila_ the nuclear
spot first seems to originate and the separation of the mirror-spot
from the subdorsal line appears to occur secondarily. It is difficult
here to draw further conclusions, since the first appearance of the
primary ring-spot has not yet been observed, and no more certain
inference respecting the history of the formation of the _primary_
ring-spots can be drawn from the manner in which the _secondary_
ring-spots are formed. Because in _Hippophaës_ the formation of the
secondary ring-spots begins with the red coloration of one or two
shagreen-dots, it does not follow that the primary spot on the eleventh
segment also originated in this manner; and this is not without
importance when we are concerned with the causes which underlie the
formation of ring-spots. In _Chærocampa_ also, the formation of the
primary eye-spots appears to differ from that of the secondary--in the
latter the black “ground-area” first appearing, and in the former the
“mirror-spot.” The secondary eye-spots certainly remain rudimentary in
this last genus, so that the evidence in support of this conclusion is
thus much weakened; but it must be admitted that we are here on ground
still too uncertain to admit of wider conclusions being based thereon.

As a final result of the investigation, we may advance the opinion
that the existing species of the genus _Deilephila_ have reached five
different phyletic stages, and that their very different external
appearance is explained by their different phyletic ages; the
appearance from these caterpillars of moths so extremely similar, can
otherwise be scarcely understood.

It may appear almost unnecessary to bring forward additional proofs in
support of this interpretation of the facts, but in a field where the
data are so scanty, no argument which can be drawn from them should be
considered as superfluous. The variations which occasionally occur
in the larvæ, however, to a certain extent furnish a proof of the
correctness of the theoretical interpretation offered.

When, in the ontogeny of these species, we actually see before us
a series of stages of phyletic development, we must admit that
ordinary reversion may occur, causing an adult caterpillar to show
the characters of the young. Forms reverting to an earlier phyletic
stage must, on the whole, occur but seldom, as this stage is removed
further back in the ontogeny. Thus, indications of the subdorsal line
must occur but rarely in the _adult_ larvæ of _Euphorbiæ_, and still
less frequently in _Nicæa_, whilst they must be expected to be of more
common occurrence in _Vespertilio_, and also, as has already been
seen, in _Dahlii_. In this last species, as also in _Vespertilio_,
the completely-developed subdorsal line is still present in the third
stage, whilst it is possessed by _Euphorbiæ_ only in the second stage,
and then in a rudimentary condition.

The state of affairs may in fact be thus described: Among several
hundred adult larvæ of _Dahlii_ found in Sardinia by Dr. Staudinger,
there were some which did not actually possess a distinct subdorsal
line, but in place thereof, and as its last indication, a feeble light
stripe. One of Dr. Staudinger’s caterpillars showed also a distinct
line between the closed eye-spots. In the last stage of _Vespertilio_
this line appears still more frequently, whilst in _Euphorbiæ_ it is
extremely rare, and when present it only appears as a faint indication.
This is the case with one of the specimens figured in Hübner’s work as
an “aberration,” and also with one in Dr. Staudinger’s collection. Of
_Nicæa_ I have at most seen only eight specimens, none of which showed
any trace of the long-vanished subdorsal line.

It must be expected that any ontogenetic stage would most readily
revert to the preceding phyletic stage, so that characters present
in the preceding stage are consequently those which would most
commonly arise by reversion. This postulate of the theory also finds
confirmation in the facts. Caterpillars which, when full grown,
belong to the _seventh_ phyletic stage, _e.g._ _D. Euphorbiæ_, not
unfrequently show variations corresponding to the _sixth_ stage,
_i.e._ only one instead of two rows of ring-spots--the upper and
first-appearing series. On the other hand, forms reverting to the
_fifth_ phyletic stage (ring-spots with connecting subdorsal line)
occur but very rarely. I have never met with such cases in adult
living caterpillars of _D. Euphorbiæ_, although in one instance such
a larva was found in the fourth ontogenetic stage; but the strikingly
dark, brownish subdorsal line which connected the otherwise perfectly
developed ring-spots, completely disappeared in the fifth stage of the
ontogeny. Those larvæ which, in the adult state, belong to the _sixth_
phyletic stage, not unfrequently show the characters of the _fifth_
stage more or less developed, as, for example, _D. Vespertilio_.[96]


THE GENUS SMERINTHUS, LATREILLE.

The caterpillars of this genus are very similar in appearance, and all
possess extremely simple markings. The occurrence of numerous stages
of development of these markings is thus excluded, and the study of the
ontogeny therefore promised to furnish less information concerning the
phyletic development of the genus than in the case of the preceding
genera. This investigation has nevertheless also yielded interesting
results, and the facts here recorded will be found of value in likewise
throwing light on the causes which have produced the markings of
caterpillars.

I shall commence, as in former cases, with the developmental history.
I have easily been able to obtain fertile eggs of all the species of
_Smerinthus_ known to me. Impregnated females laid large numbers of
eggs in confinement, and also bred females of the commoner species can
readily be made to copulate, when pinned, and exposed in a suitable
place in the open air. A male soon appears under these circumstances,
and copulation is effected as readily as though the insect were not
fastened in the way indicated.


SMERINTHUS TILIÆ, LINN.[97]

The light green eggs are nearly spherical, and after fourteen days
(beginning of July) the young larvæ emerge. These are also of a light
green colour, and are conspicuous for the great length of the caudal
horn, which is nearly half as long as the body. This horn is likewise
of a light green at first, but becomes dark violet in the course of an
hour. No trace of any markings can be detected at this stage.

As soon as the caterpillars are hatched they commence to nibble the
empty egg shells; then they run about with great activity, and after
several hours take up their position on the largest vein on the under
side of the lime leaves, where they remain for a long period. In this
situation they have the same form and colour as the leaf-vein, and
are very difficult to discover, which would not be the case if they
reposed obliquely or transversely to the vein. In about 4-5 days the
caterpillars undergo their first moult, and enter upon the second
stage. On each side of the segments 11-4, there now appear seven
oblique whitish stripes on a somewhat darker green ground; these
slope in the direction of the caudal horn. Owing to the transparency
of the skin, a dark green dorsal line appears in the position of the
underlying dorsal vessel, the green contents of the alimentary canal
being distinctly visible through the absence of adipose matter in the
tissues. The larvæ possess also a fine whitish subdorsal line, which
extends from the horn to the head. The horn at this stage becomes black
with a yellowish red base.

In the third stage, which occurs after six or seven days, the oblique
stripes appear darker, and the subdorsal line disappears.


_Fourth Stage._

After another period of 4-5 days the third moult takes place, and there
now commences a dimorphism which will perhaps be better designated
as variability, since the two extremes are connected by transitional
forms. The majority of the larvæ have, as in the preceding stage, pure
white oblique stripes, but many of them possess a blood-red spot on
the anterior side of the stripes, this spot showing all gradations
in size and depth of colour between maximum development and a mere
trace. Special interest attaches to these spots, as they are the first
rudiments of the coloured border of the oblique stripes which occurs in
so many _Sphinx_ caterpillars.

In the fifth stage--the last of the larval development--the red spots
become more strongly pronounced. Among eighty caterpillars from one
brood there were about twenty without any red whilst the remainder
were ornamented with more or less vivid blood-red spots, often large
and irregular in form. In some specimens the spots had become drawn
out into lines,[98] forming a coloured edge to the oblique white
stripes, similar to that possessed by the larva of _Sphinx Ligustri_.
The caterpillar is thus represented in many figures, but generally
the coloured stripe is made too regular, as in reality it is always
irregularly defined above, and never so sharp and even as in _Sphinx
Ligustri_. The character is here obviously not yet perfected, but is
still in a state of development.


SMERINTHUS POPULI, LINN.

From green spherical eggs there emerged larvæ 6.5 millimeters in length
without any markings. They were of a light greenish-white, the large
head and long caudal horn being of the same colour. The posterior
boundary of the segments appears as a light shining ring (Pl. VI. Fig.
55).

The characteristic markings of the genus appear on the following day
without the occurrence of any moult: seven oblique white stripes arise
from near the dorsal line, and extend along the sides in a direction
parallel to that of the horn. On the three front segments they are
represented only by three small white spots (Fig. 56). The caterpillar
likewise possesses a marking of which the adult species of the genus
retain only a trace, viz., a well-developed, pure white subdorsal line,
which is crossed by the six anterior oblique stripes, and uniting with
the upper part of the seventh extends to the caudal horn.

I long believed that the markings described were first acquired in
the second stage, as I was possessed with the generally accepted idea
that the changes of form and colour in insects could only occur at the
period of ecdysis. I at first thought that the moult had escaped my
notice, and I was only undeceived by close observation of individual
specimens.


_Second Stage._

The first moult took place after five days, the larvæ being 1.4
centimeters in length. Only unimportant changes of marking are
connected therewith. The subdorsal line loses much in thickness and
definition, and the first and last of the oblique stripes become
considerably broader than the intermediate ones (Fig. 57). The green
ground colour and also the stripes acquire a yellowish hue.

On the other hand, there occur changes in form. The head, which was at
first rounded, becomes of the characteristic triangular shape, with the
apex upwards, common to all the species of the genus, and at the same
time acquires two white lines, which unite above at the apex of the
angle. The shagreening of the skin now also takes place, and the red
spot at the base of the horn is formed.

There appears to be at this stage a general tendency for the suffusion
of red, the thoracic legs also becoming of this colour.


_Third Stage._

The second moult occurs after six or eight days, the marking only
changing to the extent of the subdorsal line becoming still more
indistinct. This line can now only be distinctly recognized on the
three front segments in a few individuals, whilst in the majority it is
completely absent. Sometimes the ferruginous red spots on the oblique
stripes now appear, but this character is not completely developed till
the fifth stage. Out of about ninety bred specimens in which I followed
the entire development, only one possessed such spots, and these were
situated on both sides of the sixth segment.


_Fourth Stage._

The third moult, which takes place after another period of six days, is
not associated with any change of marking.

In this stage also I observed in one specimen (not the one just
mentioned) the ferruginous spots, and again only on the sixth segment.
On account of the theoretical conclusions which may be drawn from this
localization of the spots--supposing it to be of general occurrence--it
becomes of importance to institute observations with different broods,
so as to investigate their first appearance, frequency, and local
limitation. It appears to me very probable that, with respect to
frequency and time of appearance, there would be great differences,
since, in the last stage, it is just this character which shows a great
variability. It would be more remarkable if it should be established
that the first appearance of the spots was always limited to a certain
segment; and there would then be a great analogy with the first
appearance of the eye-spots in _Chærocampa_ and the ring-spots in
_Deilephila_.


_Fifth Stage._

The adult caterpillar does not differ in marking to any considerable
extent from the preceding stages. The first and last stripes do not
appear larger than the intermediate ones, as the latter now increase
in size. Many specimens were entirely without red spots; in others
they were present, but were small and inconspicuous, whilst in others
again there were two spots, one above the other, of a vivid ferruginous
red, these coalescing in some cases, and thus forming one spot of a
considerable size. I have never seen these spots formed into a regular,
linear, coloured border to the white oblique stripes--as occasionally
happens in _Tiliæ_--either in living specimens, blown larvæ, or in
figures.


SMERINTHUS OCELLATUS, LINN.

The green eggs much resemble those of _Populi_, as also do the newly
hatched caterpillars, which, as in the case of this last species, are
entirely without markings. As with _Populi_, the markings are formed in
the course of the first stage, and are distinctly visible before the
first moult. The long caudal horn is of a red colour.

After two to three days the caterpillars moult, their length then being
one centimeter; the seven beautiful oblique white stripes, and the fine
white subdorsal line, are more strongly pronounced, the latter becoming
broader in front. They differ from _Populi_ in having the oblique
stripes united in the dorsal line.

The second moult occurs after another three days, and brings no
important change; only the fine subdorsal line becoming somewhat
fainter. Neither is the third moult, which takes place four days later,
associated with the appearance of any essentially new character. The
oblique stripes remain as before, but their upper portions now stand
on a somewhat darker green ground-colour, whilst the subdorsal line
vanishes, leaving distinct traces only on the three or four front
segments.

The fourth moult follows after a period of seven days, and my bred
larvæ underwent scarcely any alteration in marking. Only small
differences in coloration became perceptible in the head and horn,
these changing to bluish. Specimens occur, although but rarely, which
show in this last stage red spots in the vicinity of the oblique
stripes, just in the same manner as with _Populi_, in which species,
however, they occur more commonly. I only once found an adult larva of
_Ocellatus_ possessing reddish-brown spots above and below the oblique
stripes,[99] exactly as in one of the specimens figured by Rösel.[100]

In this stage also there remains almost always on the three to six
front segments, a more or less distinct residue of the subdorsal, which
extends backwards from the head as a whitish line intersecting the
foremost oblique stripes. (Fig. 70, Pl. VII.)


RESULTS OF THE DEVELOPMENTAL HISTORY OF SMERINTHUS TILIÆ, POPULI AND
OCELLATUS.

From the meagre materials furnished by these three obviously nearly
related species, we may at least conclude that, with respect to
marking, three stages of development can be distinguished:--(1) Simple
(green) coloration without marking; (2) subdorsal lines crossed by
seven pairs of oblique stripes; (3) more or less complete absence of
the subdorsal lines, the oblique stripes remaining, and showing a
tendency to become edged with a red border.

Which of the three species is the oldest I will not attempt to decide.
If we might venture to form any conclusion from the frequency of the
red spots, _Tiliæ_ would be the youngest, _i.e._, the species which
has made the farthest advance. But this does not agree with the fact
that the oblique stripes appear somewhat later in this species. Both
these distinctions are, however, too unimportant to enable us to
build certain conclusions on them. Neither does a comparison of the
adult larvæ with other species of _Smerinthus_ furnish any further
information of importance.

Of the genus _Smerinthus_, Latr., thirty species were catalogued by
Gray,[101] of which I am only acquainted with the larvæ of eight (five
European, and three North American). None of these in the last stage
possess a complete subdorsal line together with oblique stripes.
Neither, on the other hand, do any of them show a more advanced stage
of development in having the red spots constantly formed into coloured
border-stripes. We must therefore admit that they have all reached
nearly the same stage of phyletic development. On turning to the
doubtfully placed genus _Calymnia_, Boisduval, which is represented in
Gray by only one species, figured by Westwood[102] as a _Smerinthus_,
we first meet with an older stage of development of the genus.

The adult caterpillar of _C. Panopus_, from the East Indies, possesses,
in addition to the oblique stripes, a completely developed subdorsal
line,[103] and thus corresponds to the first stage of _S. Populi_.
This species may possibly retain in its ontogeny a stage in which
the oblique stripes are also absent, whilst the subdorsal line is
present. From the early disappearance of the subdorsal line in the
species of _Smerinthus_, we may venture to conclude that this character
appeared at an early stage of the phylogeny, whilst the oblique stripes
represent a secondary form of marking, as shall be further established
subsequently.[104]


THE GENUS MACROGLOSSA, OCHSENHEIMER.

The adult larvæ of five species are known, and to these I can now
add a sixth. In Gray the genus contains twenty-six species.[105] I
cannot find any figures or descriptions of the young stages of these
caterpillars, and I have myself only observed the complete ontogeny of
one species.

By placing a captured female _M. Stellatarum_ in a capacious
breeding-cage, in the open air, I was enabled to procure eggs. The
moth hovered about over the flowers, and laid its small, grass-green,
spherical eggs (partly when on the wing), singly, on the leaves, buds,
and stalks of _Galium Mollugo_. Altogether 130 were obtained in three
days.[106]


_First Stage._

After about eight days the caterpillars emerge. They are only two
millimeters in length, and are at first yellowish, but soon become
green, set with small single bristles, and they possess a short
greenish caudal horn, which afterwards becomes black. The head is
greenish-yellow. The young larvæ are entirely destitute of marking.
(Pl. III., Fig. 1).


_Second Stage._

The first moult takes place after four days, the caterpillar now
acquiring the marking which it essentially retains to pupation.

Fine white subdorsal and spiracular lines appear, and at the same time
a dark green dorsal line, which, however, does not arise from the
deposition of pigment, as is generally the case, but from a division in
the folds of the fatty tissue along this position. (Fig. 2, Pl. III.)

The colour is now dirty green in all specimens, the skin being finely
shagreened.


_Third Stage._

The second moult, occurring after another period of four days, does not
bring any change of marking, the colour only becoming somewhat darker.
Length, twelve millimeters.


_Fourth Stage._

The third moult (after another four days) likewise brings only a
change of colouring, which is of such a nature that the caterpillar
becomes dimorphic. At the same time that peculiar roughening of the
skin takes place which, in the case of _Chærocampa_, was designated as
“shagreening.” The colour is now light grass-green in some specimens,
and dark green in others; in these last the subdorsal line is edged
above with dark brown, and the spiracles are also of this colour.
Length, seventeen millimeters.


_Fifth Stage._

Four days later, after the fourth ecdysis, the dimorphism becomes a
polymorphism. Five chief types can be distinguished:--

_Variety I._--Light green (Fig. 7, Pl. III.); dorsal line,
blackish-green, strongly marked; subdorsal line broad, pure white,
edged above with dark green; spiracular line, chrome-yellow; horn,
black, with yellow tip and blue sides. Spiracles, blackish-brown, with
narrow yellow border; legs, and extremities of prolegs, vermilion-red.

_Variety II._--Blackish-brown (Fig. 6, Pl. III.); head and prothorax,
yellowish-brown; markings the same as above.

_Variety III._--Blackish-green or greenish-black (Figs. 10 and
11, Pl. III.); subdorsal line with blackish-green border above,
gradually passing into a light green ground-colour; spiracular line,
chrome-yellow; head and prothorax, greenish-yellow.

_Variety IV._--Light green (Figs. 4 and 12, Pl. III.); dorsal line
quite feeble; subdorsal broad, only faintly edged with dark green;
subspiracular line, faint yellowish; head and prothorax, green.

_Variety V._--Brownish-violet (Fig. 8, Pl. III.); the black dorsal line
on a reddish ground either narrow or broad.

From these five varieties we see that the different types do not
stand immediately next to one another; they are, in fact, connected
by numerous transitional forms, the ground-colour varying greatly,
being dark or light, yellowish or bluish. (Compare Figs. 4, 5, 7, and
12.) The markings remain the same in all, but may be of very different
intensities. The dorsal line is often only very feebly indicated, and
the subdorsal line is frequently but faintly edged; the latter is
also sometimes deep black above and bordered rather darkly beneath,
the sides then being of a dark green, often with blackish dots on
the yellow spiracular line (Fig. 5, Pl. III.), this likewise being
frequently edged with black. Only the horn and legs are alike in all
forms. The green ground-colour passes into blackish-green, greenish or
brownish-black, and again, from reddish-brown to lilac (Fig. 3), this
last being the rarest colour.

The designation “polymorphism” may here appear very inapplicable,
since we have no sharply distinct forms, but five very variable
ground-colours connected by numerous intermediate modes of coloration.
Should, however, the term “variability” be suggested, I am in
possession of an observation which tends to show that the different
colours have to a certain extent become fixed. I found a brown
caterpillar, the five front segments of which were light green on
the left side, and the fifth segment brown and green mixed (Fig. 9,
Pl. III.). Such parti-coloration can evidently only appear where we
have contending characters which cannot become combined; just as in
the case of hermaphrodite bees, where one half of a segment is male
and the other half female, the two characters never becoming fused so
as to produce a truly intermediate form.[107] From this observation,
I conclude that some of the chief varieties of _Stellatarum_ have
already become so far removed from one another that they must be
regarded as intermediate fixed forms, the colours of which no longer
become fused together when they occur in one individual, but are
developed in adjacent regions. Other facts agree with this conclusion.
Thus, among the 140 adult larvæ which I bred from the batch of eggs
above mentioned, the transition forms were much in the minority. There
were forty-nine green and sixty-three brown caterpillars, whilst only
twenty-eight were more or less transitional.

On these grounds I designate the phenomenon as “polymorphism,” although
it may not yet have reached, as such, its sharpest limits. This would
be brought about by the elimination of the intermediate forms.[108]

Immediately before pupation, all the caterpillars, both green and
brown, acquire a lilac coloration. The fifth stage lasts seven days,
and the whole larval development twenty-three days, the period from
the deposition of the eggs to the appearance of the moth being only
thirty-one days.

I have treated of the polymorphism of _Stellatarum_ in detail, not
only because it has hitherto remained unknown, and an analysis of such
cases has been completely ignored,[109] but more particularly because,
it appears to me, that important conclusions can be drawn therefrom.
Moreover, such an extreme multiplicity of forms is interesting, since,
so far as I know, polymorphism to this extent has not been observed in
any insect.

The theoretical bearing of this polymorphism will be treated of
subsequently. It is not in any way connected with a more advanced
development of the markings, since _M. Stellatarum_ shows in this
respect a very low state of development. This species displays only
two stages:--(1), complete absence of all markings; and (2), a simple
subdorsal, with dorsal and spiracular lines. We must therefore admit
that the phyletic development of the markings has for a long time
remained at a standstill, or, what expresses the same thing, that the
marking which the adult larva now possesses is extremely old.

In order to complete my observations on _M. Stellatarum_, I now add
some remarks on the pupa, the colour variations of which it appeared
of importance to investigate, owing to the extraordinary variability
of the caterpillar. The pupa varies but very slightly; the ochreous
yellow ground-colour sometimes passes into reddish, and sometimes
into greenish; the rather complicated blackish-brown marking of
streaky lines is very constant, especially on the wing portions, being
at most only more or less strongly pronounced. The minute colour
variations of the pupa therefore have no connection with the colour
of the caterpillar, both green and brown larvæ furnishing sometimes
reddish-yellow and sometimes greenish-yellow pupæ.

The comparison of _M. Stellatarum_ with the other known species of the
genus, brings scarcely any addition to our knowledge of the phyletic
development. Thus, the two European species of which the caterpillars
are known, viz. _M. Fuciformis_ and _Bombyliformis_,[110] show
essentially the same markings as _Stellatarum_, the chief element being
a well-developed subdorsal line. The Indian _M. Gilia_, Herrich-Schäf.,
possesses also this line,[111] and, together with the East Indian _M.
Corythus_, Walk.,[112] has oblique stripes in addition; the stripes
do not, however, cross this line, but commence underneath it, and
probably originated at a later period than the subdorsal line. Should
this be the case, we must regard _M. Corythus_ as representing a
later phyletic stage. According to Duponchel’s figures, in both _M.
Fuciformis_ and _Bombyliformis_ small oblique stripes (red) occur near
the spiracles, but these have nothing to do with the oblique stripes
of _M. Gilia_ just mentioned, as they run in a contrary direction. Of
the two European species, I have only seen the living caterpillar of
_Fuciformis_, and this possessed no oblique stripes.

To these five species I am now enabled to add a sixth, viz.
_Macroglossa Croatica_,[113] a species inhabiting Asia Minor and
Eastern Europe, of which a specimen and notice were kindly forwarded
to me by Dr. Staudinger. The adult caterpillar much resembles that of
_M. Stellatarum_ in form and marking, but the subdorsal line appears
much less distinctly defined, and the dorsal and spiracular lines seem
to be entirely absent. The colour is generally green, but varies to
red, and the subdorsal is more distinct and sharper in the young than
in the adult larva. The markings of this species do not therefore in
any way surpass those of _Stellatarum_, but are, on the contrary, much
simpler.[114]


THE GENUS PTEROGON, BOISD.[115]

Although I am acquainted with only a small portion of the developmental
history of a single species of this genus, I will here proceed to
record this fragment, since, taken in connection with two other
species, it appears to me sufficient to determine, at least broadly,
the direction of development which this genus has taken.


PTEROGON ŒNOTHERÆ, FABR.

The adult larva, as made known by many, and for the most part good
figures, has very complicated markings, which do not seem derivable
from any of the elements of marking in the _Sphingidæ_ hitherto
considered. I was therefore much surprised at finding a young
caterpillar of this species, only twelve millimeters in length, of
a light green colour, without any trace of the subsequent latticed
marking, and with a broad white subdorsal line extending along all
the twelve segments. (Pl. VII., Fig. 63). Judging from the size and
subsequent development, this caterpillar was probably in the third
stage.

The same colouring and marking remained during the following (fourth)
stage; but in the position occupied by the caudal horn in other
_Sphingidæ_, there could now be observed the rudiment of a future
ocellus in the form of a round yellowish spot (Pl. VII., Fig. 64). The
subdorsal line disappears suddenly in the fifth stage, when the larva
becomes dark green (rarely) or blackish-brown; the latticed marking
and the small oblique stripes are also acquired, together with the
beautifully developed eye-spots, consisting of a yellow mirror with
black nucleus and ground-area (Pl. VII., Fig. 65).

The North American _Pterogon Gauræ_ and _P. Abboti_[116] also show
markings precisely similar to those of this European species in the
adult state; but in the two former the markings are of special interest
as indicating the manner in which the primary Sphinx-marking has become
transformed into that of the apparently totally different adult _P.
Œnotheræ_. _P. Gauræ_ is green, with a complicated latticed marking,
which closer observation shows to arise from the dorsal line being
resolved into small black dots, whilst the subdorsal line is broken up
into black, white-bordered triangles. This caterpillar therefore gives
fresh support to the remarkable phenomenon that the animals as well as
the plants of North America are phyletically older than the European
fauna and flora, a view which also appeared similarly confirmed by
_Deilephila Lineata_, the representative form of _D. Livornica_. In
entire accordance with this is the fact that the larva of _P. Gauræ_ is
without the eye-spot on the eleventh segment, and instead thereof still
shows the original although small caudal horn. The perfect insect also
resembles our _P. Œnotheræ_ in colour and marking, but not in the form
of the wings.

That the caterpillars of the genus _Pterogon_ originally possessed the
caudal horn we learn from _P. Gorgoniades_, Hübn.,[117] a species
now inhabiting south-east Russia, and for a knowledge of which I am
indebted to Dr. Staudinger’s collection. There are in this about
eight blown specimens, from 3.7 to 3.9 centimeters in length, which
show a marking, sometimes on a red and sometimes on a green ground,
which unites this species with the young form of _P. Œnotheræ_, viz.,
a broad white subdorsal line, extending from the small caudal horn
to the head. In addition to this, however, the caterpillar possesses
an extraordinarily broad white red-bordered infra-spiracular line, a
fine white dorsal stripe, and a similar line between the subdorsal and
spiracular, _i.e._ a supra-spiracular line.

The caterpillars in Staudinger’s collection, notwithstanding their
small size, all belong to the last stage, as the moth itself does not
measure more than 2.6 centimeters in expanse, and is therefore among
the smallest of the known _Sphingidæ_. This species has therefore in
the adult condition a marking very similar to that of _Œnotheræ_ when
young--it bears to _Œnotheræ_ the same relationship that _Deilephila
Hippophaës_ does to _D. Euphorbiæ_, only in the present case the
interval between the two species is greater. _Gorgoniades_ is obviously
a phyletically older species, as we perceive from the marking and
from the possession of a horn. We certainly do not yet know whether
_Œnotheræ_ possesses a horn in its earliest stages, although in all
probability it does so; in any case the ancestor of _Œnotheræ_ had a
horn, since the closely allied _P. Gauræ_ now possesses one.

We thus see that also in the genus _Pterogon_ the marking of the
caterpillars commences with a longitudinal line formed from the
subdorsal; an infra-spiracular or also a supra-spiracular line
(_Gorgoniades_) being added. A latticed marking is developed from the
linear marking by the breaking up of the latter into spots or small
patches, which finally (in _Œnotheræ_) become completely independent,
their connection with the linear marking being no longer directly
perceptible.


THE GENUS SPHINX, LINN.

Of this genus (in the narrow sense employed by Gray) I have only been
able, in spite of all trouble, to obtain fertile eggs of one species.
The females cannot be induced to lay in confinement, and eggs can only
be obtained by chance.

I long searched in vain the literature of this subject for some account
of the young stages of these caterpillars, and at length found, in a
note to Rösel’s work, an observation of Kleemann’s on the young forms
of _Sphinx Ligustri_, which, although far from complete, throws light
on certain points.

From a female of _S. Ligustri_ Kleemann obtained 400 fertile eggs. The
caterpillars on emerging are “at first entirely light yellowish-green,
but become greener after feeding on the fresh leaves;” the horn is also
at first light green, and then becomes “darker.” The young larvæ spin
webs, by which they fasten themselves to the leaves of their food-plant
(this, so far as I know, has not been observed in any species of
_Sphingidæ_). They moult four times, the border round the head and the
purple stripes appearing after the third moult, these stripes “having
previously been entirely white.” The ecdyses follow at intervals
of about six days, increasing to about ten days after the fourth
moult.[118]

From this short account we gather that in the third stage the marking
consists of seven oblique white stripes, which acquire coloured edges
in the fourth stage, a fact which I have myself frequently observed.
On the most important point Kleemann’s observations unfortunately give
no information--the presence or absence of a subdorsal line in the
youngest stages. That he does not mention this character, can in no way
be considered as a proof of its actual absence. I am rather inclined to
believe that it is present in the first, and perhaps also in the second
stage. There occur, however, species of the genus _Sphinx_ (_sensû
strictiori_) which possess a subdorsal line when young, as I think may
be certainly inferred from the fact that the remains of such a line
are present in the adult larva of _S. Convolvuli_.

This conclusion becomes still more certain on comparing the markings
with those of a nearly allied genus; without such comparison the
separation of the genus _Macrosila_, Boisd., from _Sphinx_ is scarcely
justifiable. If to these two genera we add _Dolba_, Walk., and
_Acherontia_, Ochs., we must be principally struck with the great
similarity in the markings, which often reaches to such an extent that
the differences between two species consist entirely in small shades of
colour, while the divergence of the moths is far greater.

Of the genera mentioned, I am acquainted altogether with fourteen
species of caterpillars:--_Macrosila Hasdrubal_, _Rustica_,[119] and
_Cingulata_;[119] _Sphinx Convolvuli_, _Ligustri_, _Carolina_,[119]
_Quinquemaculata_,[119] _Drupiferarum_,[119] _Kalmiæ_,[119] and
_Gordius_;[119] _Dolba Hylæus_;[119] _Acherontia Atropos_, _Styx_,[120]
and _Satanas_.[120] With one exception all these caterpillars possess
oblique stripes of the nature of those of the _Smerinthus_ larvæ,
and most of them are without any trace of a subdorsal line; one
species--the North American _M. Cingulata_--has a completely developed
subdorsal; and the typical European species, _S. Convolvuli_, has a
rudimentary subdorsal line. The ground-colour in most of these species
is of the same green as that of the leaves of their food-plants; some
are brown, _i.e._ earth-coloured, and in these the markings do not
appear so prominently; others again possess very striking colours (_A.
Atropos_), the oblique stripes in these cases being very vivid. Only
_M. Hasdrubal_[121] separates itself completely from this system of
classification, since this species is deep black with narrow yellow
rings, the horn and last segment being red.

The large and most striking caterpillar of _M. Hasdrubal_ is the same
which Wallace has made use of for his theory of the brilliant colours
of caterpillars. The explanation of the origin of this widely divergent
mode of marking could only be furnished by the ontogeny, in which
one or another of the older phyletic stages will certainly have been
preserved.

Strictly speaking the same should be said of the other
species--nevertheless their comparison with the so similarly marked
_Smerinthinæ_, together with the circumstance that in certain species
a subdorsal line can be traced, makes it appear correct to suppose
that here also the subdorsal was the primary marking, this line being
subsequently entirely replaced by the oblique stripes. The _Sphinginæ_
would therefore be a younger group than the _Smerinthinæ_, a conclusion
which is borne out by the fact that in the former the oblique stripes
have reached a higher development, being always of two, and sometimes
even of three colours (_S. Drupiferarum_, white, red, black), whilst in
the species of _Smerinthus_ they only occasionally possess uniformly
coloured borders.


THE GENUS ANCERYX, BOISD.

Although this genus is not admitted into most of the European
catalogues--the solitary European species representing it being
referred to the genus _Sphinx_, Linn.[122]--its separation from
_Sphinx_ appears to me to be justified, not because of the striking
differences presented by the moths, but because the caterpillars,
judging from the little we know of them, likewise show a similar degree
of difference.

I have frequently succeeded in obtaining fertile eggs of _Anceryx
Pinastri_ and I will now give the developmental history of this
caterpillar, which has already been figured with great accuracy in
Ratzeburg’s excellent work on forest insects. Rösel was acquainted
with the fact that the “pine moth” laid its eggs singly on the needles
of the pine in June and July, and he described them as “yellowish,
shining, oval, and of the size of a millet seed.”

On emerging, the caterpillars are six millimeters in length, of a light
yellow colour, the head shining black with a yellow clypeus. The caudal
horn, which is forked at the tip, is also at first yellowish, but soon
becomes black. No particular marking is as yet present, but a reddish
stripe extends along the region of the dorsal vessel, and the course of
the spiracles is also marked by an orange-red line. (Fig. 53, A & B,
Pl. VI.)

As soon as the young larvæ are filled with food they acquire a greenish
streak. The first moult occurs after four days, and immediately after
this there is still an absence of distinct markings, with the exception
of a greenish-white spiracular line. In the course of some hours,
however, the original light green ground-colour becomes darker, and at
the same time a sharp, greenish-white subdorsal line appears, together
with a parallel line extending above the spiracles, which, in _Pterogon
Gorgoniades_, has already been designated as the “supra-spiracular.”
The dorsal line is absent: the head is light green, with two narrow
blackish-brown lines surrounding the clypeus; the horn and thoracic
legs are black; claspers, reddish green; length, twelve to thirteen
millimeters. (Fig. 54.)


_Third Stage._

After another period of four days the second moult occurs, neither
colour nor marking being thereby affected. Only the horn, now no longer
forked, becomes brownish with a black tip. The young caterpillars
are now, as before, admirably adapted to the pine needles, on which
they feed by day, and from which they can only be distinguished with
difficulty.


_Fourth Stage._

The third moult also brings no essential change. The ground-colour and
marking remain the same, only the spiracles, which were formerly dull
yellowish, are now of a vivid brick-red. The horn becomes yellowish-red
at the base.


_Fifth Stage._

The marking is only completely changed in the fifth and last stage.
A broad reddish-brown dorsal line replaces the subdorsal, more or
less completely. The supra-spiracular line also becomes broken up
into numerous short lengths, whilst the green ground-colour in some
specimens becomes more or less replaced by a brownish shade extending
from the back to the sides. Horn, black; the upper part of the first
segment with a corneous plate, similar to that of the _Deilephila_
larvæ.

This stage is very variable, as shown by the figures in various works.
The variations arise on the one hand from the struggle between the
green ground-colour and the reddish-brown extending from above, and,
on the other hand, from a more or less complete disappearance of the
associated longitudinal lines. The latter are sometimes completely
retained, this being the case in a caterpillar figured by Hübner
(_Sphinges_, III., _Legitimæ_ C, b), where both the subdorsal and
supra-spiracular lines are continuous from segment 11 to segment 1, an
instance which may perhaps be regarded as a reversion to the primary
form.

The entire change of the marking from the fourth to the fifth stage
depends upon the fact that the young larvæ resemble the _needles_ of
the pine, whilst the adults are adapted to the _branches_. I shall
return to this later.

The ontogeny of _A. Pinastri_ makes us acquainted with three different
forms of marking: (1) simple coloration without marking; (2) a
marking composed of three pairs of parallel longitudinal lines; (3) a
complicated marking, arising from the breaking up of the last and the
addition of a darker dorsal line.

Of the fourteen species placed by Gray in the genus _Anceryx_, I find,
in addition to the one described, notices of only two caterpillars:--

_A. Coniferarum_,[123] a North American species, lives on _Pinus
Palustris_, and was figured by Abbot and Smith. Colour and marking very
similar to _A. Pinastri_.

_A. Ello_, Linn.,[124] according to the authority of Mérian, is
described by Clemens[125] as dark brown, “with a white dorsal line,
and irregular white spots on the sides.” It lives on a “species of
_Psidium_ or _Guava_.”

Most of the species of _Anceryx_ appear to live on _Coniferæ_, to which
they show a general and decided adaptation. In the absence of decisive
information, I partly infer this from the names, as _Anceryx Juniperi_
(Africa). It has long been known that in our _A. Pinastri_ the mixture
of brown and fir-green, interspersed with conspicuous irregular light
yellowish and white spots, causes the adult larva to present a very
perfect adaptation to its environment. Of this caterpillar Rösel
states:--“After eating it remains motionless, and is then difficult
to see, because it is of the same colour as its food, since its brown
dorsal line has almost the colour of the pine twigs; and who is not
familiar with the fact that beneath the green needles there is also
much yellow to be found?”

This adaptation to the needles and twigs obviously explains why this
caterpillar in the adult condition is so far removed from those of the
genus _Sphinx_, while the moths are so nearly related that they were
only separated as a distinct genus when we became acquainted with a
large number of species.



II.

CONCLUSIONS FROM PHYLOGENY.


The considerations previously set forth are entirely based on Fritz
Müller’s and Haeckel’s view, that the development of the individual
presents the ancestral history _in nuce_, the ontogeny being a
condensed recapitulation of the phylogeny.

Although this law is generally true--all recent investigations
on development having given it fresh confirmation--it must not
be forgotten that this “recapitulation” is not only considerably
abbreviated, but may also be “falsified,” so that a searching
examination into each particular case is very desirable.

The question thus arises, in the first place, as to whether the
markings of caterpillars, so distinct at the different stages of
growth, are actually to be regarded as residual markings inherited
from the parent-form; or whether their differences do not depend upon
the fact that the caterpillar, in the course of growth, is exposed to
different external conditions of life, to which it has adapted itself
by assuming a different guise.

The former is undoubtedly the case. It can by no means be denied that
the conditions of life in young caterpillars are _sometimes_ different
to those of the adults. It will, in fact, be shown later on, that in
certain cases the assumption of a new guise at an advanced age actually
depends upon adaptation to new conditions of life; but as a rule, the
external conditions remain very similar during the development of the
larva, as follows from the fact that a change of food-plant never takes
place.[126] We should therefore rather expect a complete similarity
of marking throughout the entire larval period, instead of the great
differences which we actually observe.

Different circumstances appear to me to show that the markings of
young larvæ are only exceptionally due to a new adaptation, but that
as a rule they depend upon heredity. In the first place, there is
the fact that closely allied species, exposed to precisely similar
external conditions, as, for instance, _Chærocampa Elpenor_ and
_Porcellus_, possess exactly the same markings when young, these
markings nevertheless appearing at different stages of growth. Thus,
the subdorsal line first appears in _Elpenor_ in the second stage,
whilst in _Porcellus_ it is present during the first stage. If this
line were acquired by the young larva for adapting it at this age to
special conditions of life, it should appear in both species at the
same stage. Since this is not the case, we may conclude that it is
only an inherited character derived from the adult ancestor of the two
species, and now relegated to the young stages, being (so to speak),
pushed further back in one species than in the other.

But the strongest, and, as it appears to me, the most convincing proof
of the purely phyletic significance of the young larval markings, is
to be found in the striking regularity with which these are developed
in a similar manner in all allied species, howsoever different may
be their external conditions of life. In all the species of the
_Chærocampa_ group (the genera _Chærocampa_ and _Deilephila_) the
marking--no matter how different this may be in later stages--arises
from the simple subdorsal line. This occurs even in species which live
on the most diverse plants, and in which the markings can be of no
biological importance as long as the larvæ are so small as to be only
visible through a lens, and where there can be no possible imitation
of leaf-stalks or veins, the leaves and caterpillars being so very
distinct.

Moreover, when in the _Macroglossinæ_ (the genera _Macroglossa_,
_Pterogon_, and _Thyreus_) we see precisely the same simple marking
(the subdorsal) line retained throughout all the stages in two genera,
whilst in the _Smerinthinæ_ this line vanishes at a very early stage,
and in the _Sphinginæ_ is only present in traces, we can give but
one explanation of these facts. We have here a fragmentary series
representing the phyletic development of the Sphinx-markings, which
latter have arisen from one original plan--the simple subdorsal
line--and have then undergone further development in various
directions. As this subsequent development advanced, the older phyletic
stages would always be relegated to younger ontogenetic stages, until
finally they would be but feebly represented even in the youngest stage
(_D. Euphorbiæ_), or else entirely eliminated (most of the species of
the genus _Sphinx_). I believe that no other sufficient explanation of
these facts can be adduced. Granting that the correctness of the above
views can no longer be doubted, we may now take up the certain position
that the ontogeny of larval markings reveals their phylogeny, more or
less completely, according to the number of phyletic stages omitted,
or, in some exceptional cases, falsified. In other words, the ontogeny
of larval markings is a more or less condensed and occasionally
falsified recapitulation of the phylogeny.

Considering this to be established, we have next to deal with the
uniformity of the developmental phenomena, from which we may then
attempt to trace out the inciting causes underlying this development.

The law, or, perhaps better, the line of direction followed by the
development, is essentially the following:--

1. The development commences with a state of simplicity, and advances
gradually to one of complexity.

2. New characters first make their appearance in the last stage of the
ontogeny.

3. Such characters then become gradually carried back to the earlier
ontogenetic stages, thus displacing the older characters, until the
latter disappear completely.

The first of these laws appears almost self-evident. Whenever we
speak of development, we conceive a progression from the simple to
the complex. This result therefore does nothing but confirm the
observation, that we have actually here before us a development in
the true sense of the word, and not simply a succession of different
independent conditions.

The two following laws, on the other hand, lay claim to a greater
importance. They are not now enunciated for the first time, but were
deduced some years ago by Würtemberger[127] from a study of the
ammonites. In this case also the new characters predominate in the
later periods of life, and are then transferred back to the younger
ontogenetic stages in the course of phyletic development. “The change
in the character of the shell in ammonites, first makes itself
conspicuous in the last chamber; but in the succeeding generations
this change continually recedes towards the beginning of the spiral
chambers, until it prevails throughout the greater part of the
convolutions.”

In the same sense must also be conceived the case which Neumayr and
Paul have recently made known respecting certain forms of _Melanopsis_
from the West Sclavonian _Paludina_ bed. In _M. Recurrens_ the last
convolutions of the shell are smooth, this being a new character; the
small upper convolutions, however, are delicately ribbed, as is also
the case with the last convolution of the immediate progenitor. The
embryonic convolutions again are smooth, and the author believes (on
other grounds) that the more remote progenitor possessed a smooth shell.

In this case therefore, and in that of the ammonites, every shell
to a certain extent proclaims the ancestral history of the species;
in one and the same shell we find different phyletic stages brought
into proximity. The markings of caterpillars do not offer similar
facilities; nevertheless I believe that by their means we are led
somewhat further, and are able to enter more deeply into the causes
underlying the processes of transformation, because we can here observe
the living creature, and are thus enabled to study its life-history
with more precision than is possible with a fossil species.

When, in 1873, I received Würtemberger’s memoir, I was not only struck
with the agreement of his chief results with those which I had arrived
at by the study of larval markings, but I was almost as much astonished
at the great difference in the interpretation of the facts. The latter
indicate the gradual backward transference of a new character from the
latest to the earlier ontogenetic stages. Without further confirmation
Würtemberger assumes that it is to a certain extent self-evident
that the force producing this backward transference is the same as
that which, according to his view, first called forth the character
in question in the last stage, viz., natural selection. “Variations
acquired at an advanced age of the organism may, when advantageous,
be inherited by the succeeding generations, in such a manner that they
always appear a little earlier than in the preceding generations.”

It is certainly theoretically conceivable that a newly acquired
character, when also advantageous to the earlier stages, might be
gradually transferred to these stages, since in this case those
individuals in which this character appeared earliest would have the
greatest chance of surviving. In the case of the development of larval
markings, however, there are facts which appear to me to show that such
backward transference of a new character is, in a certain measure,
independent of the principle of utility, and that it must therefore
be referred to another cause--to the innate law of growth which rules
every organism.

When, in the larva of _C. Elpenor_, we perceive that the two eye-spots
which are first formed on the fourth and fifth segments appear
subsequently on the other segments as faint traces of no biological
value whatever, we cannot explain this phenomenon by natural selection.
We should rather say that in segmented animals there is a tendency for
similar characters to be repeated on all the segments; and this simply
amounts to the statement, that an innate law of growth is necessary for
the repetition of such newly acquired characters.

The existence of such a law of growth, acting independently of natural
selection, may therefore be considered as established, and indeed
cannot be disputed (Darwin’s “correlation of growth”). In the present
case it appears to me that an innate law of this kind, determining
the backward transference of new characters, is deducible from the
instances already quoted in another sense, viz., from the fact that in
many cases characters which are decidedly advantageous to the adult
are transferred to the younger stages, where they are at most of but
indifferent value, and can certainly be of no direct advantage. This is
the case with the oblique stripes of _Smerinthus_, which, in the adult
larvæ, resemble the leaf ribs, as will be shown more fully later on,
and, in conjunction with the green coloration, cause these caterpillars
to be very difficult of detection on their food-plants. The insects
are easily overlooked, and can only be distinctly recognized on close
inspection.

Now these oblique stripes appear, in all the _Smerinthus_ caterpillars
known to me, in the second, and sometimes even in the first stage,
_i.e._ in larvæ of from 0.7 to 1 centimeter in length. The stripes
are here much closer together than the ribs of any of the leaves of
either willow, poplar, or lime, and can therefore have no resemblance
to these leaves. The young caterpillars are certainly not rendered more
conspicuous by the oblique stripes, since they can only be recognized
on close inspection. It is for this reason that the stripes have not
been eliminated by natural selection.

The remarkable phenomenon of the backward transference of newly
acquired characters may therefore be formulated as follows:--Changes
which have arisen in the later ontogenetic stages have a tendency to
be transferred back to the younger stages in the course of phyletic
development.

The facts of development already recorded furnish numerous proofs
that this transference occurs gradually, and step by step, taking the
same course as that which led to the first establishment of the new
character in the final ontogenetic stage.

Did this law not obtain, the ontogeny would lose much of the interest
which it now possesses for us. It would then be no longer possible,
from the ontogenetic course of development of an organ or of a
character, to draw a conclusion as to its phylogeny. If, for instance,
the eye-spots of the _Chærocampa_ larvæ, which must have been acquired
at a late age, were transferred back to the younger ontogenetic stages
in the course of phyletic development, as eye-spots already perfected,
and not showing their rudimentary commencement as indentations of the
subdorsal line, the phenomenon would then give us no information as to
the manner of their formation.

It is well known to all who have studied the developmental history of
any group of animals, that no organ, or no character, however complex,
appears suddenly in the ontogeny; whereas, on the other hand, it
appears certain that new, or more advanced, but simpler characters,
predominate in the last stage of development. We are thus led to the
following modification of the foregoing conclusion:--Newly acquired
characters undergo, as a whole, backward transference, by which means
they are to a certain extent displaced from the final ontogenetic stage
by characters which appear later.

This must be a purely mechanical process, depending on that innate law
of growth, the action of which we may observe without being able to
explain fully. Under certain conditions the operation of this law may
be prevented by natural selection. Thus, for instance, if the young
caterpillars of _Anceryx Pinastri_ have not acquired the characteristic
marking of the adults, it is probably because they are better protected
by their resemblance to the green pine-needles than they would be if
they possessed the pattern of the larger caterpillars in their last
stage.

The backward transference of newly acquired characters may also
possibly be accelerated when these characters are advantageous to the
younger stages; but this transference takes place quite independently
of any advantage if the characters are of indifferent value, being then
entirely brought about by innate laws of growth.

That new characters actually predominate in the last stage of the
ontogeny, may also be demonstrated from the markings of caterpillars.
It is, of course, not hereby implied, that throughout the whole animal
kingdom new characters can only appear in the last ontogenetic stage.
Haeckel is quite correct in maintaining that the power of adaptation of
an organism is not restricted to any particular period. Under certain
circumstances transformations may occur at any period of development;
and it is precisely insects undergoing metamorphosis that prove this
point, since their larvæ differ so widely from their imagines that the
earlier stages may be completely disguised. It is here only signified
that, with respect to the development of caterpillars, new characters
first appear in the adult. The complexity of the markings, which so
frequently increases with the age of the caterpillar, can scarcely
bear any other interpretation than that the new characters were always
acquired in the last stage of the ontogeny. In certain cases we are
able, although with some uncertainty, to catch Nature in the act of
adding a new character.

I am disposed to regard the blood-red or rust-red spots which occur
in the last stage of the three species of _Smerinthus_ larvæ in the
neighbourhood of the oblique stripes as a case in point. It has
already been shown that these red spots must be regarded as the first
rudiments of the linear coloured edges which reach complete development
in the genus _Sphinx_. In some specimens of _Smerinthus Tiliæ_ the
spots coalesce so as to form an irregular coloured edge to the oblique
stripes. In _S. Populi_ they occur in many individuals, but remain
always in the spot stage; whilst _S. Ocellatus_ is but seldom, and _S.
Quercus_ appears never to be spotted.

The spots both of _S. Tiliæ_ and _Populi_ certainly do not show
themselves exclusively in the fifth (last) stage, but also in the
fourth, and sometimes in _Populi_ even as early as the third stage,
from which we might be disposed to conclude that the new character did
not first appear in the last stage. But the majority of the spotted
individuals first acquire their spots in the fifth stage, and only a
minority in the fourth; so that their occasional earlier appearance
must be ascribed to the backward transference of a character acquired
in the fifth stage. Moreover, the fourth and fifth stages of the
caterpillars are closely analogous both in size, mode of life, and
marking, and are therefore analogous with reference to the environment,
so that it is to be expected that new characters, when depending on
adaptation, would be rapidly transferred from the fifth stage to the
fourth.[128] We should thus have a case of the acceleration by natural
selection, of processes determined by innate causes. Why changes should
predominate in the last stage, is a question closely connected with
that of the causes of larval markings in general, and may therefore
be investigated later.

But if we here assume in anticipation that all new markings depend
on adaptation to the conditions of life, and arise through natural
selection, it will not be difficult to draw the conclusion that such
new characters must prevail in the last stage. There are two conditions
favouring this view; the size of the insect, and the longer duration
of the last stage. As long as the caterpillar is so small as to be
entirely covered by a leaf, it only requires a good adaptation in
colour in order to be completely hidden; independently of which, it is
also possible that many of its foes do not consider it worth attacking
at this stage. The last stage, moreover, is of considerably longer
duration than any of the four preceding ones; in _Deilephila Euphorbiæ_
this stage lasts for ten days, whilst the remaining stages have a
duration of four days; in _Sphinx Ligustri_ the last stage also extends
over ten days, and the others over six days.

In its last stage, therefore, a caterpillar is for a longer period
exposed to the danger of being discovered by its foes; and since, at
the same time, its enemies become more numerous, and its increased size
makes it more easy of detection, it is readily conceivable that a
change in the conditions of life, such, for instance, as removal to a
new food-plant, would bring about the adaptation of the _adult larva_
as its chief result.

I shall next proceed to show how far the assumption here made--that all
markings depend on natural selection--is correct.



III.

BIOLOGICAL VALUE OF MARKING IN GENERAL.


Having now described the development of larval markings, so far as
possible from their external phenomena, and having traced therefrom
the underlying law of development, I may next proceed to the main
problem--the attempt to discover those deeper inciting causes which
have produced marking in general.

The same two contingencies here present themselves as those which
relate to organic life as a whole; either the remarkably complex and
apparently incomprehensible characters to which we give the name of
markings owe their origin to the direct and indirect gradual action
of the changing conditions of life, or else they arise from causes
entirely innate in the organism itself, _i.e._ from a phyletic vital
force. I have already stated in the Introduction why the markings of
caterpillars appear to me such particularly favourable characters for
deciding this question, or, more precisely, why these characters, above
any others, appear to me to render such decision more easily possible;
repetition is here therefore unnecessary.

The whole of the present investigation had not been planned when
I joined with those who, from the first, admitted the omnipotence
of natural selection as an article of faith or scientific axiom. A
question which can only be solved by the inductive method cannot
possibly be regarded as settled, nor can further evidence be considered
unnecessary, because the first proofs favour the principle. The
admission of a mysteriously working phyletic power appears very
unsatisfactory to those who are striving after knowledge; the existence
of this power, however, is not to be considered as disproved,
because hundreds of characters can be referred to the action of
natural selection, and many others to that of the direct action of
the conditions of life. If the development of the organic world is
to be considered as absolutely dependent on the influence of the
environment, not only should we be able here and there to select at
pleasure characters which appeared the most accessible for elucidating
this point, but it becomes in the first place necessary to attempt
to completely refer _all_ characters belonging to any particular
group of phenomena, however small this group might be, to known
transforming factors. We should then see whether this were possible,
or whether there would remain residual phenomena not explicable by
known principles and compelling us to admit the existence of a force
of development innate in the organism. In any case the “phyletic vital
force” can only be got rid of by a process of elimination--by proving
that all the characters generally occurring throughout the group of
phenomena in question, must be attributed to other causes, and that
consequently nothing remains for the action of the supposed phyletic
vital force, which would in this manner be negatived, since we cannot
infer the presence of a force if the latter exerts no action whatever.

I shall here attempt such an investigation of the group of phenomena
displayed by larval markings, with special reference to those of the
_Sphingidæ_. The alternatives upon which we have to decide are the
following:--Are the markings of caterpillars purely morphological
characters, produced entirely by internal causes? or, are they simply
the response of the organism to external influences?

The solution of these questions will be arrived at by seeking to refer
all the markings present to one of the known transforming factors, and
the success or failure of this attempt will give the required decision.
The first question to be attacked is obviously this,--whether the
Sphinx-markings are actually, as they appear at first sight, purely
morphological characters. If it can be shown that all these markings
were originally of biological value, they must be attributed to the
action of natural selection.

Did I here at once proceed to establish the biological value of larval
markings--and especially of those of the _Sphingidæ_--so as to arrive
in this manner at a conclusion as to their dependence upon natural
selection, it would be impossible to avoid the consideration of the
total coloration of the caterpillars, since the marking frequently
consists only of a local strengthening of the colour, and cannot be
comprehended without coming to this understanding. The action of the
markings also often appears to be opposed to that of the colouring,
making the caterpillar again conspicuous; so that the two factors must
necessarily be considered together. I shall therefore commence the
investigation with colour in general, and then proceed to treat of
marking.



IV.

BIOLOGICAL VALUE OF COLOUR.


The general prevalence of protective colouring among caterpillars has
already been so frequently treated of that it is not here my intention
to recall particular instances. In order to judge of the effect of
marking, however, it will be well to bear in mind that these insects,
being generally defenceless and thus requiring protection, have
acquired the most diverse means of rendering themselves in some measure
secure from their foes.

The sharp spines which occur on the caterpillars of many butterflies
(_Vanessa_, _Melitæa_, _Argynnis_), and the hairs on those of
many moths, serve for protective purposes. Among other means of
protection--although in a different sense--we have in all the species
of the great family of the _Papilionidæ_ the strikingly coloured
(yellowish red) odour-emitting tentacles concealed near the head,
and suddenly protruded for terrifying foes; and likewise the forked
horn at the tail of the caterpillars of the genus of moths _Harpyia_,
the tentacles of which can be suddenly protruded in a similar
manner. Adaptive colours and forms combined with certain habits[129]
are, however, much more common than defensive weapons. Thus, the
caterpillars of the _Noctuæ_ belonging to the genus _Catocala_ and its
allies, feed only at night on the green leaves of various forest-trees;
by day they rest in crevices of the bark on the tree trunk, which they
resemble so perfectly in the colour of their peculiar glossy dull grey
or brownish skin beset with small humps, that only sharp eyes can
detect them, even when we are familiar with their habits.[130]

The striking resemblance of many moths to splinters of wood is well
known, and to this is added a habit which helps their disguise, viz.,
that of remaining stiff and motionless on the approach of danger,
just like a splinter projecting from the branch.[131] Among the moths
coming under this category may be mentioned _Cucullia Verbasci_, and
particularly those of the genus _Xylina_, which, when at rest, closely
resemble a broken splinter of wood in the colour and marking of their
fore wings, and when touched, have a habit of drawing in their legs and
falling without opening their wings as though dead.

That simple adaptive colouring prevails widely among caterpillars
is shown by the large number of green species.[132] It may be fairly
said that all caterpillars which possess no other means of protection
or defence are adaptively coloured. These facts are now well known;
so also is the explanation of the varied and striking colours of many
caterpillars given by Wallace.[133] There is, however, novelty in the
proof contained in the foregoing descriptions of larval development,
as to the manner in which the di- and polymorphism of caterpillars
can be explained from the external phenomena which they present,
these phenomena being well adapted for showing the great importance
of protective colouring to the larvæ. We have here presented a
double adaptation, although not quite of the nature of that which I
formerly admitted on hypothetical grounds.[134] In the first place,
from the developmental history there results the conclusion that all
Sphinx-larvæ which, in the adult state, are di- or polymorphic, are
unicolorous when young. Thus, the caterpillars of _Chærocampa Elpenor_
all remain green till the fourth stage, when they mostly become
light or dark brown, and only very seldom retain their green colour.
_Chærocampa Porcellus_ behaves in a precisely similar manner; as also
does _Pterogon Œnotheræ_, which inhabits the same localities, and is
found on the same food-plant, but is not very closely related to the
_Chærocampa_. In this species also (_P. Œnotheræ_) the brown is more
common than the green form in the adult state, both varieties showing a
complicated marking. The young larvæ possess only a light green colour,
and a pure white subdorsal line as the only marking; they are so well
adapted to the leaves of their food-plants, _Epilobium Hirsutum_,
and _E. Rosmarinifolium_, that they can only be detected with great
difficulty. After the third moult they become brown, and can be easily
seen when at rest on their food-plant.

Now in all known caterpillars brown colours are adaptive, sometimes
causing a resemblance to the soil, and at others to dead leaves or
branches. As soon, therefore, as the caterpillars have attained a
considerable size, they remain concealed by day.[135] The truth of this
observation not only appears from various entomological notes, but I
have frequently convinced myself of its accuracy. I well remember from
the earliest times that _C. Elpenor_, especially when the larva is
adult, always rests by day among the dead branches and leaves of its
shrub-like food-plant, _Epilobium Hirsutum_; and even when this species
lives on the low-growing _Epilobium Parviflorum_, it conceals itself
by day on the ground, among the tangled leaves and branches. I have
observed that _Sphinx Convolvuli_ has a precisely similar habit, for
which reason it is difficult to obtain, even in localities where it
occurs very commonly.

In the neighbourhood of Basle I once found at mid-day a brown
caterpillar of _Pterogon Œnotheræ_ on an isolated dead branch
of _Epilobium Rosmarinifolium_, and I was informed by Herr
Riggenbach-Stähelin--a collector of great experience who accompanied
me--that these caterpillars always rest (by day) on withered plants as
soon as they become brown, but before this change they are only to be
found on green plants.

Thus, it cannot well be doubted that the change of colour is associated
with a change in the habits of life, and the question arises as to
which has been the primary change.

If the view here entertained, that the later brown coloration is
adaptive, be correct, the species must have first acquired the habit of
concealing itself by day on the ground and among dead herbage, before
the original green colour could have been changed into brown by natural
selection. This must represent the actual facts of the case.

Nearly allied species which at an advanced age are not dimorphic, but
are darkly coloured in all individuals, are especially calculated
to throw some light on this point. For instance, the caterpillar of
_Deilephila Vespertilio_, which comes under this denomination, is light
green when young, and rests both by day and night on the leaves of the
plant on which it feeds. As soon as it acquires its dark colour--after
the third moult--it changes its habits, concealing itself by day on the
ground and feeding only by night. For this reason collectors prefer
seeking for it in the evening, or with a lantern by night.

The most instructive case, however, is that of _Deilephila Hippophaës_,
in which no change of colour is associated with age, the caterpillar,
throughout its whole life, remaining of a greyish green, which exactly
matches the colour of the leaves of its food-plant, _Hippophae
Rhamnoides_. Nevertheless this species also possesses the habit of
feeding only at night as soon as it has attained to a considerable
size, hiding itself by day at the root of its food-plant. Collectors
expressly state that this larva can scarcely be found by day, and
recommend that it should be sought for at night with a lantern.

From the foregoing facts and considerations it may fairly be concluded,
that the habit of hiding by day, possessed by these and other allied
caterpillars, was acquired when they resembled the leaves in colour,
and that the adaptation to the colour of the soil, or dead foliage and
withered branches, ensued as a secondary consequence.

But why have these caterpillars acquired such a habit, since they
appear to be perfectly protected by their resemblance in colour to
the green leaves? The answer to this question is easily given when we
consider in which species this habit generally occurs.

Does the habit prevail only among the species of the one genus
_Deilephila_, and in all the species of this genus? This is by no
means the case, since, on the one hand, many species of _Deilephila_,
such as _D. Euphorbiæ_, _Galii_, _Nicæa_, and _Dahlii_, do not possess
the habit, and, on the other hand, it occurs in species of other
genera, such as _Macroglossa Stellatarum_, _Sphinx Convolvuli_, and
_Acherontia Atropos_.

The habit in question must therefore be the result of certain external
conditions of life common to all those species which rest by day. The
mode of life common to them all is that they do not live on trees with
large leaves or with thick foliage, but on low plants or small-leaved
shrubs, such as the Sea Buckthorn.[136] I believe I do not err when
I attribute the habit possessed by the adult larvæ, of concealing
themselves by day, to the fact that the green colour is protective only
so long as they are small--or, more precisely speaking, as long as
their size does not considerably exceed that of a leaf or twig of their
food-plant. When they become considerably larger, they must become
conspicuous in spite of their adaptive colour, so that it would then be
advantageous for them to conceal themselves by day, and to feed only
by night. This habit they have acquired, and still observe, even when
the secondary adaptation to the colour of the soil, &c., has not been
brought about. We learn this from _D. Hippophaës_, which remains green
throughout its whole larval existence; and no less from the green forms
of the adult larvæ of _Sphinx Convolvuli_, _Chærocampa Elpenor_, and
_Porcellus_, all of which conceal themselves by day in the same manner
as their brown allies.

It may be objected that there are Sphinx-larvæ--instances of which I
have myself adduced--which live on low small-leaved plants, and which
nevertheless do not hide themselves by day. This is the case with the
spurge-feeding _D. Euphorbiæ_, so common in many parts of Germany. This
caterpillar must, however, be classed with those which, on account
of their distastefulness, or for other reasons to be subsequently
considered, are rejected by birds and other larger foes, and which, as
Wallace has shown, derive advantage from being coloured as vividly as
possible. I shall return to this subject later, when treating of the
biological value of special markings.

On the other hand, it is readily conceivable that, from the conditions
of life of caterpillars living on trees or shrubs with dense foliage,
the habit of resting by day and descending from the tree for
concealment would not have been acquired. Such larvæ are sufficiently
protected by their green colour among the large and numerous leaves;
and I shall have occasion to show subsequently that their markings
increase this protective resemblance.

The di- or polymorphism of the larvæ of the _Sphingidæ_ does not
therefore depend upon a _contemporaneous_ double adaptation, but
upon the replacement of an old protective colour by a new and better
one, and therefore upon a _successive_ double adaptation. The adult
caterpillars of _C. Elpenor_ are not sometimes brown and sometimes
green because some individuals have become adapted to leaves and others
to the soil, but because the anciently inherited green has not yet
been completely replaced by the newly acquired brown coloration, some
individuals still retaining the old green colour.

When, in another place,[137] I formerly stated “that a species can
become adapted in this or that manner to given conditions of life, and
that by no means can only one best adapted form be allowed for each
species,” this statement is theoretically correct speaking generally,
but not in its application to the present class of cases. A comparison
with one another of those caterpillars which repose by day, distinctly
shows that they all possess a tendency to abandon the green and assume
a dull colour, but that this process of replacement has advanced
further in some species than in others. It will not be without interest
to follow this operation in some detailed cases, since we may thus
obtain an insight into the processes by which polymorphism has arisen,
as well as into the connection between this phenomenon and simple
variability.

In _D. Hippophaës_ the process has either not yet commenced, or is
as yet in its first rudiments. If we may trust the statements of
authors, together with the ordinary green form there occurs, rarely,
a silver-grey variety, which may be regarded as the beginning of a
process of colour substitution. Among thirty-five living specimens of
this scarce species which I was able to procure, the grey form did not
occur, neither have I found it in collections.

In _Macroglossa Stellatarum_ we see the transforming process in full
operation. A large number of individuals (about thirty-five per cent.)
are still green; the number of dark-coloured individuals reaches
forty-six per cent., these, therefore, preponderating; whilst between
the two extremes there are about nineteen per cent. of transition
forms, showing all possible shades between light green and dark
blackish-brown or brownish-violet, and even, in solitary individuals,
pure violet (See Figs. 3-12, Pl. III.). The relatively small number
of the intermediate forms, taken in connection with the fact that all
the 140 specimens employed in my investigation were obtained from one
female, leads to the conclusion that these forms owe their existence to
cross-breeding. It would be superfluous to attempt to prove this last
conclusion with reference to the before-mentioned case, in which a
caterpillar was streaked with brown and green (Fig. 9, Pl. III.).

The process of transformation, as already mentioned, advances in such
a manner that the intermediate forms diminish relatively to the dark
individuals. This is found to be the case with _Sphinx Convolvuli_,
and almost to the same extent with _Chærocampa Elpenor_, in both of
which species the green caterpillars are the rarest.[138] Forms truly
intermediate in colour between green and brown no longer occur, but
apparently only different shades of light and dark brown, passing into
brownish-black.

The process has again made a further advance in _Chærocampa Porcellus_
and _Celerio_ as well as in _Pterogon Œnotheræ_. In all these species
the green form occurs,[139] but so rarely that very few collectors have
seen it. The brown form has therefore in these cases nearly become the
predominant type, and the solitary green specimens which occasionally
occur, may be regarded as reversions to an older phyletic stage.

_Deilephila Livornica_ appears to have reached a similar stage, but the
caterpillar of this species has been so imperfectly observed, that it
is difficult to determine, even approximately, the relative proportion
of the brown to the green individuals. I have only seen one of the
latter in Dr. Staudinger’s collection (Compare Fig. 62, Pl. VII.).

In _Deilephila Vespertilio_, _Euphorbiæ_, _Dahlii_, _Mauritanica_,
_Nicæa_, and _Galii_, the green form has completely disappeared. The
blackish olive-green colour shown by many caterpillars of the two last
species, can be considered as a faint retention of the light green
colour which they formerly possessed, and which they both show at the
present time in their young stages.

Beginning with the appearance of single darker individuals, we pass
on in the first place to a greater variability of colouring, and
from this, by the greater diminution of the intermediate forms,
to polymorphism; the complete extermination of these forms ending
in dimorphism. The whole process of transformation has been thus
effected:--As the new colouring always prevailed over the old, the
latter was at length completely displaced, and the caterpillars, which
were at first simply variable, became polymorphic and then dimorphic,
finally returning to monomorphism.

We thus see the process of transformation still going on, and no
doubt can arise as to its inciting causes. When a character can with
certainty be ascribed to adaptation, we can explain its origin in no
other way than by the action of natural selection. If, as I believe,
it can and has been shown, not only that caterpillars in general
possess adaptive colours, but that these colours can change during the
lifetime of one and the same species, in correspondence with external
conditions, we must certainly gain a very high conception of the power
which natural selection exerts on this group of living forms.[140]



V.

BIOLOGICAL VALUE OF SPECIAL MARKINGS.


The following questions now present themselves: Have the markings
of caterpillars any biological value, or are they in a measure only
sports of nature? Can they be considered as partially or entirely the
result of natural selection, or has this agency had no share in their
production?

The problem here offers itself more distinctly than in any other group
of living forms, because it presents an alternative without a third
possibility. In other words, if it is not possible to show that larval
markings have a distinct biological significance, there remains only
for their explanation the assumption of a phyletic force, since the
direct action of the environment is insufficient to account for such
regularity of development throughout a series of forms. The explanation
by sexual selection is excluded _ab initio_, since we are here
concerned with larvæ, and not with reproductive forms.[141]

The biological significance of marking--if such significance it
possess--will be most easily investigated by examining whether species
with similar markings have any conditions of life in common which would
permit of any possible inference as to the significance of the markings.

Among the _Sphingidæ_ we find four chief forms of marking; (1) complete
absence of all marking; (2) longitudinal stripes; either a simple
subdorsal or this together with a spiracular and dorsal line; (3)
oblique stripes; (4) eye-spots and ring-spots, single, paired, or in
complete rows.

Now if we consider in which species these four kinds of marking are of
general occurrence, not only in the small group of the _Sphingidæ_
but in the whole order Lepidoptera, we shall arrive at the following
results:--

1. _Complete absence of marking_, so common in the larvæ of
other insects, such as the Coleoptera, is but seldom found among
Lepidopterous caterpillars.

To this category belong all the species of _Sesiidæ_ (the genera
_Sesia_, _Trochilia_, _Sciapteron_, _Bembecia_, &c.), the larvæ of
which, without exception, are of a whitish or yellowish colour, and
live partly in the wood of trees and shrubs and partly in the shoots
of herbaceous plants. Subterranean larvæ also, living at the roots
of plants, such as _Hepialus Humuli_ at the roots of hop, and _H.
Lupulinus_ at those of _Triticum Repens_, possess neither colour nor
marking. These, like the foregoing, are yellowish-white, evidently
because they are deprived of the influence of light.[142] The larvæ of
certain small moths, such as _Tortrix Arbutana_ and _Pomonana_, which
live in fruit, and many case-bearing _Tineina_, are likewise without
marking and devoid of bright colour, being generally whitish. Many
of the small caterpillars which feed exteriorly are also--so far as my
experience extends--without definite markings, these being among the
most minute, such as the greenish leaf-mining species of _Nepticula_.
It is among the larger species that we first meet with longitudinal
and oblique stripes. Eye-spots do not occur in any of these larvæ, a
circumstance of the greatest importance for the biological significance
of this character, as will be shown subsequently. The small size of
the caterpillars cannot be the sole cause of the absence of such
eye-spots, since in young _Smerinthus_ caterpillars one centimeter
long, the oblique stripes are beautifully developed, and the larvæ of
many of the smaller moths considerably exceed this size. The surface of
these caterpillars therefore, _i.e._, the field on which markings are
displayed, is not absolutely too small for the development of such a
character.

Besides the larvæ of the Micro-lepidoptera and of those species living
in the dark, there is also a complete absence of marking in the young
stages of many caterpillars. Thus, all the _Sphingidæ_ of which I have
been able to observe the development, show no markings immediately
after emergence from the egg; in many they appear very soon, even
before the first moult, and, in other species, after this period.

2. _The second category of markings, longitudinal stripes_, is very
widely distributed among the most diverse families. This character
is found among the larvæ of butterflies, _Sphingidæ_, _Noctuæ_,
Micro-lepidoptera, &c., but in all these groups it is absent in many
species. This last fact is opposed to the view that this character
is purely morphological, and leads to the supposition that it may
have a biological value, being of service for the preservation of the
individual, and therefore of the species.

I find that such marking is of service, stripes extending
longitudinally along the upper surface of the caterpillar generally
making the latter less conspicuous. This, of course, does not hold
good under all circumstances, since there are many species with very
striking colours which possess longitudinal stripes. Let us consider,
however, a case of adaptive colouring, such as a green caterpillar,
which, on this account only, is difficult to see, since it accords
with the colour of the plant on which it lives. If it is a small
caterpillar, _i.e._, if its length and thickness do not considerably
exceed that of the parts of its food-plant, it can scarcely be better
concealed--stripes would hardly confer any special advantage unless the
parts of the plant were also striped. But the case is quite different
if the caterpillar is considerably larger than the parts of the plant
(leaves, stalks, &c.). The most perfect adaptive colouring would not
now prevent it from standing out conspicuously as a larger body, among
the surrounding parts of the plants. It must be distinctly advantageous
therefore to such a caterpillar to be striped, since these markings
to a certain extent divide the large body into several longitudinal
portions--they no longer permit it to be seen as a whole, and thus act
more effectively than mere assimilative colouring in causing it to
escape detection. This protection would be the more efficacious if the
stripes resembled the parts of the plant in colour and size, such, for
instance, as the lines of light and shadow produced by stalks or by
long and sharp-edged leaves.

If this view be correct, we should expect longitudinal stripes to be
absent in the smallest caterpillars, and to be present more especially
in those species which live on plants with their parts similarly
disposed, _i.e._, on plants with numerous thin, closely-growing stalks
and grass-like leaves, or on plants with needle-shaped leaves.

It has already been mentioned that the smallest species are devoid of
longitudinal striping. The larvæ of the Micro-lepidoptera show no such
marking, even when they do not live in the dark, but feed either on the
surface or in superficial galleries of the leaves (_Nepticula_, &c.),
in which they must be exposed to almost as much light as when living
on the surface. The fact that the subdorsal line sometimes appears in
very young Sphinx-larvæ is explained, as has already been shown, by the
gradual backward transference of adaptational characters acquired in
the last stage of development.

It can easily be demonstrated that longitudinally striped caterpillars
mostly live on plants, of which the general appearance gives the
impression of a striped arrangement. We have only to consider in
connection with their mode of life, any large group of adaptively
coloured species marked in this manner. Thus, among the butterflies,
nearly all the _Satyrinæ_ possess larvæ conspicuously striped--a fact
which is readily explicable, because all these caterpillars live on
grasses. This is the case with the genera _Melanargia_, _Erebia_,
_Satyrus_, _Pararge_, _Epinephele_, and _Cænonympha_, no species of
which, so far as the larvæ are known, is without longitudinal stripes,
and all of which feed on grasses. It is interesting that here also,
as in certain _Sphingidæ_, some species are brown, _i.e._, adapted to
the soil, whilst the majority are green, and are therefore adapted to
living grass. Just as in the case of the _Sphingidæ_ also, the brown
species conceal themselves by day on the earth, whilst some of the
green species have likewise acquired this habit. I have already shown
how this habit originates from the increasing size of the growing
larva, which would otherwise become too conspicuous, in spite of
adaptive colour and marking. A beautiful confirmation of this view is
found in the circumstance that only the largest species of _Satyrus_,
such as _S. Proserpinus_, _Hermione_, _Phædrus_, &c., possess brown
caterpillars. I should not be surprised if a more exact investigation
of these species, which have hitherto been but seldom observed,
revealed in some cases a dimorphism similar to that of the _Sphingidæ_;
and I believe that I may venture to predict that the young stages
of all these brown larvæ--at present quite unknown--are, as in the
last-named group, green.

Besides the _Satyrinæ_, most of the larvæ of the _Pierinæ_ and
_Hesperidæ_ possess longitudinal stripes, which are generally less
strongly pronounced than in the former subfamily. Some of the _Pierinæ_
live on _Cruciferæ_, of which the narrow leaves and thin leaf- and
flower-stalks present nothing but a linear arrangement; other species
of this group, however, feed on _Leguminosæ_ (_Lathyrus_, _Lotus_,
_Coronilla_, _Vicia_), and some few on broad-leaved bushes (_Rhamnus_).
This last fact may appear to be opposed to the theory; but light
lateral stripes, such for example, as those possessed by _Gonepteryx
Rhamni_, can never be disadvantageous, and may be of use, even on large
leaves, so that if we consider them as an inherited character, there
is no reason for natural selection to eliminate them. In the case of
caterpillars living on vetch, clover, and other _Leguminosæ_, it must
not be forgotten that, although their food-plants do not present any
longitudinal arrangement of parts, they always grow among grasses, the
species feeding on such plants always resting between grass stems, and
very frequently on the grass itself, so that they can have no better
protective marking than longitudinal stripes. The striping of the
_Hesperidæ_ larvæ, which partly feed on grasses but mostly on species
of _Leguminosæ_, can be explained in a similar manner.

It is not here my intention to go through all the groups of Lepidoptera
in this manner. The instances adduced are quite sufficient to prove
that longitudinal stripes occur wherever we should expect to find them,
and that they really possess the biological significance which I have
ascribed to them. That these markings are occasionally converted into
an adaptive imitation of certain special parts of a plant, is shown
by the larvæ of many moths, such for example as _Chesias Spartiata_,
which lives on broom (_Spartium Scoparium_), its longitudinal stripes
deceptively resembling the sharp edges of the stems of this plant.[143]

3. _Oblique striping._ Can the lilac and white oblique stripes on the
sides of a large green caterpillar, such as those of _Sphinx Ligustri_;
or the red and white, or white, black, and red stripes of _Smerinthus
Tiliæ_ and _Sphinx Drupiferarum_ respectively, be of any possible use?
Have we not here just one of those cases which clearly prove that such
a character is purely morphological, and worthless for the preservation
of the individual? Does not Nature occasionally sport with purposeless
forms and colours; or, as it has often been poetically expressed, does
she not here give play to the wealth of her phantasy?

At first sight this indeed appears to be the case. We might
almost doubt the adaptive importance of the green ground-colour
on finding coloured stripes added thereto, and thus--as one might
suppose--abolishing the beneficial action of this ground-colour, by
making the insect strikingly conspicuous. But this view would be
decidedly incorrect, since oblique stripes are of just the same
importance as longitudinal stripes. The former serve to render the
caterpillar difficult of detection, by making it resemble, as far as
possible, a leaf; they are imitations of the leaf-veins.

Nobody who is in the habit of searching for caterpillars will
doubt that, in cases where the oblique stripes are simply white or
greenish-white, it is extremely difficult to see the insect on its
food-plant, _e.g._ _S. Ocellatus_ on _Salix_; not only because it
possesses the colour of the leaves, but no less because its large body
does not present an unbroken green surface, which would bring it into
strong contrast with the leaves, and thus arrest the attention. In the
case of the species named, the coloured area of the body is divided by
oblique parallel stripes, just in the same manner as a willow leaf.
In such instances of course we have not presented to us any special
imitation of a leaf with all its details--there is not a perfect
resemblance of the insect to a leaf, but only an arrangement of lines
and interspaces which does not greatly differ from the division of a
leaf by its ribs.

That this view is correct is shown by the occurrence of this form
of marking. It is on the whole rare, being found, besides in
many _Sphingidæ_, in isolated cases in various families, but is
always confined to those larvæ which live on ribbed leaves, and
never occurring in species which feed on grasses or on trees with
needle-shaped leaves. This has already been shown with respect to
the _Sphingidæ_, in which the oblique stripes are only completely
developed in the subfamilies _Smerinthinæ_ and _Sphinginæ_. The species
of _Smerinthus_ all live on trees such as willows, poplars, lime,
oak, &c., and all possess oblique stripes. The genus _Anceryx_ also
belongs to the _Sphinginæ_, and these caterpillars, as far as known,
live on trees with needle-shaped leaves. The moths of this last genus
are very closely allied to the species of _Sphinx_, not only in form
and colour, but also in many details of marking. The larvæ are however
different, this distinction arising entirely from their adaptation
to needle-shaped leaves, the _Sphinx_ caterpillars being adapted
to ordinary foliage. The species of _Anceryx_, as has been already
shown, are brown mixed with green, and never possess even a trace of
the oblique stripes, but have a latticed marking, consisting of many
interrupted lines, which very effectively serves to conceal them among
the needles and brown bark of the _Coniferæ_.

Of the _Sphinginæ_ living on plants with ordinary foliage, not a single
species is without oblique stripes. I am acquainted with ten species of
caterpillars and their respective food-plants, viz. _Sphinx Carolina_,
_Convolvuli_, _Quinquemaculata_, _Prini_, _Drupiferarum_, _Ligustri_;
_Macrosila Rustica_ and _Cingulata_; _Dolba Hylæus_ and _Acherontia
Atropos_.

Besides among the _Sphinginæ_, oblique stripes occur in the larvæ of
certain butterflies, viz. _Apatura Iris_, _Ilia_, and _Clytie_, all of
which live on forest trees (aspen and willows), and are excellently
adapted to the leaves by their green colour. In addition to these, I am
acquainted with the larvæ of some few moths, viz. of _Aglia Tau_ and
_Endromis Versicolora_, both of which also live on forest trees.

Oblique stripes also occasionally occur in the smaller caterpillars
of _Noctuæ_, _Geometræ_, and even in those of certain _Pyrales_, in
all of which they are shorter and differently arranged. In these cases
also, my theory of adaptation holds good, but it would take us too far
if I attempted to go more closely into them. I will here only mention
the extraordinary adaptation shown by the caterpillar of _Eriopus
Pteridis_. This little Noctuid lives on _Pteris Aquilina_; it possesses
the same green colour as this fern, and has double oblique white
stripes crossing at a sharp angle on each segment, these resembling the
lines of _sori_ of the fern-frond so closely, that the insect is very
difficult to perceive.

After all these illustrations it can no longer remain doubtful that
the oblique stripes of the _Sphingidæ_ are adaptive. But how are
the coloured edges bordering these stripes in so many species to be
explained?

I must confess that I long doubted the possibility of being able to
ascribe any biological value to this character, which appeared to me
only conspicuous, and not protective. Cases may actually occur in which
the brightly coloured edges of the oblique stripes make the caterpillar
conspicuous--just in the same manner as any marking may bring about a
conspicuous appearance by presenting a striking contrast of colour.
I am acquainted with no such instance, however. As a rule, in all
well-adapted caterpillars, considering their colour in its totality,
this is certainly not the case. The coloured edges, on the contrary,
enhance the deceptive appearance by representing the oblique shadows
cast by the ribs on the under-side of the leaf; all these caterpillars
rest underneath the leaves, and never on the upper surface.

This explanation may, perhaps, at first sight appear far-fetched,
but if the experiment be made of observing a caterpillar of _Sphinx
Ligustri_ on its food-plant, not immediately before one’s eyes in a
room, but at a distance as under natural conditions, it will be found
that the violet edges do not stand out brightly, but show a colour
very similar to that of the shadows playing about the leaves. The
coloured edges, in fact, produce a more effective breaking up of the
large green surface of the caterpillar’s body, than whitish stripes
alone. Of course if the insect was placed on a bare twig in the sun, it
would be easily visible at a distance; the larva never rests in such
a position, however, but always in the deep shadow of the leaves, in
which situation the coloured edges produce their peculiar effect. It
may be objected that the oblique white stripes, standing simply on
a dark green ground-colour, would produce the same effect, and that
my explanation therefore leaves the bright colouring of these edges
still unaccounted for. I certainly cannot say why in _Sphinx Ligustri_
these edges are lilac, and in _S. Drupiferarum_, _S. Prini_, and
_Dolba Hylæus_ red, nor why they are black and green in _Macrosila
Rustica_, and blue in _Acherontia Atropos_. If we knew exactly on what
plants these caterpillars fed originally, we might perhaps indulge in
comparing with an artistic eye the shadows playing about their leaves,
seeing in one case more red, and in another more blue or violet. The
coloured stripes of the _Sphingidæ_ must be regarded as the single
strokes of a great master on the countenance of a human portrait.
Looked into closely, we see red, blue, or even green spots and strokes;
but all these colours, conspicuous when close, disappear on retreating,
a general effect of colour being then produced, which cannot be
precisely described by words.

Quite in accordance with this explanation, we see caterpillars with
the brightest coloured stripes concealing themselves in the earth by
day, and betaking themselves to their food-plants only in the dusk of
the evening or dawn of morning and even during the night; _i.e._ in a
light so faint that feeble colours would produce scarcely any effect.
The bright blue of _Acherontia Atropos_, for example, would give the
impression of oblique shadows without any distinctive colour.

It is precisely the case of this last caterpillar, which formerly
appeared to me to present insurmountable difficulties to the
explanation of the coloured stripes by adaptation, and I believed that
this insect would have to be classed with those species which are
brightly coloured because they are distasteful, and are avoided by
birds. But although we have no experiments on this point, I must reject
this view. Unfortunately, we know scarcely anything of the ontogeny
of this caterpillar; but we know at least that the young larvæ (stage
four) are greener than the more purely yellow ones of the fifth stage
(which, however, are also frequently green), and we know further that
some adults are of a dark brownish-grey, without any striking colours.
From analogy with the dimorphism of the species of _Chærocampa_ and
_Sphinx_, fully considered previously, it must therefore be concluded
that in this case also, a new process of adaptation has commenced--that
the caterpillar is becoming adapted to the soil in and on which it
conceals itself by day.[144] An insect which acquires undoubted
protective colours cannot, however, be classed with those which possess
an immunity from hostile attacks.

That the coloured edges are correctly explained as imitations of the
oblique shadows of the leaf-ribs, may also be proved from another point
of view. Let us assume, for the sake of argument, that these coloured
stripes are not adaptive, and that they have not been produced by
natural selection, but by a hypothetical phyletic force. We should
then expect to see them appear at some period in the course of the
phyletic development--perhaps at first only in solitary individuals,
then in several, and finally in all; but we certainly could not expect
that at first single, irregular, coloured spots should arise in the
neighbourhood of the oblique white stripes--that these spots should
then multiply, and fusing together, should adhere to the white stripes,
so as to form an irregular spot-like edge, which finally becomes formed
into a straight, uniformly broad stripe. The phyletic development of
the coloured edges takes place, however, in such a manner, the species
of _Smerinthus_, as has already been established, showing this with
particular distinctness. In _S. Tiliæ_ the course of development can
be followed till the somewhat irregular red border is formed. In the
species of _Sphinx_ this border has become completely linear. It is
very possible that the ontogeny of _S. Ligustri_ or _Drupiferarum_
would reveal the whole process, although it may also be possible that
owing to the contraction of the development, much of the phylogeny is
already lost.

I have now arrived at the consideration of the last kind of marking
which occurs in the _Sphingidæ_, viz.:--

4. _Eye-spots and Ring-spots._--These markings, besides among the
_Sphingidæ_, are found only in a very few caterpillars, such as certain
tropical _Papilionidæ_ and _Noctuæ_. I know nothing of the conditions
of life and habits of these species, however, and without such
knowledge it is impossible to arrive at a complete explanation.

With Darwin, I take an eye-spot to be “a spot within a ring of another
colour, like the pupil within the iris,” but to this central spot
“concentric zones” maybe added. In the _Chærocampa_ larvæ and in
_Pterogon Œnotheræ_, in which complete ocelli occur, there are always
three zones--a central spot, the pupil, or, as I have called it, the
“nucleus;” then a light zone, the “mirror;” and, surrounding this
again, a dark zone (generally black), the “ground-area.”

As ring-spots I will consider those ocelli which are without the
nucleus (pupil), and which are not therefore, strictly speaking,
deceptive imitations of an eye, but present a conspicuous light spot
surrounded by a dark zone.

Between these two kinds of markings there is, however, no sharp
boundary, and morphologically they can scarcely be separated. Species
with ring-spots sometimes have nuclei, and ocellated larvæ in some
cases possess only a pale spot instead of a dark pupil. I deal here
with the two kinds separately, because it happens that they appear
in two distinct genera, in each of which they have their special
developmental history. Ring-spots originate in a different position,
and in another manner than eye-spots; but it must not, on this
account, be assumed without further inquiry, that they are called
into existence by the same causes; they must rather be investigated
separately, from their origin.

Eye-spots are possessed by the genera _Chærocampa_ and _Pterogon_;
ring-spots by the genus _Deilephila_. In accordance with the data
furnished by the above-given developmental histories, the origination
of these markings in the two genera may be thus represented:--

In the genera named, eye-spots and ring-spots are formed by the
transformation of single portions of the subdorsal line.

In _Chærocampa_ the primary ocelli originate on the fourth and fifth
segments by the detachment of a curved portion of the subdorsal, this
fragment becoming the “mirror,” and acquiring a dark encircling zone
(“ground-area”). The nucleus (pupil) is added subsequently.

In _Deilephila_ we learn from the development of _D. Hippophaës_, that
the primary annulus arises on the segment bearing the caudal horn (the
eleventh) by the deposition of a red spot on the white subdorsal line,
which is somewhat enlarged in this region. The formation of a dark
“ground-area” subsequently occurs, and with this, at first the partial,
and then the complete, detachment of the mirror-spot from the subdorsal
line takes place.

In both genera the spots arise at first locally on one or two segments,
from which they are transferred to the others as a secondary
character. In _Chærocampa_ this transference is chiefly backwards, in
_Deilephila_ invariably forwards.

We have now to inquire whether complete eye-spots--such as those of the
_Chærocampa_ larvæ--have any significance at all, and whether they are
of biological importance. It is clear at starting, that these spots do
not belong to that class of markings which make their possessors more
difficult of detection; they have rather the opposite effect.

We might thus be disposed to class ocellated caterpillars with those
“brightly coloured” species which, like the _Heliconinæ_ and _Danainæ_
among butterflies, possess a disgusting taste, and which to a certain
extent bear the signal of their distastefulness in their brilliant
colours. But even if I had not found by experiment that our native
_Chærocampa_ larvæ were devoured by birds and lizards, and that they
are not therefore distasteful to these insect persecutors, from the
circumstance that these caterpillars are all protectively coloured,
it could have been inferred that they do not belong to this category.
It has been found that all adaptively coloured caterpillars are
eaten, and one and the same species cannot possibly be at the same
time inconspicuously (adaptively) and conspicuously coloured; the one
condition excludes the other.

What other significance can eye-spots possess than that of making the
insects conspicuous? Had we to deal with sexually mature forms, we
should, in the first place, think of the action of sexual selection,
and should regard these spots as objects of taste, like the ocelli
on the feathers of the peacock and argus-pheasant. But we are here
concerned with larvæ, and sexual selection is thus excluded.

The eye-spots must therefore possess some other significance, or else
they are of no importance at all to the life of the insect, and are
purely “morphological characters;” in which case, supposing this could
be proved, they would owe their existence exclusively to forces innate
in the organism itself--a view which very closely approaches the
admission of a phyletic vital force.

I am of opinion, however, that eye-spots certainly possess a biological
value as a means of terrifying--they belong to that numerous class of
characters which occur in the most diverse groups of animals, and which
serve the purpose of making their possessors appear as alarming as
possible.

The caterpillars of the _Sphingidæ_ are known to behave themselves in
different manners when attacked. Some species, such, for instance,
as _Sphinx Ligustri_ and _Smerinthus Ocellatus_, on the approach of
danger assume the so-called Sphinx attitude; if they are then actually
seized, they dash themselves madly to right and left, by this means not
only attempting to get free, but also to terrify their persecutor.
This habit frequently succeeds with men, and more especially with
women and children; perhaps more easily in these cases than with their
experienced foes, birds.

The ocellated _Chærocampa_ larvæ behave differently. They remain quiet
on being attacked, and do not put on a Sphinx-like attitude, but only
withdraw the head and three small front segments into the large fourth
segment, which thus becomes much swollen, and is on this account taken
for the head of the insect by the inexperienced.[145] Now the large
eye-spots are situated on the fourth segment, and it does not require
much imagination to see in such a caterpillar an alarming monster with
fiery eyes, especially if we consider the size which it must appear to
an enemy such as a lizard or small bird. Fig. 28 represents the larva
of _C. Porcellus_ in an attitude of defence, although but imperfectly,
since the front segments can be still more withdrawn.

These facts and considerations do not, however, amount to scientific
demonstration, and I therefore made a series of experiments, in order
to determine whether these caterpillars did actually frighten small
birds. The first experiment proved but little satisfactory. A jay,
which had been domesticated for years, to which I threw a caterpillar
of _Chærocampa Elpenor_, did not give the insect any time for
manœuvring, but killed it immediately by a strong blow with its bill.
This bird had been tame for years, and was in the habit of pecking at
everything thrown to him. Perhaps a wild jay (_Garrulus Glandarius_)
would have treated the insect differently, but it is hardly possible
that such a large and courageous bird would have much respect for
our native caterpillars. I now turned to wild birds. A large brown
_Elpenor_ larva was placed in the food-trough of an open fowl-house
from which the fowls had been removed. A flock of sparrows and
chaffinches (_Fringilla Domestica_ and _Cœlebs_) soon flew down from
the neighbouring trees, and alighted near the trough to pick up stray
food in their usual manner. One bird soon flew on to the edge of the
trough, and was just about to hop into it when it caught sight of the
caterpillar, and stood jerking its head from side to side, but did not
venture to enter. Another bird soon came, and behaved in a precisely
similar manner; then a third, and a fourth; others settled on the perch
over the trough, and a flock of ten or twelve were finally perched
around. They all stretched their heads and looked into the trough, but
none flew into it.

I now made the reverse experiment, by removing the caterpillar and
allowing the birds again to assemble, when they hopped briskly into the
trough.

I often repeated this experiment, and always with the same result. Once
it could be plainly seen that it was really fear and not mere curiosity
that the birds showed towards the caterpillar. The latter was outside
the trough amongst scattered grains of food, so that from one side it
was concealed by the trough. A sparrow flew down obliquely from above,
so that at first it could not see the caterpillar, close to which it
alighted. The instant it caught sight of the insect, however, it turned
in evident fright and flew away.

Of course these experiments do not prove that the larger insectivorous
birds are also afraid of these caterpillars. Although I have not
been able to experiment with such birds, I can certainly prove that
even fowls have a strong dislike to these insects. I frequently
placed a large _Elpenor_ larva in the poultry yard, where it was soon
discovered, and a fowl would run hastily towards it, but would draw
back its head just when about to give a blow with the bill, as soon
as it saw the caterpillar closely. The bird would now run round the
larva irresolutely in a circle--the insect in the meantime assuming
its terrifying attitude--and stretching out its head would make ten
or twenty attempts to deal a blow with its bill, drawing back again
each time. All the cocks and hens acted in a similar manner, and it
was often five or ten minutes before one particularly courageous bird
would give the first peck, which would soon be followed by a second
and third, till the caterpillar, appearing palatable, would finally be
swallowed.

These experiments were always made in the presence of several persons,
in order to guard myself against too subjective an interpretation of
the phenomena; but they all invariably considered the conduct of the
birds to be as I have here represented it.[146]

If it be admitted that the ocelli of caterpillars are thus means of
exciting terror, the difficulty of their occurring in protectively
coloured species at once vanishes. They do not diminish the advantage
of the adaptive colouring, because they do not make the caterpillars
conspicuous, or at least any more easily visible at a distance,
excepting when the insects have assumed their attitude of alarm. But
these markings are of use when, in spite of protective colouring, the
larva is attacked by an enemy. The eye-spots accordingly serve the
caterpillar as a second means of defence, which is resorted to when the
protective colouring has failed.

By this it must not be understood that the ocelli of the _Chærocampa_
larvæ invariably possess only this, and no other significance for the
life of the insect. Every pattern can be conceived to render its
possessor in the highest degree conspicuous by strongly contrasted
and brilliant colouring, so that it might be anticipated that perfect
eye-spots in certain unpalatable species would lose their original
meaning, and instead of serving for terrifying become mere signals of
distastefulness. This is perhaps the case with _Chærocampa Tersa_ (Fig.
35), the numerous eye-spots of which make the insect easily visible.
Without experimenting on this point, however, no certain conclusion
can be ventured upon, and it may be equally possible that in this case
the variegated ocelli with bright red nuclei resemble the blossoms of
the food-plant (_Spermacoce Hyssopifolia_).[147] I here mention this
possibility only in order to show how an inherited form of marking,
even when as well-defined and complicated as in the present case, may,
under certain circumstances, be turned in quite another direction
by natural selection, for the benefit of its possessor. Just in the
same manner one and the same organ, such, for instance, as the limb
of a crustacean, may, in the course of phyletic development, perform
very different functions--first serving for locomotion, then for
respiration, then for reproduction or oviposition, and finally for the
acquisition of food.

I now proceed to the consideration of the biological value of
incomplete eye-spots, or, as I have termed them, ring-spots. Are these
also means of terrifying, or are they only signals of distastefulness?

I must at the outset acknowledge that on this point I am able to offer
but a very undecided explanation. The decision is only to be arrived
at by experiments conducted with each separate species upon which
one desires to pronounce judgment. It is not here legitimate to draw
analogical inferences, and to apply one case to all, since it is not
only possible, but very probable, that the biological significance
of ring-spots changes in different species. Nothing but a large
series of experiments could completely establish this. Unfortunately
I have hitherto failed in obtaining materials for this purpose. I
would have deferred the publication of this essay for a year, could
I have foreseen with certainty that such materials would have been
forthcoming in sufficient quantity during the following summer; but
this unfortunately depends very much upon chance, and I believed that
a preliminary conclusion would be preferable to uncertainty. Perhaps
some entomologist to whom materials are more easily accessible, may, by
continuing these experiments, accomplish this object.

The experiments hitherto made by other observers, are not sufficient
for deciding the question under consideration. Weir,[148] as is
well known, showed that certain brightly coloured and conspicuous
larvæ were refused by insectivorous birds; and Butler[149] proved the
same for lizards and frogs. These experiments are unfortunately so
briefly described, that in no case is the species experimented with
mentioned by name, so that we do not know whether there were any Sphinx
caterpillars among them.[150] I have likewise experimented in this
direction with lizards, in order to convince myself of the truth of
the statement that (1) there are caterpillars which are not eaten on
account of their taste, and (2) that such larvæ possess bright colours.
I obtained positive, and on the whole, very decided results. Thus,
the common orange and blue striped caterpillars of _Bombyx Neustria_
enjoyed complete immunity from the attacks of lizards, whilst those of
the nearly allied _Eriogaster Lanestris_ and _L. Pini_ were devoured,
although not exactly relished. That the hairiness is not the cause
of their being unpalatable, is shown by the fact that _L. Pini_ is
much more hairy than _B. Neustria_. The very conspicuous yellow and
black ringed caterpillar of _Euchelia Jacobææ_ gave also most decided
results. I frequently placed this insect in a cage with _Lacerta
Viridis_, but they would never even notice them, and I often saw the
caterpillars crawl over the body, or even the head of the lizards,
without being snapped at. On every occasion the larvæ remained for
several days with the lizards without one being ever missed. The
reptiles behaved in a precisely similar manner with respect to the moth
of _E. Jacobææ_, not one of which was ever touched by them. The yellow
and black longitudinally striped caterpillars of _Pygæra Bucephala_
were also avoided, and so were the brightly coloured larvæ of the
large cabbage white (_Pieris Brassicæ_), which when crushed give a
disagreeable odour. This last property clearly shows why lizards reject
this species as distasteful. Both caterpillar and butterfly possess
a blood of a strong yellow colour and oily consistency, in which,
however, I could not detect such a decided smell as is emitted by that
of the _Heliconinæ_ and _Danainæ_.[151]

I next made the experiment of placing before a lizard a caterpillar
as much as possible like that of _E. Jacobææ_. Half grown larvæ of
_Bombyx Rubi_ likewise possess golden yellow (but narrower) transverse
rings on a dark ground, and they are much more hairy than those of
_E. Jacobææ_. The lizard first applied its tongue to this caterpillar
and then withdrew it, so that I believed it would also be avoided;
nevertheless it was subsequently eaten. The caterpillars of _Saturnia
Carpini_ were similarly devoured in spite of their bristly hairs,
and likewise cuspidate larvæ (_Dicranura Vinula_), notwithstanding
their extraordinary appearance and their forked caudal horn.[152]
These lizards were by no means epicures, but consumed large numbers
of earth-worms, slugs, and great caterpillars, and once a specimen of
the large and powerfully biting Orthopteron, _Decticus Verucivorus_.
Creatures which possessed a strongly repugnant odour were, however,
always rejected, this being the case with the strongly smelling
beetle, _Chrysomela Populi_, as also with the stinking centipede,
_Iulus Terrestris_, whilst the inodorous _Lithobius Forficatus_ was
greedily eaten. I will call particular attention to these last facts,
because they favour the supposition that with rejected caterpillars a
disgusting odour--although perhaps not always perceptible by us--is the
cause of their being unpalatable.

Striking colours are of course only signals of distastefulness, and the
experiment with _Bombyx Rubi_ shows that the lizards were from the
first prejudiced against such larvæ, the prejudice only being overcome
on actually trying the specimen offered. A subsequent observation
which I made after arriving at this conclusion, is most noteworthy.
After the lizard had learnt by experience that there might be not only
distasteful caterpillars (_E. Jacobææ_), but also palatable ones banded
with black and yellow (_B. Rubi_), it sometimes tasted the _Jacobææ_
larvæ, as if to convince itself that the insect was actually as it
appeared to be, viz., unpalatable!

A striking appearance combined with a very perceptible and penetrating
odour is occasionally to be met with, as in the caterpillar of the
common Swallow-tail, _Papilio Machaon_. I have never seen a lizard
make the slightest attempt to attack this species. I once placed two
large specimens of this caterpillar in the lizard vivarium, where they
remained for five days, and finally pupated unharmed on the side of the
case.

I have recorded these experiments, although they do not thus far
relate to Sphinx-caterpillars, with the markings of which we are
here primarily concerned, because it appeared to me in the first
place necessary to establish by my own experiments that signals of
distastefulness did occur in caterpillars.

I now come to my unfortunately very meagre experience with _Deilephila_
larvæ, with only two species of which have I been able to experiment,
viz., _D. Galii_ and _Euphorbiæ_.

The first of these was constantly rejected. Two large caterpillars, one
of the black and the other of the yellow variety, were left for twelve
hours in the lizard vivarium, without being either examined or touched.
It thus appears that _D. Galii_ is a distasteful morsel to lizards;
and the habits of the caterpillar are quite in accordance with this,
since it does not conceal itself, but rests fully exposed by day on a
stem, so that it can scarcely escape being detected. It is almost as
conspicuous as _D. Euphorbiæ_.

I was much surprised to find, however, that this last species was not
rejected by lizards. On placing a large caterpillar, six to seven
centimeters long, in the vivarium, the lizard immediately commenced
to watch it, and as soon as it began to crawl about, seized it by the
head, and, after shaking it violently, commenced to swallow it. In
spite of its vigorous twisting and turning, the insect gradually began
to disappear, amidst repeated shakings; and in less than five minutes
was completely swallowed.[153] With regard to lizards, therefore, the
prominent ring-spots of this larva are not effective as a means of
alarm, nor are they considered as a sign of distastefulness.

Unfortunately I have not hitherto been able to make any experiments
with birds. It would be rash to conclude from the experience with
lizards that ring-spots were of no biological value. There is scarcely
any one means of protection which can render its possessor secure
against _all_ its foes. The venom of the most poisonous snakes does
not protect them from the attack of the secretary bird (_Serpentarius
Secretarius_) and serpent eagle (_Spilornis Cheela_); and the adder,
as is well known, is devoured by hedgehogs without hesitation. It
must therefore be admitted that many species which are protected by
distastefulness, may possess certain foes against which this quality
is of no avail. Thus, it cannot be said that brightly coloured
caterpillars, which are not eaten by birds and lizards, are also spared
by ichneumons. It is readily conceivable therefore, that the larva of
_D. Euphorbiæ_ may not be unpalatable to lizards, because they swallow
it whole; whilst it is perhaps distasteful to birds, because they must
hack and tear in order to swallow it.

From these considerations it still appears most probable to me that
_D. Euphorbiæ_, and the nearly allied _D. Dahlii_ and _Mauritanica_,
bear conspicuous ring-spots as signs of their being unpalatable to the
majority of their foes. The fact that these species feed on poisonous
_Euphorbiaceæ_, combined with their habit of exposing themselves openly
by day, so as to be easily seen at a distance, may perhaps give
support to this view. As these insects are not protectively coloured,
this habit would long ago have led to their extermination; instead
of this, however, we find that in all situations favourable to their
conditions of life they are among the commonest of the _Sphingidæ_.

Thus, _D. Euphorbiæ_ occurs in large numbers both in South and North
Germany (Berlin); and Dr. Staudinger informs me that in Sardinia the
larvæ of _D. Dahlii_ were brought to him by baskets full.

But if the conspicuous ring-spots (combined of course with the other
bright colours) may be regarded as signals of distastefulness in many
species of _Deilephila_, this by no means excludes the possibility that
in some species these markings play another part, and are effective as
a means of alarm. It even appears conceivable to me that in one and the
same caterpillar they may play both parts against different foes, and
it would certainly be of interest to confirm or refute this supposition
by experiment.

In the light yellow variety of the caterpillar of _D. Galii_ the
ring-spots may serve as means of alarm, and still more so in that of
_D. Nicæa_, the resemblance of which to a snake has struck earlier
observers.[154]

In those species of _Deilephila_ which conceal themselves by day, the
ring-spots cannot be considered as signals of distastefulness, and they
must therefore have some other meaning. As examples of this class may
be mentioned _D. Vespertilio_, which is protectively coloured both in
the young and in the adult stages; and likewise _D. Hippophaës_, in
which this habit of concealment is associated with adaptive colouring.
In the case of the first-named species, it appears possible that the
numerous large ring-spots may serve to alarm small foes, but the
truth of this supposition could only be decided by experiment. In _D.
Hippophaës_, on the other hand, such an interpretation must be at once
rejected, since most individuals possess but a single ring-spot, which
shows no resemblance whatever to an eye.

I long sought in vain for the meaning of this ring-spot, the
discovery of which would in this particular case be of the greatest
value, because we have here obviously the commencement of the whole
development of ring-spots before us--the initial stage from which the
marking of all the other species of _Deilephila_ has proceeded.

I believe that I have now found the correct answer to this riddle, but
unfortunately at a period of the year when I am unable to prove it
experimentally. I consider that the ring-spots are crude imitations of
the berries of the food-plant. The latter are orange-red, and exactly
of the same colour as the spots; the agreement in colour between the
latter and the berries is quite as close as that between the leaves and
the general colouring of the caterpillar. I know of no species which
more closely resembles the colour of the leaves of its food-plant,
the dark upper side and light under side corresponding in the leaves
and caterpillars. The colour of the _Hippophae_ is not an ordinary
green, but a grey-green, which shade also occurs, although certainly
but rarely, in the larvæ. I may expressly state that I have repeatedly
shown to people as many as six to eight of the large caterpillars on
one buckthorn branch, without their being able at once to detect them.
It is not therefore mere supposition, but a fact, that this species
is protected by its general colouring. At first the orange-red spots
appear rather to diminish this protection--at least when the insects
are placed on young shoots bearing no berries. But since at the same
time when the berries become red (end of July and the beginning of
August) the caterpillars are in their last stage of development (_i.e._
possess red spots), it appears extremely probable that these spots are
vague representations of the berries. For the same reason that these
caterpillars have acquired the habit of feeding only at dusk and during
the morning twilight, or at night, and of concealing themselves by
day, it must be advantageous for them to have the surface of their
large bodies not only divided by white stripes, but also interrupted
in yet another manner. How could this be better effected than by two
spots which, in colour and position, represent the grouping of the red
berries on the branches? When feeding, the insect always rests with
the hind segments on a branch, the front segments only being more or
less raised and held parallel to the leaves; the red spots thus always
appear on the stem, where the berries are likewise situated. It might
indeed be almost supposed that the small progress which the formation
of secondary ring-spots on the other segments has made up to the
present time, is explicable by the fact that such berry-like spots on
other portions of the caterpillar would be rather injurious than useful.

It may, however, be asked how an imitation of red berries, which are
eaten by birds just as much as other berries, can be advantageous to a
caterpillar, since by this means it would rather attract the attention
of its enemies?

Two answers can be given to this. In the first place, the berries are
so numerous on every plant that there is but a very small chance of the
smaller and less conspicuous berry-spots catching the eye of a bird
before the true berries; and, secondly, the latter, although beginning
to turn red when the caterpillars are feeding, do not completely
ripen till the autumn, when the leaves are shed, and the yellowish-red
clusters of berries can be seen at a distance. The caterpillar,
however, pupates long before this time.

I have considered this case in such detail because it appears to me
of special importance. It is the only instance which teaches us that
the rows of ring-spots of the _Deilephila_ larvæ proceed from one
original pair--the only instance which permits of the whole course of
development being traced to its origin. Were it possible to arrive at
the causes of the formation of these spots, their original or primary
significance would thereby be made clear.

I will now briefly summarise the results of the investigation of the
biological value of the _Deilephila_ ring-spots.

In the known species of the genus now existing these spots have
different meanings.

In some species (certainly in _Galii_, and probably in _Euphorbiæ_
and _Mauritanica_) the conspicuous ring-spots serve as signals of
distastefulness for certain enemies (not for all).

In a second group of species they serve as a means of alarm, like the
eye-spots of the _Chærocampa_ larvæ (_Nicæa_? light form of _Galii_?).

Finally, in a third group, of which I can at present only cite
_Hippophaës_, they act as an adaptive resemblance to a portion of a
plant, and enhance the efficacy of the protective colouring.


5. _Subordinate Markings._--If, from the foregoing
considerations, it appears that the three chief elements of the
Sphinx-markings--longitudinal and oblique stripes, and spot
formations--are not purely morphological characters, but have a very
decided significance with respect to their possessors, there should be
no difficulty in referring the whole of the markings of the _Sphingidæ_
to the action of natural selection, supposing that these three kinds of
marking were the only ones which actually occurred.

In various species, however, there appear other patterns, which I have
comprised under the term “subordinate markings,” some of which I will
select, for the purpose of showing the reasons which permit of their
being thus designated.

I ascribe to this category, for example, that fine network of dark
longitudinal streaks which often extends over the whole upper side
of the caterpillar, and which is termed the “reticulation.” This
character is found chiefly in the adult larvæ of _Chærocampa_, being
most strongly pronounced in the brown varieties: it occurs also in
_Deilephila Vespertilio_, _Pterogon Œnotheræ_, and _Sphinx Convolvuli_.
As far as I know, it is only associated with adaptive colours, and
indeed occurs only in those caterpillars which rest periodically at
the base of their food-plants among the dead leaves and branches. I do
not consider this reticulation to be a distinct imitation, but only as
one of the various means of breaking up the large uniform surface of
the caterpillar so as to make it present inequalities, and thus render
it less conspicuous. There can be no doubt as to the dependence of this
character upon natural selection.

There is, however, a second group of markings, which must be referred
to another origin. To this group, for instance, belong those light dots
in _Chærocampa Porcellus_ and _Elpenor_ which have been termed “dorsal
spots.” I know of no other explanation for these than that they are the
necessary results of other new formations, and depend on correlation
(Darwin), or, as I may express it, they are the result of the action of
the law governing the organization of these species.

As long as we are confined to the mere supposition that the character
in question may be the outward expression of an innate law of growth,
it is permissible to attempt to show that a quite similar formation in
another species depends upon such a law.

Many of the dark specimens of _Sphinx Convolvuli_ show whitish dots
on segments six to eleven, one being situated on the front edge of
each of these segments, at the height of the completely vanished
subdorsal line (Fig. 52). These spots vary much in size, lightness,
and sharpness of definition. Now it might be difficult to attribute
any biological significance to this character, but its origin becomes
clear on examining light specimens in which the oblique white stripes
are distinct on the sides and the subdorsal line is retained at least
on the five or six anterior segments. It can then be seen that the
spots are located at the points of intersection of the subdorsal and
the oblique stripes (Fig. 16, Pl. III.), and they can accordingly be
explained by the tendency to the deposition of light pigment being
twice as great in these positions as in other portions of the two
systems of light lines. Light spots are thus formed when the lines
which cross at these points are partially or completely extinct
throughout their remaining course.

A marking is therefore produced in this case by a purely innate law of
growth--by the superposition of two ancient characters now rudimentary.
Many other unimportant details of marking must be regarded as having
been produced in a similar manner, although it may not be possible to
prove this with respect to every minute spot and stripe. The majority
of “subordinate markings” depend on the commingling of inherited, but
now meaningless, characters with newly acquired ones.

It would be quite erroneous to attribute to natural selection only
those characters which can be demonstrated to still possess a
biological value in the species possessing them. They may be equally
due to heredity. Thus, it is quite possible that the faint and
inconspicuous ring-spots of _Deilephila Vespertilio_ are now valueless
to the life of the species--they may be derived from an ancestral form,
and have not been eliminated by natural selection simply because they
are harmless. I only mention this as a hypothetical case.

In the case of markings of the second class, _i.e._ oblique stripes,
a transference to later phyletic stages can be demonstrated, although
the stripes thereby lose their original biological value. Thus, the
_Chærocampa_ larvæ, when they were green throughout their whole life
and adapted to the leaves, appear to have all possessed light oblique
stripes in imitation of the leaf-ribs. All the species of the older
type of colouring and marking, such as _Chærocampa Syriaca_ (Fig. 29)
and _Darapsa Chœrilus_ (Fig. 34), and also the light green young forms
of _C. Elpenor_ (Fig. 20), and _Porcellus_ (Figs. 25 and 26), show
these oblique stripes. In these last species the foliage imitation
is abandoned at a later stage, and a dark brown, or blackish-brown,
ground-colour acquired. Nevertheless the oblique stripes do not
disappear, but show themselves--in the fourth stage especially, and
sometimes in the fifth--as distinct dirty yellow stripes, although not
so sharply defined as in the earlier stages. These persistent stripes,
in accordance with their small biological value, are very variable,
since they are only useful in so far as they help to break up the
large surface presented by the caterpillar, and are of no value as
imitations of surrounding objects.

The oblique stripes of _Sphinx Convolvuli_ offer a precisely similar
case; and it may be safely predicted that the young forms of this
species would possess sharply defined light oblique stripes, since
more or less distinct remnants of these markings occur in all the
adult larvæ, and especially in the green form. The entire pattern of
this caterpillar depends essentially on the commingling of characters
persisting from an earlier period, _i.e._ of residues of the subdorsal
and oblique stripes, both these markings being extraordinarily
variable. The black reticulation was added to the ground-colour as
a new means of adaptation, this character appearing only in the
phyletically younger brown form, and being entirely absent, or only
faintly indicated, in the older green variety.



VI.

OBJECTIONS TO A PHYLETIC VITAL FORCE.


It has been shown in the previous section that the three elements
composing the markings of the Sphinx-larvæ originally possessed a
distinct significance with respect to the life of the species, and
that they were by this means called into existence. It has likewise
been shown, that in most of the species which possess these characters
at the present time they still have a decided, although sometimes a
different use, for their possessors, so that from this point of view no
objection can be raised to their being considered as having arisen by
natural selection.

On looking at the phenomenon as a whole, however, certain instances
occur which appear quite irreconcilable with this view.

The most formidable objection is offered by the genus _Deilephila_.
The row of ring-spots which nearly all the existing species have more
or less developed, has arisen from a simple subdorsal line. It would
not, therefore, be surprising if a species were discovered which
possessed this line without any ring-spots as its only marking. If
_D. Hippophaës_ were thus marked, there would be no objection to the
theoretical assumption that this[155] was the ancestor of the other
species. It would then be said that ring-spots were first developed
in a later species by natural selection, and that they had been
transmitted to all succeeding and younger species.

Certain individuals of _D. Hippophaës_, however, possess small
ring-spots, some of which are well developed on several segments.
In this species the row of ring-spots is therefore comprised in the
development. The remaining species, which are much younger phyletically
than _Hippophaës_, could not have inherited their ring-spots from the
latter, since this species itself only possesses them occasionally,
and, so to speak, in a tentative manner. The spots would therefore
appear to have arisen spontaneously in this species, and independently
of those in the other species. But if this were the case, how should
we be able to prove that in the other species also the ring-spots did
not arise independently; and if, moreover, a large number of species
showed the same character without its being referable to inheritance
from a common ancestor, how could this be otherwise explained than as
the result of a force innate in these species and producing similar
variations? But this is nothing but Askenasy’s “fixed direction of
variation”--_i.e._, a phyletic vital force.

The only escape from this difficulty is perhaps to be found in proving
that _D. Hippophaës_ formerly possessed ring-spots, and that these
have been subsequently either partially or completely lost, so that
their occasional appearance in this species would therefore depend upon
reversion. The ontogeny, however, teaches us that this is not the case,
since the young caterpillar does not possess a greater number of more
distinct ring-spots, but wants them altogether with the exception of a
red spot on the eleventh segment, which is, however, much fainter than
in the last stage.

This last-mentioned fact contains the solution of the problem. The
premises from which this reasoning set out were all incorrect--the one
red spot on the eleventh segment is likewise a ring-spot, and indeed
the most important one of all, being primary, or the first to come
into existence. Now all specimens, without exception, possess this
first ring-spot, which is useful, and has therefore been called forth
by natural selection; it is not inherited, but newly acquired by this
species; at least, if the explanation of these spots which I have
previously offered is correct.

The primary pair of spots may have been transferred from this to later
species by heredity; and since, in all segmented animals there is a
tendency for the peculiarities of one segment to be repeated on the
others, this repetition must have occurred with greater frequency and
more completely in the later species--the more so if the process were
favoured by natural selection, _i.e._ if the row of ring-spots which
originated in this manner could in any way be turned to the use of the
species.

In _Hippophaës_ itself there must also be a tendency to the formation
of secondary ring-spots, and indeed in a number of specimens we
actually see series of such ring-spots, the latter being present in
varying numbers, and in very different states of development. The fact
that the ring-spots have not become a constant and well-developed
character, is simply explained by the circumstance that as such they
would have endangered the existence of the species.

In this case there is therefore no necessity for assuming a phyletic
vital force. The ring-spots of the genus _Deilephila_ rather furnish
us with an excellent explanation of a fact which might otherwise have
been adduced in support of a phyletic vital force, viz., the strict
uniformity in the development of larval markings.

Before I had been led to the discovery, by the study of the marking and
development of _Hippophaës_, that the spots of the genus _Deilephila_
originated on one segment only, from which they were transferred
secondarily to the others, this astonishing regularity appeared to me
an incomprehensible problem, which could only be solved by assuming
a phyletic vital force. If it be attempted, for the ten species here
considered, to construct a genealogical tree based on the supposition
that it is the _rows of spots_ which have been inherited in cases
where they occur, and not the _mere tendency to their production_
by the transference of the one originally inherited primary spot to
the remaining segments, the attempt will fail. The greater number of
the species would have to be arranged in one row, since one species
always bears a perfected form of marking, which appears in the young
stages of the following species. But it is very improbable that nine
different species, derived directly the one from the other, would
contemporaneously survive.[156] One species, _D. Vespertilio_, could
not be inserted at all in the genealogical tree, since it wants one
character which occurs in all the other species, viz., the caudal horn,
which is absent even in the third stage, and must therefore have been
lost at a very early period of the phyletic development, so that we
may consider it to be on this account genetically allied to the oldest
known form. But the markings of this larva pass through precisely the
same stages of development as do those of the other species. Now if
the ring-spots were inherited as such, the existence of a hornless
species with ring-spots would be an insoluble riddle, and would favour
the admission of parallel developmental series, which again could be
scarcely otherwise explained than by a “fixed direction of variation.”
We have here one of that class of cases which the supporters of a
phyletic vital force have already so often made use of in support of
their view.

The explanation of such a case--_i.e._ its reference to known causes of
species transformation--is never easy, and is indeed impossible without
a precise knowledge of the ontogeny of many species, as well as of the
original significance of the characters in question. In the case of the
_Deilephila_ larvæ, however, such knowledge is still wanting. It is
true that they present us with parallel developmental series, but these
do not depend on an unknown phyletic force--the parallelism can be
referred to the action of the imperfectly known laws of growth innate
in segmented organisms. Because the characters of one segment have
a tendency to repeat themselves on the others, from one parent-form
possessing ring-spots on one segment only, there may have proceeded
several developmental series, all of which developed rows of such spots
independently of each other.

From these considerations we may venture to construct the following
genealogical tree:--

[Illustration: POSSIBLE GENEALOGICAL TREE OF THE GENUS DEILEPHILA

The circles indicate the phyletic stages IV.-VIII.; the eighth is only
reached by _Nicæa_, and is distinguished from the seventh chiefly by
the ontogeny, in the third stage of which the seventh phyletic stage is
reached, whilst in _Euphorbiæ_ and _Dahlii_ this stage is reached in
the fourth ontogenetic stage. The phyletic stages indicated by queries
are extinct, and only known through the ontogeny of existing species.
It must be understood that this pedigree expresses only the ideal and
not the actual relations of the species to one another. Thus, it is
possible that _Hippophaës_ is not the parent-form, but an unknown or
extinct species, which must, however, have possessed the same marking,
and so on.]

Four parallel series here proceed from the parent-form _Hippophaës_;
there may have been five, or possibly only three, but the incomplete
state of our knowledge of the ontogeny does not permit of any certain
conclusion. For the point under consideration this is, however, quite
immaterial. The distance from the central point (the parent-form)
indicates the grade of phyletic development which the respective
species have at present reached.

There is another case which is no less instructive, because it reveals,
although in a somewhat different manner, the action of a law of growth
innate in the organism itself, but which can nevertheless by no means
be regarded as equivalent to a phyletic vital force. I refer to the
coloured edges of the oblique stripes which occur in most of the
species of the genus _Sphinx_. It has already been insisted upon in
a previous section, that the mode in which this character originates
negatives the assumption of a phyletic force, because these coloured
edges are gradually built up out of irregularly scattered spots. There
is no occasion for a “developmental force” to grope in the dark; if
such a power exists, we should expect that it would add new characters
to old ones with the precision of a master workman.

If, however, the coloured edges certainly depend on natural selection,
this agency causing the scattered spots to coalesce and become linear,
we have here the proof that such spots first arose in a precisely
similar manner in several species, quite independently of one
another--that, in fact, a “fixed direction of variation” in a certain
sense exists.

In three species of _Smerinthus_-larvæ, red spots appear towards
the end of the ontogeny; in _S. Populi_ and _Ocellatus_ in only a
minority of individuals, and always separate (not coalescent), and in
_S. Tiliæ_ in a majority of specimens, the spots frequently becoming
fused into one large, single, longish marking. These three species
cannot have inherited the spots from a common ancestor, since they are
absent in the younger ontogenetic stages, or occur only exceptionally,
becoming larger and more numerous in the last stage; they obviously
form a character which must be considered as a case of “anticipated
development.”

How is it then that three species vary independently of each other
in an analogous manner? I know of no other answer to this question
than that similar variations must necessarily arise from similar
physical constitutions--or, otherwise expressed, the three species have
inherited from an unknown parent species, devoid of spots, not this
last character itself, but a physical constitution, having a tendency
to the formation of red spots on the skin.[157] The case offers many
analogies to that of the colour varieties of _Lacerta Muralis_,
to which Eimer[158] briefly calls attention in his interesting
communications on the blue lizard of the Faraglioni Rocks at Capri.
The South Italian lizards, although having differently formed skulls,
show the same brilliantly coloured varieties as those of North Italy;
and Eimer believes that these parallel variations in widely separated
localities, some of which have long been isolated, must be referred
to a tendency towards fixed directions of variation innate in the
constitution of the species.

I long ago insisted[159] that it should not be forgotten that natural
selection is, in the first place, dependent upon the variations which
an organism offers to this agency, and that, although the number of
possible variations may be very great for each species, yet this
number is by no means to be considered as literally infinite. For
every species there may be _impossible_ variations. For this reason I
am of opinion that the physical nature of each species is of no less
importance in the production of new characters than natural selection,
which must always, in the first place, operate upon the results of this
physical nature, _i.e._ upon the variations presented, and can thus
call new ones into existence.

It requires but a slight alteration of the definition to make out of
this “restricted” or “limited variability,” which is the necessary
consequence of the physical nature of each species, a “fixed direction
of variation” in the sense of a phyletic vital force. Instead of--the
_Smerinthus_-larvæ show a tendency to produce red spots on the skin, it
is only necessary to say--these larvæ tend to produce red borders to
the oblique stripes. The latter statement would, however, be incorrect,
since the red borders first arose by the coalescence of red spots
through the action of natural selection. It is not even correct to say
that _all_ the species of _Smerinthus_ show this tendency to produce
spots, since this character does not seem to occur either in _S.
Quercus_ or _S. Tremulæ_.

The distinction between the two modes of conception will become clear
if we ask, as an example, whether those _Chærocampa_-larvæ which do not
at present possess eye-spots will subsequently acquire these markings,
supposing that they maintain their existence on the earth for a
sufficient period?

The supporters of a “fixed direction of variation” would answer this
question in the affirmative. Ocelli constitute a character which occurs
in nearly all the species of the group--they are the goal towards
which the phyletic force is urging, and which must sooner or later
be reached by each member of the group. On the other hand, I cannot
express so decidedly my own opinion, viz., that such complicated
characters as the many-coloured oblique stripes or eye-spots are never
the results of purely internal forces, but always arise by the action
of natural selection, _i.e._ by the combination of such minute and
simple variations as may present themselves. It may be replied that the
formation of eye-spots in those species which are at present devoid of
them, cannot indeed be considered impossible, but that they would only
appear if the constitution of these species had a tendency to give rise
to the production of darker spots on the edge of the subdorsal line,
and if at the same time, the possession of eye-spots would be of use to
the caterpillar under its special conditions of life.

The condition of affairs would be quite different if we were simply
concerned with the transference of a character from one segment
on which it was already present, to the remaining segments. The
transference would, in this case, result from causes purely innate in
the organism--from the action of laws of equilibration or of growth
(correlation), and the external conditions of life would play only a
negative part, since they might prevent the complete reproduction of
a character, such, for example, as eye-spots, on all the segments,
in cases where it was disadvantageous to the species. The fact that
our species of _Chærocampa_ have only faint indications, and not a
completely-developed eye-spot, on the remaining segments, may perhaps
be explained in this manner. It is conceivable that the two pairs
of ocelli on the front segments are more effective as a means of
alarm than if the insects were provided with two long rows of such
markings; but nothing can be stated with certainty on this point until
experiments have been made with caterpillars having rows of eye-spots.

The question raised above--whether the species of _Chærocampa_ at
present devoid of eye-spots are to be expected to acquire this
character in the course of their further phyletic development--brings
with it another point, which cannot be here passed over.

If the _utility_ of the four kinds of markings in their perfected form
is demonstrated, their origination through natural selection is not,
strictly speaking, thereby proved. It must also be shown that the first
rudiments of these characters were also of use to their possessors. The
question as to the utility of the “initial stages” of useful characters
must here be set at rest.

In the case of markings such as longitudinal and oblique stripes, it is
quite evident that the initial stages of these simple characters do not
differ greatly from the perfected marking, but this is certainly not
the case with eye- and ring-spots. The most light is thrown upon this
question by the latter, because a species which has remained at the
initial stage of the formation of ring-spots here presents itself for
examination, viz. _Deilephila Hippophaës_.

I have attempted to show that the orange-red spots, which, as a rule,
adorn only the eleventh segment, enhance the adaptive colouring of this
caterpillar by their resemblance to the berries of the sea-buckthorn,
whilst the general surface resembles the leaves in colour. If this be
admitted, the origination of these spots by natural selection offers no
difficulty, since a smaller spot, or one of a fainter red, must also be
of some use to its possessor.

This case is of importance, as showing that a “change of function” may
occur in markings, just as it does in certain organs among the most
diverse species of animals, in the course of phyletic development. The
spots which in _Hippophaës_ are imitations of red berries, in species
which have further advanced phyletically play quite another part--they
serve as means of alarm, or signals of distastefulness.

It appears to me very improbable, however, that the perfect ocelli of
the _Chærocampa_-larvæ have also undergone such a “functional change”
(Dohrn). I rather believe that the first rudiments of these markings
produced the same effect as that which they now exercise, viz.,
terror. We are certainly not so favourably circumstanced in this case
in knowing a species which shows the initial steps of this character
in its last stage of life; but in the initial steps which the second
stage of certain species present, we see preserved the form under which
the eye-spots first appeared in the phylogeny, and from this we are
enabled to judge with some certainty of the effect which they must have
produced at the time.

In the ontogeny of _C. Elpenor_ and _Porcellus_ we see that a small
curvature of the subdorsal line first arises, the concavity of which
becomes filled with darker green, and soon afterwards with black; the
upwardly curved piece of the subdorsal then becomes detached and more
completely surrounded by black. The white fragment of the subdorsal
which has become separated, in the next place broadens, and a black
(dark) pupil appears in its centre.

Now the first rudiments of the eye-spot certainly appear very
insignificant in a caterpillar two centimeters long, but we must
not forget that in the ancestors of the existing _Chærocampa_-larvæ
this character appeared in the adult state. If we conceive the
curvature of the white subdorsal with the underlying dark pigment
to be correspondingly magnified, its importance as a means of alarm
can scarcely be denied, particularly when we consider that this
marking stands on the enlarged fourth segment, which alone invests
the caterpillar with a singular, and, to smaller foes, an alarming
appearance. We know that in the case of those _Chærocampa_-larvæ which
possess no eye-spots, the distension of this segment is employed
against hostile attacks. (See the illustration of _Darapsa Chærilus_,
Pl. IV., Fig. 34.) Those markings which even only remotely resembled an
eye must, in such a position, have increased the terrifying action. On
these grounds I believe that it may be safely admitted, that this kind
of marking possessed the same significance in its initial stages as
it now does when fully perfected. No functional change has here taken
place.

Among all the facts brought together in the first section I only
know of one group of phenomena which at least permit of an attempt
to refer them to a phyletic vital force. This is the occurrence of
dark ground-colours in adult larvæ which are of light colours in
their young condition. I have already attempted to show that in
the _Chærocampa_-larvæ this change of colour depends on a double
adaptation, the young caterpillars being adapted to the green
colour of the plant and the adults to the soil and dead leaves.
This interpretation appears the more correct when we find the same
process, viz. the gradual replacement of the original green by brown
colours, among species of widely different genera, which, with the
dark colouring, possess the necessarily correlated habit of hiding
themselves by day when in the adult condition. This is the case with
_Sphinx Convolvuli_, _Deilephila Vespertilio_, and _Acherontia Atropos_.

Thus far all has been easily explicable by natural selection; but
when we also see a “tendency” to acquire a dark colour in the course
of development, in those species which neither conceal themselves
nor are adaptively coloured, but are very conspicuously marked--and
if, further, it can be shown that these species, such for instance
as _Deilephila Galii_, actually possess immunity from the attacks of
foes,--how can this tendency to the formation of a dark colour be
otherwise explained than by the admission of a phyletic vital force
urging the variations in this direction?

Nevertheless I believe that also on this point an appeal to unknown
forces can be dispensed with. In the first place, dark ground-colours
can be of use to a species otherwise than as means of adaptation. In
_D. Galii_, as well as in _D. Euphorbiæ_, the light ring-spots appear
rather at their brightest on the pitchy-black ground; and if this
caterpillar must (_sit venia verbo!_) become conspicuous, this purpose
would be best attained by acquiring a dark ground-colour, such as that
of _D. Euphorbiæ_.

The tendency, apparently common to all these _Sphingidæ_, to acquire
a dark colour with increasing age, depends therefore on two quite
distinct adaptations--first, in species sought by enemies, on an
adaptation to the colour of the soil; and secondly, in species rejected
by foes, on the endeavour to produce the greatest possible contrast of
colour.

Moreover, the supposition from which this last plea for a vital
force set out is not universally correct, since there are species,
such for instance as _D. Nicæa_, which never acquire a dark colour;
and in _D. Galii_ also, although all the individuals abandon the
protective green of the young stages, they by no means all acquire a
dark hue in exchange for this colour; many individuals in their light
ochreous-yellow colouring rather strikingly resemble the snake-like
caterpillar of _D. Nicæa_.



VII.

PHYLETIC DEVELOPMENT OF THE MARKINGS OF THE SPHINGIDÆ: SUMMARY AND
CONCLUSION.


If, from the form possessed by many of the caterpillars of the
_Sphingidæ_ on their emergence from the egg, we may venture to draw
a conclusion concerning the oldest phyletic stage, these larvæ were
originally completely destitute of marking. The characteristic caudal
horn must be older than the existing markings, since it is present in
the younger stages (except in cases where it is altogether wanting),
and is generally even larger than at a later age.

There is, however, further evidence that there were once Sphinx-larvæ
without any markings. Such a species now exists. I do not mean the
boring caterpillars of the _Sesiidæ_,[160] which live in the dark,
and are therefore colourless, but I refer to a large larva (over six
centimeters long) preserved in spirit in the Berlin Museum,[161] which,
from its form, belongs to the _Smerinthus_ group. It possesses a caudal
horn, and on the whole upper surface is covered with short and sparsely
scattered bristles, such as occur in the _Sesiidæ_. The colour of this
unknown insect appears to have been light green, although it now shows
only a yellowish shade. Every trace of marking is absent, and it thus
corresponds exactly with the youngest stages of the majority of the
existing Sphinx-larvæ--even to the short bristles sparsely scattered
over the whole upper surface of its body. We have therefore, so to
speak, a living fossil before us, and it would be of great interest to
ascertain its history.

All the data furnished by the developmental history go to show that
of the three kinds of markings which occur in the _Sphingidæ_, viz.,
longitudinal and oblique stripes and spots, the first is the oldest.
Among the species which are ornamented with oblique stripes or spots
there are many which are longitudinally striped in their young stages,
but the reverse case never occurs--young larvæ never show spots or
oblique stripes when the adult is only striped longitudinally.

The first and oldest marking of the caterpillars of the _Sphingidæ_ was
therefore the longitudinal striping, or, more precisely speaking, the
subdorsal, to which dorsal and spiracular lines may have been added.
That this second stage of phyletic development has also been preserved
in existing species has already been sufficiently shown; the greater
portion of one group, the _Macroglossinæ_, has indeed remained at this
stage of development.

From the biological value which must be attributed to this kind of
marking, its origination by natural selection presents no difficulty.
The first rudiments of striping must have been useful, since they must
have broken up the large surface of the body of the caterpillar into
several portions, and would thus have rendered it less conspicuous to
its enemies.

Thus it is not difficult to perceive how a whole group of genera could
have made shift with this low grade of marking up to the present time.
Colour and marking are not the only means of offence and defence
possessed by these insects; and it is just such simply-marked larvæ as
those of the _Macroglossinæ_ which have the protective habit of feeding
only at night, and of concealing themselves by day. Moreover, under
certain conditions of life the longitudinal stripes may be a better
means of protection, even for a Sphinx-larva, than any other marking;
and all those species in which this pattern is retained at the present
time live either among grasses or on _Coniferæ_.

It cannot be properly said that the second form of marking--the
oblique stripes--has been developed out of the first. If these had
arisen by the transformation of the longitudinal stripes, the two
forms could not exist side by side. This is the case, however, both
in certain species in the adult state (_Calymnia Panopus_[162]), as
well as in others during their young stages (most beautifully seen
in _Smerinthus Populi_, Fig. 56). Various facts tend to show that
the oblique stripes appeared in the phyletic development _later_
than the longitudinal lines. In the first place they appear later
than the latter in the ontogeny of certain species. This is the case
with _Chærocampa Elpenor_ and _Porcellus_, in which, however, they
certainly do not reach a high state of development. Then again, the
longitudinal lines disappear completely in the course of the ontogeny,
whilst the oblique stripes alone maintain their ground. Thus, the
subdorsal line vanishes at a very early stage, with the exception of
a small residue,[163] in all native species of _Smerinthus_. I have
already attempted to show that new characters are only acquired _in the
last stage_, and that if still newer ones are then added, the former
disappear from the last stage, and are transferred back to a younger
one. _Characters vanish therefore from a stage in the same order as
they were acquired._

Finally, among the genera with longitudinal stripes (_e.g.
Macroglossa_) we know certain species which, when at an advanced
age, possess oblique stripes (_M. Fuciformis_), although these slant
in a direction opposite to those of most of the other larvæ of the
_Sphingidæ_. These are, however, always species which differ from their
allies in their mode of life, not feeding on grasses or low plants, but
on large-leaved shrubs. If we were able to ascertain the ontogeny of
these species, we should find that the oblique stripes appeared late in
life, as has already been shown in the case of _Pterogon Œnotheræ_.

If it be asked why the longitudinal lines were first formed, and then
the oblique stripes, it may be replied that the physical constitution
of these caterpillars would be more easily able to give rise to simple
longitudinal lines than to complicated oblique stripes crossing their
segments.[164] It may perhaps also be suggested that the oldest
_Sphingidæ_ lived entirely on low plants among grasses, and in the
course of time gradually took to shrubs and trees. At the present time
the majority of the Sphinx-larvæ still live on low plants, and but few
on trees, such caterpillars generally belonging to certain special
genera.

The character of oblique stripes becomes perfected by the addition
of coloured edges, the latter, as is self-evident, having been added
subsesequently.

The third chief constituent of the Sphinx-markings, _i.e._ the
spots--whether perfect ocelli or only ring-spots--in two of the special
genera here considered, arise on the subdorsal, where they are either
deposited (_Deilephila_), or built up from a fragment of this line
(_Chærocampa_). That these markings can, however, also originate
independently of the subdorsal, is shown by the ocellus of _Pterogon
Œnotheræ_, situated on the segment bearing the caudal horn. In this
case, however, the ontogeny teaches us that the spot also succeeds the
subdorsal, so that we can state generally that all these spot-markings
are of later origin than the longitudinal striping.

The question as to the relative ages of the oblique stripes and the
spot-marking does not admit of a general answer. In some cases (_C.
Elpenor_ and _Porcellus_) the oblique stripes disappear when the ocelli
reach complete development, and we may therefore venture to conclude
that in these cases the former appeared earlier in the phylogeny.
But it is very probable that oblique stripes arose independently at
different periods, just as longitudinal lines occur irregularly in
quite distinct families. It would be a great error if we were to
ascribe the possession of oblique stripes solely to descent from a
common ancestor. The oblique markings found on certain species of
_Macroglossa_ (_M. Corythus_ from India) have not been inherited from
a remote period, but have been independently acquired by this or by
some recent ancestral species. They have nothing to do _genetically_
with the oblique stripes which occur in some species of _Chærocampa_
(_e.g._ in _C. Nessus_, from India), or with those of the species of
_Smerinthus_ and _Sphinx_. They depend simply on analogous adaptation
(Seidlitz[165]), _i.e._ on adaptation to an analogous environment.

The case is similar with the spot-markings. I have already shown that
under certain conditions ring-spots may assume the exact appearance of
eye-spots by the formation of a nucleus in the “mirror,” such as occurs
occasionally in _Deilephila Euphorbiæ_ (Fig. 43), more frequently
in _D. Galii_, and as a rule in _D. Vespertilio_. Nevertheless,
these markings arise in quite another manner to the eye-spots of the
_Chærocampinæ_, with which they consequently have no genetic relation;
the two genera became separated at a time when they neither possessed
spot-markings. Further, in _Pterogon Œnotheræ_ we find a third kind
of spot-marking, which is most closely allied to the ocelli of the
_Chærocampa_-larvæ, but is situated in quite another position, and must
have originated in another manner, and consequently quite independently
of these eye-spots.

It can also be readily understood why the first and second elements of
the markings of the _Sphingidæ_ should be mutually exclusive, and not
the second and third or the first and third.

A light longitudinal line cutting the oblique stripes, considerably
diminishes that resemblance to a leaf towards which the latter have a
tendency, and it is therefore only found in cases where an adaptive
marking can be of no effect on account of the small size of the
caterpillar, _i.e._ in quite young stages. (See, for instance, Fig.
56, the first stage of _S. Populi_.) At a later period of life the
old marking must give way to the new, and we accordingly find that
the subdorsal line vanishes from all the segments on which oblique
stripes are situated, and is only retained on the anterior segments
where the latter are wanting. In some few cases both elements of
marking certainly occur together, such as in _Calymnia Panopus_ and
_Macroglossa Corythus_; but the oblique stripes are, under these
circumstances, shorter, and do not extend above the subdorsal line, and
in _Darapsa Chœrilus_ even become fused into the latter.[166]

In certain cases there may also be a special leaf structure imitated
by the longitudinal lines, but on the whole the latter diminish the
effect of the oblique stripes; and we accordingly find that not only
has the subdorsal disappeared from those segments with oblique stripes,
but that most larvæ with this last character are also without the
otherwise broad spiracular and dorsal lines. This is the case with all
the species of _Smerinthus_[167] known to me, as well as with all the
species of the genera _Sphinx_, _Dolba_, and _Acherontia_.

Oblique stripes and spot-markings are not, however, necessarily
mutually exclusive in their action, and we also find these in certain
cases united in the same larva, although certainly never in an equal
state of perfection. Thus, _Chærocampa Nessus_[168] possesses strongly
marked oblique stripes, but feebly developed ocelli; and, on the other
hand, _Chærocampa Elpenor_ shows strongly developed eye-spots, but the
earlier oblique stripes are at most only present as faint traces. This
is easily explained by the mode of life. These caterpillars--at least
such of them as are perfectly known--do not live on plants with large,
strongly-ribbed leaves, and are even in the majority of individuals
adapted to the colour of the soil; the oblique stripes have therefore
in these cases only the significance of rudimentary formations.

That the first and third forms of markings also are not always mutually
prejudicial in their action is shown by the case of _Chærocampa
Tersa_, in which the eye-spots certainly appear to possess some
other significance than as a means of causing terror. In most of the
_Chærocampa_-larvæ the subdorsal line disappears in the course of the
phylogeny, and it can be understood that the illusive appearance of the
eye-spots would be more perfect if they did not stand upon a white
line.

If we consider the small number of facts with which I have here been
able to deal, the result of these investigations will not be deemed
unsatisfactory. It has been possible to show that each of the three
chief elements of the markings of the _Sphingidæ_ have a biological
significance, and their origin by means of natural selection has thus
been made to appear probable. It has further been possible to show
that the first rudiments of these markings must also have been of
use; and it thus appears to me that their origin by means of natural
selection has been proved to demonstration. Moreover, it has not been
difficult to understand the displacement of the primary elements of the
markings by secondary characters added at a later period, as likewise
an essential effect of natural selection. Finally, it has been possible
to explain also the subordinate or accessory elements of the markings,
partly by the action of natural selection, and partly as the result of
markings formerly present acting by correlation.

From the origin and gradual evolution of the markings of the
_Sphingidæ_ we may accordingly sketch the following picture:--

The oldest Sphinx-larvæ were without markings; they were probably
protected only by adaptive colouring, and a large caudal horn, and by
being armed with short bristles.

Their successors, through natural selection, became longitudinally
striped; they acquired a subdorsal line extending from the horn to the
head, as well as a spiracular, and sometimes also a dorsal, line. The
caterpillars thus marked must have been best hidden on those plants
in which an arrangement of parallel linear parts predominated; and we
may venture to suppose that at this period most of the larvæ of the
_Sphingidæ_ lived on or among such plants (grasses).

At a later period oblique stripes were added to the longitudinal
lines, the former (almost always) slanting across the seven hindmost
segments from the back towards the feet in the direction of the
caudal horn. Whether these stripes all arose simultaneously, or, as
is more probable, whether only one at first appeared, which was then
transferred to the other segments by correlation assisted by natural
selection, cannot, at least from the facts available, at present be
determined.

On the whole, as the oblique stripes became lengthened towards the
back, the longitudinal lines disappeared, since they injured the
deceptive effect of the stripes. In many species also there were formed
dark or variegated coloured edges to the oblique stripes, in imitation
of the shadow lines cast by the leaf-ribs.

Whilst one group of _Sphingidæ_ (_Sphinx_, _Smerinthus_) were thus
striving to make their external appearance approximate more and more to
that of a ribbed leaf, others of the longitudinally striped species
became developed in another manner.

Some of the latter lived indeed on bush-like leaved plants, but no
oblique stripes were developed, because these would have been useless
among the dense, narrow, and feebly-ribbed leaves of the food-plants.
These caterpillars, from the earlier markings, simply retained the
longitudinal lines, which, combined with a very close resemblance to
the colour of the leaves, afforded them a high degree of protection
against discovery. This protection would also have been enhanced if
other parts of the food-plant, such as the berries (_Hippophaës_), were
imitated in colour and position in such a manner that the large body
of the caterpillar contrasted still less with its environment. In this
way the first ring-spot probably arose in some species on only one--the
penultimate segment.

As soon as this first pair of ring-spots had become an established
character of the species, they had a tendency to become repeated
on the other segments, advancing from the hind segments towards
the front ones. Under certain conditions this repetition of the
ring-spots might have been of great disadvantage to the species, and
would therefore have been as far as possible prevented by natural
selection (_Hippophaës_); in other cases, however, no disadvantage
would have resulted--the caterpillar, well adapted to the colour of its
food-plant, would not have been made more conspicuous by the small
ring-spots, which might thus have become repeated on all the segments
(_Zygophylli_). In cases like the two latter, striking colours must
have been eliminated when inherited from an immediate ancestor; but on
this point nothing can as yet be said with certainty.

In other cases the repetition of the ring-spots with strongly
contrasted colours was neither prejudicial nor indifferent, but could
be turned to the further advantage of the species. If a caterpillar fed
on plants containing acrid juices (_Euphorbiaceæ_) which, by permeating
its alimentary system, rendered it repulsive to other animals, the
ring-spots commencing to appear (by repetition) would furnish an
easy means for natural selection to adorn the species with brilliant
colours, which would protect it from attack by acting as signals of
distastefulness.

But if the dark spots stood on a light ground (_Nicæa_), they would
present the appearance of eyes, and cause their possessors to appear
alarming to smaller foes.

From the developmental histories and biological data at present before
us, it cannot with certainty be said which of these two functions of
the ring-spots was first acquired in the phylogeny, but we may perhaps
suppose that their significance as a means of causing alarm was arrived
at finally.

It may also be easily conceived that as the ring-spots became more and
more complicated, they would occasionally have played other parts,
being fashioned once again in these stages into imitations of portions
of plants, such as a row of berries or flower-buds. For this, however,
there is as yet no positive evidence.

As the ring-spots became detached from the subdorsal line out of which
they had arisen, the latter disappeared more and more completely from
the last ontogenetic stage, and receded towards the younger stages of
life of the caterpillar--it became _historical_. This disappearance
of the subdorsal may also be explained by the fact that the original
longitudinal stripe imitating the linear arrangement of leaves would
become meaningless, even if it did not always diminish the effect of
the ring-spots. But characters which have become worthless are known
in the course of time to become rudimentary, and finally to disappear
altogether. I do not believe that disuse alone causes such characters
to vanish, although in the case of active organs it may have a large
share in this suppression. With markings it cannot, however, be a
question of use or disuse--nevertheless they gradually disappear as
soon as they become meaningless. I consider this to be the effect of
the arrest of the controlling action of natural selection upon these
characters (suspension of the so-called “conservative adaptation,”
Seidlitz). Any variations may become of value if the character
concerned is met with in the necessary state of fluctuation. That this
process of extinction does not proceed rapidly, but rather with extreme
slowness, is seen in the ontogeny of several species of _Deilephila_,
which retain the now meaningless subdorsal line through a whole series
of stages of life.

In another group of Sphinx-larvæ with longitudinal stripes, an eye-spot
became developed independently of the subdorsal line, in the position
of the caudal horn, which has here vanished with the exception of a
small knob-like swelling. This character--which we now see perfected in
_Pterogon Œnotheræ_--undoubtedly serves as a means of causing terror;
but whether the incipient stages possessed the same significance,
cannot be decided in the isolated case offered by the one species of
the genus _Pterogon_ possessing this marking.

In a third group of longitudinally striped caterpillars, the younger
genus _Chærocampa_, eye-spots were developed directly from portions of
the subdorsal line, at first only on the fourth and fifth segments. It
can be here positively asserted that this character served as a means
of alarm from its very commencement. It is certainly for this reason
that we see the subdorsal line in the immediate neighbourhood of the
spots disappear at an early stage, whilst it is retained on the other
segments for a longer period. A portion of the younger (tropical)
species of this group then developed similar, or nearly similar, ocelli
on the remaining segments by correlation; and it may now have occurred
that in solitary cases the eye-spots acquired another significance
(_C. Tersa_?), becoming of use as a disguise by resembling berries or
flower-buds. It is also conceivable that the eye-spots may in other
cases have been converted into a warning sign of distastefulness.

In all those larvæ which possessed purely terrifying markings,
however, not only was the original protective colouring preserved,
but in most of them this colour gradually became replaced by a
better one (adaptation of the adult larva to the soil). The oblique
stripes imitating the leaf-ribs also are by no means lost, but are
almost always present, although but feebly developed, and often only
temporarily.

The pattern formed by the oblique stripes may also be retained, even
with perfect adaptation to the soil, and may be converted to a new use
by losing its sharpness, and, instead of imitating definite parts of
plants, may become transformed into an irregular and confused marking,
and thus best serve to represent the complicated lights and shadows,
stripes, spots, &c., cast on the ground under low-growing plants from
between the stems and dead leaves.

Just as in the case of ocellated species where caterpillars without
eye-spots may retain and newly utilize their older markings, so larvæ
having oblique stripes with the most diversely coloured edges may show
the same markings in allied (younger?) species, both in a rudimentary
and in a transformed condition. These markings may thus contribute to
the formation of a latticed or reticulated pattern. Even the oldest
marking, the subdorsal line, may still play a part, since its remnants
cause certain portions of the complicated pattern to appear more
strongly marked (_S. Convolvuli_). Finally, when an adaptation to a
changing environment intersected by lights and shadows is required,
new markings may be here added as in other cases, viz., dark streaks
extending over the light surface of the whole caterpillar.

In concluding this essay, I may remark that, with respect to the
wide and generally important question which gave rise to these
investigations, a clearer and simpler result has been obtained than
could have been expected, considering the complexity of the characters
requiring to be traced to their causes, as well as our still highly
imperfect knowledge of ontogenetic and biological facts.

For a long time I believed that it was not possible to trace all the
forms of marking and their combinations to those causes which are
known to produce transformation; I expected that there would be an
inexplicable residue.

But this is not the case. Although it cannot yet be stated at first
sight with certainty in every single instance how far any particular
element of marking may have a biological value in the species
possessing it, nevertheless it has been established that each of
the elements of marking occurring in the larvæ of the _Sphingidæ_
originally possessed a decided biological significance, which was
produced by natural selection.

In the case of the three chief elements of the markings of the
_Sphingidæ_, it can be further shown that not only the initial stages
but also their ultimate perfection--the highest stages of their
development, are of decided advantage to their possessors, and have
a distinct biological value, so that the gradual development and
improvement of these characters can be traced to the action of natural
selection.

But although natural selection is the factor which has called into
existence and perfected the three chief forms and certain of the
subsidiary markings, in the repetition of the local character on the
other segments, as well as in the formation of new elements of marking
at the points of intersection of older characters now rudimentary, we
can recognize a second factor which must be entirely innate in the
organism, and which governs the uniformity of the bodily structure
in such a manner that no part can become changed without exerting a
certain action on the other parts--an innate law of growth (Darwin’s
“correlation”).

Only once during the whole course of the investigations was it for an
instant doubtful whether a phyletic vital force did not make itself
apparent, viz., in the red spots accompanying the oblique stripes
in several _Smerinthus_-larvæ. Closer analysis, however, enabled us
to perceive most distinctly the wide gulf that separates “analogous
variation” from the mystic phyletic vital force. Nothing further
remains therefore for the action of this force in respect to the
marking and colouring of the _Sphingidæ_, since several even of the
subordinate markings can be traced to their causes, only the “dorsal
spots” of our two native species of _Chærocampa_ having been referred
to correlation without decided proof. From the temporary inability
to explain satisfactorily such an insignificant detail, no one will,
however, infer the existence of such a cumbrous power as a phyletic
vital force.

The final result to which these investigations have led us is
therefore the following:--The action of a phyletic vital force cannot
be recognized in the marking and colouring of the _Sphingidæ_; the
origination and perfection of these characters depend entirely on the
known factors of natural selection and correlation.



=II.=

ON PHYLETIC PARALLELISM IN METAMORPHIC SPECIES.


INTRODUCTION.

In the previous essay I attempted to trace a whole group of apparently
“purely morphological” characters to the action of known factors of
transformation, to explain them completely by these factors, and in
this manner I endeavoured to exclude the operation of an internal power
inciting change (phyletic vital force).

In this second study I have attempted to solve the problem as to
whether such an innate inciting power can be shown to exist by
comparing the forms of the two chief stages of metamorphic species, or
whether such a force can be dispensed with.

Nobody has as yet apparently entertained the idea of testing this
question by those species which appear in the two forms of larva
and imago (insects), or, expressed in more general terms, by those
species the individuals of which successively possess quite different
forms (metamorphosis), or in which the different forms that occur
are distributed among different individuals alternating with and
proceeding from one another (alternation of generation). Nevertheless,
it is precisely here that quite distinct form-relationships would be
expected according as the development of the organic world depended
on a phyletic vital force, or was simply the response of the specific
organism to the action of the environment.

Assuming the first to be the case, there must have occurred, and must
still occur, what I designate “phyletic parallelism,” _i.e._ the two
stages of metamorphic species must have undergone a precisely parallel
development--every change in the butterfly must have been accompanied
or followed by a change in the caterpillar, and the systematic groups
of the butterflies must be also found in a precisely corresponding
manner in a systematic grouping of the caterpillars. If species are
able to fashion themselves into new forms by an innate power causing
periodic change, this re-moulding cannot possibly affect only one
single stage of development--such as the larva only--but would rather
extend, either contemporaneously or successively, to all stages--larva,
pupa, and imago: each stage would acquire a new form, and it might
even be expected that each would change to the same extent. At least,
it cannot be perceived why a purely internal force should influence
the development of one stage more than that of another. The larvæ
and imagines of two species must differ from one another to the
same extent, and the same must hold good for the larvæ and imagines
of two genera, families, and so forth. In brief, a larval system
must completely coincide with the system based entirely on imaginal
characters, or, what amounts to the same thing, the form-relationships
of the larvæ must correspond exactly with the form-relationships of the
imagines.

On the other hand, the condition of affairs must be quite different
if an internal power causing phyletic remodelling does not exist, the
transformation of species depending entirely on the action of the
environment. In this case dissimilarities in the phyletic development
of the different stages of life must be expected, since the temporary,
and often widely deviating, conditions of life in the two stages can
and must frequently influence the one stage whilst leaving the other
unacted upon--the former can therefore undergo remodelling while the
latter remains unchanged.[169]

By this means there would arise an unequal difference between the two
stages of two species. Thus, the butterflies, supposing these to have
become changed, would bear a more remote form-relationship to each
other than the caterpillars, and the differences between the former
(imagines) would always be greater than that between the larvæ if the
butterflies were, at several successive periods, affected by changing
influences whilst the larvæ continued under the same conditions and
accordingly remained unaltered. The two stages would not coincide
in their phyletic development--the latter could not be expressed by
parallel lines, and we should accordingly expect to find that there
was by no means a complete congruity between the systems founded on
the larval and imaginal characters respectively, but rather that the
caterpillars frequently formed different systematic groups to the
butterflies.[170]

Accordingly, the problem to be investigated was whether in those
species which develope by means of metamorphosis, and of which the
individual stages exist under very different conditions of life, a
complete phyletic parallelism was to be found or not. This cannot
be decided _directly_ since we cannot see the phyletic development
unfolded under our observation, but it can be established _indirectly_
by examining and comparing with each other the form-relationships
of the two separate stages--by confronting the larval and imaginal
systematic groups. If the phyletic development has been parallel and
perfectly equal, so also must its end-results--the forms at present
existing--stand at equal distances from one another; larval and
imaginal systems must coincide and be _congruent_. If the course of
the phyletic development has not been parallel, there must appear
inequalities--incongruences between the two systems.

I am certain that systematists of the old school will read these
lines with dismay. Do we not regard it as a considerable advance in
taxonomy that we have generally ceased to classify species simply
according to one or to some few characters, and that we now take
into consideration not merely the last stage of the development (the
imago), but likewise the widely divergent young stages (larva and
pupa)? And now shall it not be investigated whether caterpillars and
butterflies do not form quite distinct systems? In the case of new
species of butterflies of doubtful systematic position was not always
the first question:--what is the nature of the caterpillars? and did
not this frequently throw light upon the relationships of the imago?
Assuredly; and without any doubt we have been quite correct in taking
the larval structure into consideration. But in so doing we should
always keep in mind that there are two kinds of relationship--form- and
blood-relationship--which might possibly not always coincide.

It has hitherto been tacitly assumed that the degree of relationship
between the imagines is always the same as that between the larvæ,
and if blood-relationship is spoken of this must naturally be the
case, since the larva and the imago are the same individual. In all
groups of animals we have not always the means of deciding strictly
between form- and blood-relationship, and must accordingly frequently
content ourselves by taking simply the form-relationship as the
basis of our systems, although the latter may not always express the
blood-relationship. But it is exactly in the case of metamorphic
species that there is no necessity for, nor ought we to remain
satisfied with, this mode of procedure, since we have here two kinds
of form-relationship, that of the larvæ and that of the imagines, and,
as I have just attempted to show, it is by no means self-evident that
these always agree; there are indeed already a sufficient number of
instances to show that such agreement does not generally exist.

This want of coincidence is strikingly shown in a group of animals
widely remote from the Insecta, viz. the Hydromedusæ, the systematic
arrangement of which is quite different according as this is based on
the polypoid or on the medusoid generation. Thus, the medusoid family
of the oceanic Hydrozoa springs from polypites belonging to quite
different families, and in each of these polypoid families there are
species which produce _Medusæ_ of another family.

Similarly, the larvæ of the Ophiuroidea (_Pluteus_-form) among the
Echinodermata are not the most closely related in form to those of
the ordinary star-fishes, but rather to the larvæ of quite a distinct
order, the sea-urchins.

I will not assert that in these two cases the dissimilarity in the
form-relationship, or, as I may designate it, the incongruence
of the morphological systems, must depend on an unequal rate of
phyletic development in the two stages or generations, or that this
incongruence can be completely explained by the admission of such an
unequal rate of development: indeed it appears to me probable that,
at least in the _Ophiureæ_, quite another factor is concerned--that
the form-relationship to the larvæ of the sea-urchins does not depend
upon blood-relationship, but on convergence (Oscar Schmidt), _i.e._ on
adaptation to similar conditions of life. These two cases, however,
show that unequal form-relationship of two stages may occur.

From such instances we certainly cannot infer off-hand that a phyletic
force does not exist; it must first be investigated whether and to
what extent such dissimilarities can be referred to unequal phyletic
development and, should this be the case, whether deviations from a
strict congruence of the morphological systems are not compatible
with the admission of an internal transforming power. _That a certain
amount_ of influence is exerted by the environment on the course of the
processes of development of the organic world, will however be acceded
to by the defenders of the phyletic vital force. It must therefore be
demonstrated that deviations from complete congruence occur, which,
from their nature or magnitude, are incompatible with the admission of
innate powers, and, on the other hand, it must likewise be attempted to
show that the departures from this congruence as well as the congruence
itself can be explained without admitting a phyletic vital force.

In the following pages I shall attempt to solve this question for the
order Lepidoptera, with the occasional assistance of two other orders
of insects. Neither the Echinodermata nor the Hydromedusæ are at
present adapted to such a critical examination; the number of species
in these groups of which the development has been established with
certainty is still too small, and their biological conditions are
still to a great extent unknown. In both these respects they are far
surpassed by the Lepidoptera. In this group we know a large number
of species in the two chief stages of their development and likewise
more or less exactly the conditions under which they exist during each
of these phases. We are thus able to judge, at least to a certain
extent, what changes in the conditions of life produce changes of
structure. Neither in the number of known species of larvæ, nor in the
intimate knowledge of their mode of life, can any of the remaining
orders of insects compete with the Lepidoptera. There is no Dipterous
or Hymenopterous genus in which ten or more species are so intimately
known in the larval stage that they can be employed for the purposes
of morphological comparison. Who is able to define the distinctions
between the life-conditions of the larvæ of twenty different species
of _Culex_ or of _Tipula_? The caterpillars of closely allied species
of Lepidoptera, on the other hand, frequently live on different
plants, from which circumstance alone a certain difference in the
life-conditions is brought about.

The chief question which the research had to reply to was the
following:--Does there exist a complete phyletic parallelism among
Lepidoptera or not? or, more precisely speaking:--Can we infer, from
the form-relationships which at present exist between larvæ on the one
hand and imagines on the other, an exactly parallel course of phyletic
development in both stages; or do incongruences of form-relationship
exist which point to unequal development?

Before I proceed to the solution of this question it is indispensable
that one point should be cleared up which has not been hitherto touched
upon, but which must be settled before the problem can be formally
stated in general terms. Before it can be asked whether larvæ and
imagines have undergone a precisely parallel development, we must
know whether unequal development is possible--whether there does not
exist such an intimate structural relationship between the two stages
that every change in one of these must bring about a change in the
other. Were this the case, every change in the butterfly would cause
a correlative change in the caterpillar, and _vice versâ_, so that an
inequality of form-relationship between the larvæ on one hand and the
imagines on the other would be inconceivable--systems based on the
characters of the caterpillars would completely coincide with those
based on the characters of the butterflies and we should arrive at a
false conclusion if we attributed the phyletically parallel development
of the two stages to the existence of an internal phyletic force,
whilst it was only the known factor, correlation, which caused the
equality of the course of development.

For these reasons it must first be established that the larva and
imago are not respectively fixed in form, and the whole of the first
section will therefore be devoted to proving that the two stages change
independently of one another. Conclusions as to the causes of change
will then be drawn, and these will corroborate from another side a
subsequent inquiry as to the presence or absence of complete congruence
in the two morphological systems. The two questions the answers to
which will be successively attempted are by no means identical,
although closely related, since it is quite conceivable that the first
may be answered by there being no precise correlation of form, or only
an extremely small correlation, between the caterpillar and the imago,
whilst, at the same time, it would not be thereby decided whether
the phyletic development of the two stages had kept pace uniformly
or not. A perfect congruence of morphological relationships could
only take place if transformations resulted from an internal power
instead of external influences. The question:--Does there exist a fixed
correlation of form between the two stages? must therefore be followed
by another:--Do the form-relationships of the two stages coincide or
not--has their phyletic development been uniform or not?



  STUDIES IN THE THEORY
  OF DESCENT

  BY

  DR. AUGUST WEISMANN
  PROFESSOR IN THE UNIVERSITY OF FREIBURG

  _WITH NOTES AND ADDITIONS BY THE AUTHOR_

  TRANSLATED AND EDITED, WITH NOTES, BY

  RAPHAEL MELDOLA, F.C.S.
  LATE VICE-PRESIDENT OF THE ENTOMOLOGICAL SOCIETY OF LONDON

  WITH A PREFATORY NOTICE BY

  CHARLES DARWIN, LL.D., F.R.S.
  _Author of “The Origin of Species,” &c._

  IN TWO VOLUMES
  VOL. II.

  WITH EIGHT COLOURED PLATES

  London:

  SAMPSON LOW, MARSTON, SEARLE, & RIVINGTON
  CROWN BUILDINGS, 188, FLEET STREET

  1882

  [_All rights reserved_]



I.

LARVA AND IMAGO VARY IN STRUCTURE INDEPENDENTLY OF EACH OTHER.


It would be meaningless to assert that the two stages above mentioned
were _completely_ independent of one another. It is obvious that the
amount of organic and living matter contained in the caterpillar
determines the size of the butterfly, and that the quantity of organic
matter in the egg must determine the size of the emergent larva.
The assertion in the above heading refers only to the structure;
but even for this it cannot be taken as signifying an absolute, but
only a relative independence, which, however, certainly obtains in
a very high degree. Although it is conceivable that every change of
structure in the imago may entail a correlative change of structure
in the larva, no such cases have as yet been proved; on the contrary,
all facts indicate an almost complete independence of the two stages.
It is quite different with cases of _indirect_ dependence, such, for
example, as are brought about by ‘nurse-breeding.’ This phenomenon is
almost completely absent in Lepidoptera, but is found in Diptera, and
especially in Hymenoptera in every degree. The larvæ of ichneumons
which live in other insects, require (not always, but in most
instances) that the female imago should possess a sharp ovipositor,
so that in this case also the structure and mode of life of the larva
influences the perfect insect. This does not depend, however, on
inherent laws of growth (correlation), but on the action of external
influences, to which the organism endeavours to adapt itself by natural
selection.

I will now let the facts speak for themselves.

It is shown by those species in which only one stage is di- or
polymorphic that not every change in the one stage entails a
corresponding change in the other. Thus, in all seasonally dimorphic
species we find that the caterpillars of butterflies which are often
widely different in the colour and marking of their successive
generations are absolutely identical. On the other hand, many species
can be adduced of which the larvæ are dimorphic whilst the imagines
occur only in one form (compare the first and second essays in this
volume).

There are however facts which directly prove that any one stage can
change independently of the others; I refer to the circumstance that
any one stage may become independently variable--that the property of
greater variability or of greater constancy by no means always occurs
in an equal degree in all the three stages of larva, pupa, and imago,
but that sometimes the caterpillar is very variable and the pupa and
imago quite constant. On the other hand, all three stages may be
equally variable or equally constant, although this seldom occurs.

If variability is to be understood as indicating the period of
re-modelling of a living form, whether in its totality or only in
single characters or groups of characters, from the simple fact of the
heterochronic variability of the ontogenetic stages, it follows that
the latter can be modified individually, and that the re-modelling
of one stage by no means necessarily entails that of the others. It
cannot however be doubted that variability, from whatever cause it may
have arisen, is in all cases competent to produce a new form. From the
continued crossing of variable individuals alone, an equalization of
differences must at length take place, and with this a new, although
not always a widely deviating, constant form must arise.

That the different stages of development of a species may actually be
partly variable and partly constant, and that the variable or constant
character of one stage has no influence on the other stages, is shown
by the following cases, which are, at the same time, well adapted to
throw light on the causes of variability, and are thus calculated to
contribute towards the solution of the main problem with which this
investigation is concerned.

When, in the following pages, I speak of _variability_, I do not
refer to the occurrence of local varieties, or to variations which
occur in the course of time, but I mean a high degree of individual
variability--a considerable fluctuation of characters in the
individuals of one and the same district or of the same brood. I
consider a species to be constant, on the other hand, when the
individuals from a small or large district differ from one another only
to a very slight extent. Constant forms are likewise generally, but not
invariably, such as are poor in local varieties, whilst variable forms
are those which are rich in such variations. Since the terms “variable”
and “constant” are but relative, I will confine myself to the most
extreme cases, those in which the individual peculiarities fluctuate
within very wide or very narrow limits.

As no observations upon the degree of variability shown by a species in
the different stages of its development were available, I was obliged
to fall back upon my own, at least so far as relates to the larval and
pupal stages, whilst for the imaginal stage the wide experience of my
esteemed friend Dr. Staudinger has been of essential service to me.

Let us in the first place confine our attention to the three chief
forms which every Lepidopteron presents, viz. larva, pupa, and imago.
With respect to the constancy or variability of these three forms, we
actually find in nature all the combinations which are theoretically
conceivable.

(1.) There are species which possess a high degree of constancy in
all three stages, such, for example, as _Limenitis Camilla_, _Pieris
Brassicæ_,[171] _Sphinx Ligustri_, and _Euchelia Jacobææ_.

(2.) There are species showing a high degree of variability in all
three stages. This case must be of rare occurrence, as I am only able
to adduce _Araschnia Prorsa-Levana_, a fact which arises from the
circumstance that the pupal stage is, as a rule, but seldom variable.

(3.) There are species which are variable in two stages and constant
in the third. To this class, for example, belongs _Smerinthus Tiliæ_,
of which the larva and imago are very variable, whilst the pupa is
quite constant. The same is the case with _Lasiocampa Pini_, the
well-known fir moth. Many butterflies show this same phenomenon in
other combinations, such, for instance, as _Vanessa Urticæ_ and
_Polychloros_, in which the larva and pupa are very variable, and the
imago very constant. In a less degree the same is also the case with
_Vanessa Atalanta_, whilst in _Pieris Napi_ the pupa and imago are
variable, and the caterpillar remarkably constant, this likewise being
the case with the local form _Bryoniæ_, which, according to my theory,
is to be regarded as the parent form of _Napi_ (See Part I. of the
present volume).

(4.) There are species which are constant in two stages, and variable
only in the third. Thus, a few species can be found in which the
larva and pupa are constant and the imago variable. This is the case
with _Saturnia Yamamai_, the imago of which is well known to present
numberless shades of colour, varying from light yellow to greyish
black, whilst the green caterpillar shows only slight individual
differences of marking, and scarcely any differences of colour. The
pupa of this species is quite constant. _Arctia Caja_ and _Hebe_, and
_Chelonia Plantaginis_ belong to this same category.

There are a very large number of species which possess very constant
imagines and pupæ, but extremely variable larvæ. The following are
the cases known to me:--_Macroglossa Stellatarum_, _Fuciformis_
and _Bombyliformis_; _Chærocampa Elpenor_, _Celerio_, and _Nerii_;
_Deilephila Galii_, _Livornica_, Hübn., _Hippophaës_, _Vespertilio_,
and _Zygophylli_; _Sphinx Convolvuli_; _Acherontia Atropos_;
_Smerinthus Ocellatus_ and _Tiliæ_; _Callimorpha Hera_; _Cucullia
Verbasci_ and _Scrophulariæ_.

Cases in which the variability depends entirely upon the pupa, while
the larva and imago are extremely constant, are of great rarity.
_Vanessa Io_ is a case in point, the pupa being light or dark brown, or
bright golden green, whilst in the two other stages scarcely any light
shades of colour or variations in the very complicated marking are to
be met with.

The facts thus justify the above view that the individual stages of
development change independently--that a change occurring in one stage
is without influence on the preceding and succeeding stages. Were this
not the case no one stage could possibly become variable without all
the other stages becoming so. Did there exist a correlation between
larvæ, pupæ, and imagines of such a nature that every change in the
larva entailed a corresponding change in the imago, as soon as a large
number of larval characters became fluctuating (_i.e._ as soon as this
stage became variable), a large number of imaginal characters would
necessarily also become fluctuating (_i.e._ this stage would also
become correspondingly variable).

There is one other interpretation which might perhaps be attempted from
the point of view of the old doctrine of species. It might be said that
it is a special property of certain larval or imaginal markings to be
variable whilst others are constant, and since the larval and imaginal
markings of a species are generally quite distinct, it may easily
happen that a butterfly possessing markings having the property of
constancy may belong to a caterpillar having variable markings.

There is a soul of truth underlying this objection, since it is
true that the various forms of markings which occur in Lepidoptera
apparently reach different degrees of constancy. If we speak of the
constancy or variability of a species, a different meaning is attached
to these expressions according as we are dealing _e.g._ with a species
of _Sphinx_ or a species of _Arctia_. That which in the latter would
be estimated as a high degree of constancy, in the former would be
taken as a considerable amount of variability. It is of interest, in
connection with the question as to the causes of constancy, to note
that the power of any form of marking to attain to a high degree of
constancy is by no means inversely proportional to the complication of
the marking, as would have been expected _à priori_.

Thus, the species of _Sphinx_ and of allied genera possess on their
fore-wings, which are mostly coloured with a mixture of dull grey,
white and black, an exceedingly complicated arrangement of lines
which, in constant species, show a high degree of uniformity: on the
other hand, the checquered fore-wings of our _Arctiidæ_, which are far
more coarsely marked, always show, even in the most constant species,
well-marked individual differences. The different types of marking must
therefore be measured by different standards.

But in granting this, we decidedly refute the statement that constancy
and variability are inherent properties of certain forms of marking.

This reasoning is based on the simple fact that a given type of marking
comprises both species of great constancy and of (relatively) great
variability.

Thus, the fore-wings of _Sphinx Ligustri_ and _S. Convolvuli_ are
extremely constant, whilst the very similarly marked _Anceryx
(Hyloicus) Pinastri_ is exceedingly variable. Similarly _Deilephila
Euphorbiæ_ is known by its great variability of colouring and marking,
whilst _D. Galii_, which resembles this species so closely as to be
sometimes confounded with it, possesses a high degree of constancy,
and further, the Corsican and Sardinian _D. Dahlii_ is very variable.
Among the family _Arctiidæ_, _Callimorpha Hera_ and the Alpine _Arctia
Flavia_ are cases of constancy, whilst _A. Caja_, which is so similar
to the last species, is so generally variable that two perfectly
identical specimens can scarcely be found together.

The same can be shown to hold good for the markings of caterpillars.
Thus, the larva of _D. Dahlii_ shows very considerable variability,
whilst that of _D. Galii_ is very constant in marking (disregarding the
ground-colour). So also the larva of _Vanessa Urticæ_ is very variable
and that of _V. Antiopa_ very constant, &c.

The great differences with respect to constancy or variability which
are displayed by the different stages of one and the same species,
must therefore find their explanation elsewhere than in the type of
the marking itself. The explanation must be found in the circumstance
that each stage changes independently of the others, and at different
periods can enter a new phase of variability.

We are here led in anticipation to the main question:--Are changes
produced by internal or external causes? is it the physical nature of
the organism which is compelled to become remoulded spontaneously after
the lapse of a certain period of time? or does such modification only
occur when produced directly or indirectly by the external conditions
of life?

In the cases before us the facts undoubtedly indicate a complete
dependence of the transformations upon external conditions of life.

The independent appearance of variability in the separate stages of the
metamorphosis might, however, be regarded as only apparent. It might
still be attempted to attribute the changes to a purely inherent cause,
_i.e._, to a phyletic vital force, by assuming that the latter acts
periodically in such a manner that at first one and then the following
stage becomes variable, until finally the entire species is transformed.

There is but little to be said in reply to this if we once take refuge
in entirely unknown forces, the operation of which can be arbitrarily
conceived to be either constant or periodic.

But granting that such a transforming power exists and acts
periodically, the variability must always pass over the different
stages in a fixed direction, like a wave over the surface of
water--imago, pupa, and larva, or larva, pupa, and imago, must
_successively_ become variable. Cases like that of _Araschnia
Prorsa_, in which all three stages are variable, may certainly be
thus explained, but those instances in which the larva and imago
are extremely variable, and the pupa quite constant, are entirely
inexplicable from this point of view.

The latter can, however, be very simply explained if we suppose the
changes to be dependent upon external influences. From this standpoint
we not only see how it is possible that an intermediate stage should
remain uninfluenced by the changes which affect the two other stages,
but we can also understand why it should just be the pupal stage that
plays this part so frequently. If we ask why most pupæ are constant
and are relatively but very slightly variable, the answer will be
found in the facts that all pupæ which remain concealed in the earth
or inside plants (_Sesiidæ_), or which are protected by stout cocoons,
show complete constancy, whilst any considerable amount of variability
occurs only in those pupæ which are suspended or openly exposed. This
is closely connected with a fact to which I have called attention on a
former occasion,[172] viz., that dimorphism occurs in certain pupæ,
but only in those which are openly exposed and which are therefore
visible to their foes. I am only acquainted with such cases among the
pupæ of butterflies, and it is likewise only among these that I have
found any considerable amount of variability.

Facts of this kind indicate that Nature does not uselessly sport with
forms, but that at any rate changes of this sort result from external
influences. The greater frequency of variability among larvæ and its
comparative rarity in imagines is also undoubtedly in favour of this
view.

It has already been shown that species with variable larvæ and constant
imagines are extremely common, but that those with constant larvæ
and variable imagines are very rare. This confirms the conclusions,
already drawn above, first, that the variability of the imago cannot
owe its existence to the variability of the larvæ, and secondly, that
the causes which produce variability affect the larval condition more
commonly than that of the imago.

Where can these causes be otherwise sought than in the external
conditions of life, which are so widely different in the two stages,
and which are much more variable for the larva than for the imago?

Let us take the species of one genus, _e.g._ those of _Deilephila_.
The imagines of our European species--as far as we know--all live in
precisely the same manner; they all fly at twilight,[173] showing a
preference for the same flowers and very often frequenting the same
spots, so that in the haunts of one species the others are almost
always to be met with, supposing them to occur in the same locality.
They conceal themselves by day in similar places, and are attacked by
similar foes.

It is quite different with the caterpillars. These, even in the case of
the most closely allied species, live under different conditions, as
appears from the fact that they feed on different plants. The latter
can, however, produce changes both directly and indirectly. The larvæ
may acquire adaptive colours and markings, and these would vary in
accordance with the colour and structure of the food-plant; or they may
become brightly coloured as a sign of distastefulness in cases where
they are inedible. Then again the colour of the soil on which the larvæ
live would act upon their colours making these adaptive. Certain habits
of the caterpillars may also be dependent upon the nature of their
food-plants. Thus, _e.g._ _Deilephila Hippophaës_ feeds only at night,
and conceals itself by day under moss and among the leaves at the base
of the food-plant; but _D. Euphorbiæ_ could not acquire such a habit,
because _Euphorbia Cyparissias_ generally grows on arid soil which is
poor in vegetation, and which therefore affords no concealment, and
furthermore, because a caterpillar, as long as it continues to feed,
cannot, and as a matter of fact does not, ever wander far from its
food-plant. A habit of concealment by burying in the earth also, such
for example as occurs in _Acherontia Atropos_, could not be acquired by
_D. Euphorbiæ_, because its food-plant generally grows on hard, dry,
and stony ground.

In addition to these considerations, the foes would be different
according as the caterpillar lived on plants which formed dense
thickets covering large extents of the shore (_Hippophae_) or grew
isolated on dry hillocks and declivities where the herbage was scanty
or altogether absent; or again, according as the insect, in conjunction
with such local differences, fed by day or had acquired the habit
of feeding only by night. It must in fact be admitted that new and
improved adaptations, or, in more general terms, that inducements to
change, when depending on the environment, must be more frequently
dissimilar for larvæ than for the imagines. We must accordingly expect
to find actual change, or that condition of variability which may be
regarded as initiative to change, occurring more commonly in larvæ than
in perfect insects.

Since facts are in complete accordance with the results of these _à
priori_ considerations we may also venture to conclude that the basis
of the considerations is likewise correct, viz., the supposition that
the changes of colour and marking in caterpillars, pupæ, and imagines
result from external influences only.

This must not be taken as signifying that the single stages of the
larval development are also only able to change through the action of
external influences. The larval stages are correlated with each other,
as has already been shown (see the previous essay): new characters
arise in the adult caterpillar at the last stage and are then gradually
transferred back to the younger stages quite independently of external
influences, this recession being entirely brought about by the laws of
correlation. Natural selection here only exerts a secondary action,
since it can accelerate or retard this transference, according as the
new characters are advantageous or disadvantageous to the younger
stages.

Now as considerable individual differences appear in the first
acquisition of a new character with respect to the rapidity and
completeness with which the individuals acquire such a character, the
same must obtain for the transference of an improvement acquired in
the last stage to the next younger stage. The new character would be
acquired by different individuals in different degrees and at different
rates--it would have, to a certain extent, to struggle with the older
characters of the stage; in brief, the younger stage would become
variable.

Variability of this kind might well be designated as _secondary_,
in contradistinction to _primary_ variability; the latter (primary)
depends upon an unequal reaction of the individual organisms to
external influences, the former (secondary) results from the unequal
strength and rate of the action of the innate laws of growth governing
the organism. In both cases alike exceeding variability may occur, but
the causes producing this variability are dissimilar.

The different stages of larval development would thus frequently
display independent variability in a manner similar to the pupal or
imaginal stages, since they can show individual variability while
the other stages of development remain constant. This appearance
of independent variability in the different stages of the larval
development, however, is in truth deceptive--we have here in fact
a kind of wave of variability, which passes downwards through the
developmental stages, becoming gradually weaker, and finally dying out
completely.

In accordance with this, we very frequently find that only the last or
two last stages are variable, while the younger stages are constant.
Thus in _Macroglossa Stellatarum_, the larvæ are constant in the first,
second, and third stages, but become variable in the fourth, and in
the fifth stage first show that high degree of variability which has
already been described in detail (See. Pl. III., Figs. 3-12). The
larvæ, of _Vanessa Cardui_ also, according to my notes, are extremely
constant in the first four stages in spite of their complicated
marking, but become variable in the fifth stage, although to no very
great extent.

In _Smerinthus Tiliæ_, _Ocellatus_ and _Populi_ also, the greatest
larval variability is shown only in the last stage, the preceding
stages being very constant. These cases by no means depend upon the
marking of the young stages being simpler and therefore being less
capable of varying. The reverse case also occurs. In a somewhat similar
manner as the young of the tapir and wild hog are striped, while the
adult animals are plainly coloured, the young caterpillars of _Saturnia
Yamamai_ possess longitudinal black lines on a yellow ground, while as
early as in the second stage a simple green colour appears in the place
of this complicated but perfectly constant marking. If the young stages
are so frequently constant, this rather depends upon the fact that the
transference of a new character to these stages not only takes place
gradually, but also with continually diminishing energy, in a manner
somewhat similar to physical motion, which continually diminishes in
speed by the action of resistance till it is completely arrested. This
constancy of the younger stages may further be due to the circumstance
that the characters would only be transferred when they had become
fixed in the last stage, and were consequently no longer variable. The
transferred characters may thus have acquired a greater regularity,
_i.e._ a less degree of variability, than they possessed at their first
origination. Extensive investigations in this special direction must
be made if the precise laws, in accordance with which the backward
transference of new characters takes place, are to be discovered. By
such researches only should we arrive with certainty at the causes
which determine the lesser variability of the young larval stages.

It may also occur that the early stages are variable, whilst the
later stages are constant, although this case appears to happen less
frequently. Thus, the caterpillars of _Gastropacha Quercifolia_ vary
considerably in the second stage but are constant at a later period,
and the same is the case with _Spilosoma Urticæ_, which in the second
stage may be almost considered to be dimorphic, but which subsequently
becomes constant.

Cases in which the first stage is variable appear to be of the least
frequent occurrence. I know of only one such instance, viz., _Anceryx
Pinastri_, of which the newly hatched larvæ (Pl. VI., Fig. 53) show
considerable differences in the brownish-black crescentic spots. The
second (Fig. 54), third, and fourth stages are then tolerably constant,
while the fifth stage again is very variable.

An instance of this kind can be easily explained by two waves of
variation, the first of which now affects only the first stage,
while the second has just commenced to affect the fifth stage. Such
a supposition is not opposed to any theoretical considerations, but
rather has much probability in its favour, since we know that species
are from time to time subject to be remodelled; and further, that the
coalescence of several stages of phyletic development in the ontogeny
of one and the same species (see p. 226, development of the genus
_Deilephila_) shows that during the backward transference of one
character, new characters may appear in the last stage of the ontogeny,
and indeed very frequently at a time when the next youngest character
has not been transferred back so far as to the first stage.

That this secondary variability is to a certain extent brought about by
the conflict between the old and new characters, the latter striving to
suppress the former, is shown by the caterpillar of _Saturnia Carpini_
which I have observed for many years from this point of view, and than
which I do not know a more beautiful illustration.

When these larvæ leave the egg they are black, but in the adult state
are almost bright green--this at least being the case in a local form
which, from the district in the vicinity of Genoa where it is found,
I will designate as the var. _Ligurica_. Now whilst these two extreme
stages of development are relatively constant, the intermediate stages
show a variability which becomes greater the nearer the last stage
is approached, this variation in the marking depending simply on the
struggle between the green colour and the more anciently inherited
black. In this manner there arises, especially in the fourth stage
of the German local form, an incredible mixture of the most diverse
markings, all of which can, however, be very easily explained from the
foregoing point of view.

The simpler and, as I am inclined to believe, the older form of the
transformation is presented to us in the local variety _Ligurica_. In
the last stage, when 7.5 centimeters long, this form is of a beautiful
bright green colour without any trace of black marking[174] (Pl. VIII.,
Fig. 77). The colour of the six orange warts which are situated on
each segment is also similar in all specimens, so that this stage is
perfectly constant.

Our German _S. Carpini_ shows different characters in the fifth stage.
It is true that individual specimens occur which are entirely green
without any black, but these are rare; the majority possess a more or
less broad black ring encircling the middle of each segment (Pl. VIII.,
Figs. 78 and 79). Those specimens in which the black ring has become
broken up into large or small spots surrounding the base of the warts
constitute intermediate forms (Fig. 80). The last stage of the German
local form, unlike that of the Genoese local form, is therefore very
variable.

The two forms, moreover, do not simply differ in being more or less
advanced in phyletic development, but also in several other points. As
it is of great theoretical interest to show that a species can develop
local differences only in the stage of larva, I will here subjoin the
plain facts.

The differences consist in that the Genoese local form goes through
five moults whilst the German local form, like most caterpillars,
has only four moults. Further, in the Genoese form the light green,
which is also possessed by the German form in the fourth stage, when
it once appears, is retained to the end of the larval development,
whilst in the fifth stage of the German form this colour is replaced
by a dull greyish-green (compare Figs. 77 and 78). There is further
a very considerable difference in the earlier stages which shows
that the phyletic transforming process has taken a quite independent
course in the two forms. Since the struggle between the green and
black--retaining this idea--appears to be quite finished in the last
stage of the Genoese form, we should expect that the new colour, green,
would now also have encroached further upon the younger stages than
in the German form. Nevertheless, this is not the case, but quite
the reverse happens, the black maintaining its ground longer in the
Italian than in the German form.

In the Genoese form the two first stages are completely black, and
in the third stage an orange-yellow lateral stripe first appears. In
the German form this stripe appears in the second stage, and there
is not subsequently added, at least on the middle segments, a yellow
border surrounding some of the warts of the median series. In the
third stage, however, the yellow (which is but the precursor of the
later green colour) becomes further extended, so that the caterpillars
often appear of an orange colour, some or all of the warts and certain
spots and stripes only being black (Figs. 66 and 68). The warts are
also often yellow while the ground remains in most part black--in
brief, the bright colour is in full struggle with the black, and an
endless series of variations is the result of this conflict, whilst in
the corresponding stage of the Genoese form almost complete constancy
prevails.

This constancy remains also in the following (fourth) stage, the
caterpillar still being deep black, only the yellow (sulphur-coloured)
lateral stripe, which has now become brighter, indicating the impending
change (Fig. 67). This takes place in the fifth stage, in which the
ground-colour suddenly becomes bright green, the black remaining at
most only in traces on the anterior edges of the segments.

This is the same marking as is shown by the fourth stage of the German
form, only in this case individuals quite destitute of black do not
occur. In many specimens indeed black forms the ground-colour, the
green only appearing in certain spots (Figs. 71 to 75); in others the
green predominates, and these two extremes are connected by innumerable
intermediate forms, so that this stage must be regarded as the most
variable of all.

The sixth stage of the Genoese and the fifth of the German form have
already been compared together. The results may be thus tabulated:--

  _A. German form._                  _B. Genoese form._

  STAGE I. 9 days.                 9 days.
    Black; constant.                   Black; constant.

  STAGE II. 8 days.                11 days.
    Black, with orange-yellow          Black; constant.
      lateral stripe; variable.

  STAGE III. 5 days (in some       12 days.
    cases as much as 16 days).
    Black, with yellow; very           Black, with orange-yellow
      variable.                          lateral stripes; constant.

  STAGE IV. 16 days (in some       6 days.
    cases only 5 days).
    Bright green and black,            Black, with bright yellowish
      mixed; very variable.              lateral stripe; constant.

  STAGE V. 6 days (frequently      6 days.
    longer).
    Dark green, with or without        Bright green, small traces
      black bands; variable.             of black; variable.

  STAGE VI. Pupation.              18 days.
                                       Bright green, without any
                                         black; constant.

                                   STAGE VII. Pupation.

From this comparison we perceive that the process of transformation
has at least become preliminarily concluded in the Genoese form. Why
the backward transference of the newly-acquired character to the young
stages has not yet occurred, or, at least, why it is not in progress,
does not appear; neither can it be stated whether this will take place
later, although we may venture to suppose that such will be the case.
At first sight but a relatively short time appears necessary for the
single stage V., which is still in a state of fluctuation (variable),
to become constant by continued crossing, like all the other stages.

That the transformation is still in full progress in the German form,
is shown by the fact that in this case all the stages are variable with
the exception of the first--the second stage being only variable to a
small extent, the third to a much greater extent, and the fourth to the
highest degree conceivable, whilst the fifth and last stage is again
less variable--so that the greatest struggle between the old and new
characters takes place in the fourth stage.

Among the innumerable variations presented by this last stage a
complete series of transitional forms can be arranged so as to show the
gradual conquest of the black by the green, and thus indicating, step
by step, the course which the latter colour has taken.

In the blackest specimens there is nothing green but the lateral
(infra-spiracular) line which was yellow in the preceding stage, and a
crescent-shaped streak at the base of the middle warts together with
a still smaller crescent at the base of the upper warts (Figs. 71 and
81). These spots become extended in lighter specimens and approximate
so as to leave only narrow black bridges, a third spot being added at
the posterior edge of the warts (Figs. 72 and 82). The three spots
then extend on all sides, still leaving for a long period narrow
black lines at the boundaries where their growth has caused them to
abut. In this manner there frequently arises on the green ground a
true hieroglyphic-like marking (Figs. 85 and 86). Finally the black
disappears from the anterior edge and diminishes on the middle line of
the back where it still partly remains as a T-shaped figure (Figs. 73
and 74), although generally replaced elsewhere by the green with the
exception of small residues.

One point remained for a long time inexplicable to me, viz., the change
of the light green into dark grey-green which appeared in the last
stage in connection with a total change of the black marking.

Supposing that new characters are actually acquired only in the last
stage, and that from this they are transferred to the younger stages,
we should expect to find completely developed in the last stage the
same colouring and markings as are possessed more or less incompletely
in the fourth stage. Now since the developmental tendency to the
removal of black and to the predominance of green--if we may thus
venture to express it--is obvious in the fourth stage, we may expect to
find in the fifth stage a bright green ground-colour, either without
any mixture of black or with such black spots and streaks as were
retained in the fourth stage as residues of the original ground-colour.
But instead of this the fifth stage shows a dark green colour, and a
more or less developed black marking which cannot in any way be derived
from that of the fourth stage.

The Genoese local form observed last year first gave me an explanation
to the extent that in this form the last stage is actually only the
potential penultimate stage, or, more correctly expressed, that
the same characters which at present distinguish the last stage of
this form, are already more or less completely transferred to the
penultimate stage.

The apparently paradoxical behaviour of the German form can be
explained by supposing that before the pure bright green had become
completely transferred to the penultimate stage a further change
appeared in the last stage, the green ground-colour becoming darker,
and black transverse bands being formed. The marking of the last
stage would then be regarded as the reverse of that of the preceding
stage; the absence of black would be the older, simple black spots at
the base of the warts the next in succession, and a connected black
transverse band the most advanced state of the development.

Whether this explanation is correct, and if so, what causes have
produced the second change, may perhaps be learnt at some future
time by a comparison with the ontogeny of other _Saturniidæ_; in the
meantime this explanation receives support from another side by the
behaviour of the Genoese local form. If the last stage of the German
form has actually commenced to be again re-modelled, then this variety
is further advanced in phyletic development than the Genoese form; and
this corresponds entirely with the theory that in the former the light
colour (the orange considered as preliminary to the transformation into
green) has already been carried down into the second stage, whilst in
the Genoese variety even in the fourth stage only the first rudiments
of the colour-transformation show themselves.

The Genoese form is to a certain extent intermediate between the
German form of _Saturnia Carpini_ and the nearly related _S. Spini_,
a species inhabiting East Germany. In this latter the larvæ, even in
the adult state, are completely black with yellow warts. This form of
caterpillar must therefore be regarded as phyletically the oldest, and
this very well agrees with the character of the moth, which differs
essentially from _S. Carpini_ only in not being sexually dimorphic. In
_Carpini_ the male possesses a far more brilliant colouring than the
female, the latter agreeing so completely with the female of _Spini_
that it can hardly be distinguished therefrom, especially in the case
of the somewhat larger South European specimens of the last species.
Now as the more simple colouring of the female must in any case be
regarded as the original form, we must consider _Spini_, both sexes
of which possess this colouring, to be phyletically the older form,
and _Carpini_, the male of which has become differently coloured, must
be considered as the younger type. This completely accords with the
characters of the larvæ.

I must here mention that I have also asked myself the question whether
the variations of the different larval stages are connected together as
cause and effect--whether the lightest specimens of the fifth stage may
perhaps not also have been the lightest individuals of the third and
fourth stages.

Such relationship is only apparent between the third and fourth stages;
the darkest larvæ of the third stage become the darker varieties of the
fourth stage, although it is true that the lighter forms of the third
sometimes also become dark varieties in the fourth stage. Between the
fourth and fifth stages there is scarcely any connection of this kind
to be recognized. Thus, the darkest varieties of the fourth stage
sometimes become the lightest forms of the fifth stage, whilst in other
cases from the lightest individuals of the fourth stage there arise
all the possible modifications of the fifth stage. Further details may
be omitted: the negative result cannot cause any surprise, as it is a
necessary consequence of the continued crossing that must take place.

We thus see that the three chief stages of development (larva, pupa,
and imago) actually change in colour independently of each other,
the single stages of the larval development being however in greater
dependence upon one another, and being connected indeed in such a
manner that a new character cannot be added to the last stage without
being transferred in the course of time to the preceding stage, and at
a later period from this again even to the youngest stage, supposing
it not to be previously delayed in the course of its transference by
unknown opposing forces. On this last point, however, the facts at
present available do not admit of any certain decision.

But why do the individual larval stages behave in this respect so
very differently to the chief stages of the whole development? why
are the former so exactly correlated whilst the latter are not? If
new characters have a general tendency to become transferred to the
younger ontogenetic stages, why are not new imaginal characters first
transferred to the pupa, and finally to the larva?

The answer to these questions is not far to find. The wider two stages
of a species differ in structure, the less does correlation become
possible; the nearer the two stages are morphologically related, the
more powerful does the action of correlation become. It is readily
conceivable that the more widely two succeeding stages deviate
in structure and mode of life, the less possible does it become
for characters to be transferred from one to the other. How is it
possible, for example, that a new character in the proboscis or on the
wings of a butterfly can be transferred to the caterpillar? If such
correlation existed it could only manifest itself by some other part
of the caterpillar changing in correspondence with the change of the
proboscis or wings of the butterfly. That this is not the case has, in
my opinion, been conclusively shown by all the foregoing considerations
respecting the independent variability of the chief stages of the
metamorphosis.

There are, moreover, an endless number of facts which prove the
independence of the individual stages of development--I refer to
the multitudinous phenomena presented by metamorphosis itself.
The existence of that form of development which we designate as
metamorphosis is alone sufficient to prove incontestably that the
single stages are able to change independently of one another to a most
remarkable extent.

If we now ask the question: how has the so-called “complete”
metamorphosis of insects arisen? the answer can only be: through the
gradual adaptation of the different stages of development to conditions
of life which have continually deviated more and more widely from each
other.[175]

But if individual stages of the post-embryonic development can finally
attain to such complete diversity of structure as that of the larva and
imago through gradual adaptations to continually diverging conditions
of life, this shows that the characters acquired by the single stages
are always only transferred to the same stages of the following
generation, whilst the other stages remain uninfluenced thereby. This
depends upon that form of heredity designated by Darwin “inheritance at
corresponding periods of life,” and by Haeckel “homochronic heredity.”



II.

DOES THE FORM-RELATIONSHIP OF THE LARVA COINCIDE WITH THAT OF THE IMAGO?


Having thus established the independence in the variability of
the individual stages of metamorphosis, I will now turn to the
consideration of the question as to how far a parallelism is displayed
in the phyletic development of these stages. Is there a complete
congruence of form-relationship between larvæ on the one hand and
imagines on the other? does the classification founded on the
morphology of the imagines agree with that based on the morphology of
the larvæ or not?

If, according to Claus,[176] we divide the order Lepidoptera into six
great groups of families, it is at once seen that these groups, which
were originally founded exclusively on imaginal characters, cannot by
any means be so clearly and sharply defined by the larval characters.

This is certainly the case with the _Geometræ_, of which the larvæ
possess only ten legs, and on this account progress with that peculiar
“looping” movement which strikes even the uninitiated. This group,
which is very small, is however the only one which can be founded on
the morphology of the larvæ; it comprises only two nearly related
families (_Phytometridæ_ and _Dendrometridæ_), and it is not yet
decided whether these should not be united into one group comprising
the family characters of the whole of the “loopers.”

Neither the group of Micro-lepidoptera, nor those of the _Noctuina_,
_Bombycina_, _Sphingina_, and _Rhopalocera_, can be based
systematically on larval characters. Several of these groups are indeed
but indistinctly defined, and even the imagines present no common
characteristics by which the groups can be sharply distinguished.

This is well shown by the _Rhopalocera_ or butterflies. These insects,
in their large and generally brilliantly coloured wings, which are
usually held erect when at rest, and in their clubbed antennæ, possess
characters which are nowhere else found associated together, and
which thus serve to constitute them a sharply defined group.[177]
The caterpillars, however, show a quite different state of affairs.
Although the larval structure is so characteristic in the individual
families of butterflies, these “larval-families” cannot be united
into a larger group by any common characters, and the “_Rhopalocera_”
would never have been established if only the larvæ had been known.
It is true that they all have sixteen legs, that they never possess a
Sphinx-like horn, and that they are seldom hairy, as is the case with
many _Bombycidæ_,[178] but these common _negative_ characters occur
also in quite distinct groups.

In the butterflies, therefore, a perfect congruence of
form-relationship does not exist, inasmuch as the imagines constitute
one large group of higher order whilst the larvæ can only be formed
into families. If it be admitted that the common characters of
butterflies depend on their derivation from a common ancestor, the
imagines must have retained certain common characters which enable them
to be recognized as allies, whilst the larvæ have preserved no such
characters from the period at which the families diverged.

Without going at present into the causes of these phenomena I will pass
on to the consideration of further facts, and will now proceed to
investigate both the form-relationships within the families. Here there
can be no doubt that in an overwhelmingly large majority of cases the
phyletic development has proceeded with very close parallelism in both
stages; larval and imaginal families agree almost completely.

Thus, under the group _Rhopalocera_ there is a series of families
which equally well permit of their being founded on the structure of
the larva or on that of the imago, and in which the larvæ and imagines
therefore deviate from one another to the same extent. This is the
case, for instance, with the families of the _Pieridæ_, _Papilionidæ_,
_Danaidæ_, and _Lycænidæ_.

But there are also families of which the limits would be very different
if the larvæ were made the basis of the classification instead of the
butterflies as heretofore. To this category belongs the sub-family
_Nymphalinæ_. Here also a very characteristic form of caterpillar
indeed prevails, but it does not occur in all the genera, being
replaced in some by a quite different form of larva.

In the latest catalogue of Diurnal Lepidoptera, that of Kirby (1871),
112 genera are comprised under this family. Of these most of the larvæ
possess one or several rows of spines on most or on all the segments, a
character which, as thus disposed, is not met with in any other family.

This character is noticeable in genera 1 to 90, if, from those
genera of which the larvæ are known, we may draw a conclusion with
reference to their allies. I am acquainted with larvæ of genus 2,
_Agraulis_, Boisd. (_Dione_, Hübn.); of genus 3, _Cethosia_, Fabr.; 10,
_Atella_, Doubl.; 12, _Argynnis_, Fabr.; 13, _Melitæa_,[179] Fabr.;
19, _Araschnia_, Hübn.; 22, _Vanessa_, Fabr.; 23, _Pyrameis_, Hübn.;
24, _Junonia_, Hübn.; 31, _Ergolis_, Boisd.; 65, _Hypolimnas_, Hübn.
(_Diadema_, Boisd.); 77, _Limenitis_, Fabr.; 81, _Neptis_, Fabr.; 82,
_Athyma_, Westw.; and finally with those of genus 90, _Euthalia_,
Hübn.--which, according to Horsfield’s figures, possess only two rows
of spines, these being remarkably long and curved, and fringing both
sides. It may be safely assumed that the intermediate genera would
agree in possessing this important character of the Nymphalideous
larvæ, viz., spines.

After the genus 90 there are 22 more genera, and these are spineless,
at least in the case of the two chief genera, 93, _Apatura_, and
104, _Nymphalis_. Of the remainder I know neither figures nor
descriptions.[180] In the two genera named the larvæ are provided with
two or more spine-like tentacles on the head, and the last segment
ends in a fork-like process directed backwards. The body is otherwise
smooth, and differs also in form from that of the larvæ of the other
_Nymphalinæ_, being thickest in the middle, and tapering anteriorly and
posteriorly; neither is the form cylindrical, but somewhat flattened
and slug-shaped. If therefore we were to arrange these butterflies by
the larvæ instead of by the imagines, these two genera and their allies
would form a distinct family, and could not remain associated with the
90 other Nymphalideous genera.

We have here a case of _incongruence_; the imagines of the genera 1-90
and 91-112 are more closely allied than their larvæ.

From still another side there arises a similar disagreement. The larvæ
of the genera _Apatura_ and _Nymphalis_ agree very closely in their
bodily form and in their forked caudal appendage with the caterpillars
of another sub-family of butterflies, the _Satyrinæ_, whilst their
imagines differ chiefly from those of the latter sub-family in the
absence of an enlargement of certain veins of the fore-wings, an
essential character of the _Satyrinæ_.

This double disagreement has also been noticed by those systematists
who have taken the form of the caterpillar into consideration.
Thus, Morris[181] attempted to incorporate the genera _Apatura_ and
_Nymphalis_ into the family _Libytheidæ_, placing the latter as
transitional from the _Nymphalidæ_ to the _Satyridæ_. But although the
imagines of the genera _Apatura_, _Nymphalis_, and _Libythea_ may be
most closely related--as I believe they actually are--the larvæ are
widely different, being at least as different as are those of _Apatura_
and _Nymphalis_ from the remaining _Nymphalinæ_.

Now if we could safely raise _Apatura_ and _Nymphalis_ into a distinct
family--an arrangement which in the estimation of Staudinger[182] is
correct--and if this were interpolated between the _Satyridæ_ and
_Nymphalidæ_, such an arrangement could only be based on the larval
structure, and that of the imagines would thus remain unconsidered,
since no other common characters can be found for these two genera than
those which they possess in common with the other Nymphalideous genera.

The emperor-butterflies (_Apatura_), by the ocelli of their fore-wings
certainly put us somewhat in mind of the _Satyrinæ_, in which such
spots are always present; but this character does not occur in the
genus _Nymphalis_, and is likewise absent in most of the other genera
of this group. The genus _Apatura_ shows in addition a most striking
similarity in the markings of the wings to the purely Nymphalideous
genus _Limenitis_, and it is therefore placed, by those systematists
who leave this genus in the same family, in the closest proximity to
_Limenitis_. This resemblance cannot depend upon mimicry, since not
only one or another but _all_ the species of the two genera possess a
similar marking; and further, because similarity of marking alone does
not constitute mimicry, but a resemblance in colour must also be added.
The genus _Limenitis_ actually contains a case of imitation, but in
quite another direction; this will be treated of subsequently.

It cannot therefore be well denied that in this case the larvæ show
different relationships to the imagines.

If the “natural” system is the expression of the genetic relationship
of living forms, the question arises in this and in similar cases
as to whether the more credence is to be attached to the larvæ
or to the imagines--or, in more scientific phraseology, which
of the two inherited classes of characters have been the most
distinctly and completely preserved, and which of these, through its
form-relationship, admits of the most distinct recognition of the
blood-relationship, or, inversely, which has diverged the most widely
from the ancestral form? The decision in single instances cannot but be
difficult, and appears indeed at first sight impossible; nevertheless
this will be arrived at in most cases as soon as the ontogeny of the
larvæ, and therewith a portion of the phylogeny of this stage, can be
accurately ascertained.

As in the _Rhopalocera_ most of the families show a complete congruence
in the form-relationship of the caterpillars and perfect insects, so
a similar congruence is also found in the majority of the families
belonging to other groups. Thus, the two allied families of the group
_Sphingina_ can also be very well characterized by their larvæ;[183]
both the _Sphingidæ_ and the _Sesiidæ_ possess throughout a
characteristic form of larva.

Of the group _Bombycina_ the family of the _Saturniidæ_ possess
thick cylindrical caterpillars, of which the segments are beset with
a certain number of knob-like warts. It is true that two genera of
this family (_Endromis_ and _Aglia_) are without these characteristic
warts, but the imagines of these genera also show extensive and common
differences from those of the other genera. A distinct family has in
fact already been based on these genera (_Endromidæ_, Boisd.). Thus the
congruence is not thereby disturbed.

So also the families _Liparidæ_, _Euprepiidæ_, and _Lithosiidæ_ appear
sharply defined in both forms; and similar families occur likewise
under the _Noctuina_, although in this group the erection of families
presents great difficulties owing to the near relationship of the
genera, and is always to some extent arbitrary. It is important,
however, that it is precisely the transitional families which present
intermediate forms both as larvæ and as imagines.

Such an instance is offered by the _Acronyctidæ_, a family belonging
to the group _Noctuina_. The imagines here show in certain points an
approximation to the group _Bombycina_; and their larvæ, which are
thickly covered with hairs, likewise possess the characteristics of
many of the caterpillars of this group.[184]

A second illustration is furnished by the family _Ophiusidæ_, which is
still placed by all systematists under the _Noctuina_, its affinity to
the _Geometrina_, however, being represented by its being located at
the end of the _Noctuina_. The broad wings and narrow bodies of these
moths remind us in fact of the appearance of the “geometers;” and the
larvæ, like the imagines, show a striking resemblance to those of the
_Geometrina_ in the absence of the anterior abdominal legs. For this
reason Hübner in his work on caterpillars has termed the species of
this family “_Semi-Geometræ_.”

All these cases show a complete congruence in the two kinds of
form-relationship; but exceptions are not wanting. Thus, the family
_Bombycidæ_ would certainly never have been formed if the larval
structure only had been taken into consideration, since, whilst the
genera _Gastropacha_, _Clisiocampa_, _Lasiocampa_, _Odonestis_, and
their allies, are thickly covered with short silky hairs disposed in
a very characteristic manner, the caterpillars of the genus _Bombyx_,
to which the common silkworm, _B. Mori_, belongs, are quite naked and
similar to many Sphinx-caterpillars (_Chærocampa_). Are the imagines of
the genera united under this family, at any rate morphologically, as
unequally related as their larvæ? Whether it is correct to combine them
into one family is a question that does not belong here; we are now
only concerned with the fact that the two stages are related in form in
very different degrees.

An especially striking case of incongruence is offered by the family
_Notodontidæ_, under which Boisduval, depending only on imaginal
characters, united genera of which the larvæ differed to a very great
extent. In O. Wilde’s work on caterpillars this family is on this
account quite correctly characterized as follows:--“Larvæ of various
forms, naked or with thin hairs, sixteen or fourteen legs.”[185]
In fact in the whole order Lepidoptera there can scarcely be found
associated together such diverse larvæ as are here placed in one
imago-family; on one side the short cylindrical caterpillars of
the genus _Cnethocampa_, Steph. (_C. Processionea_, _Pithyocampa_,
&c.), which are covered with fine, brittle, hooked hairs, and are
very similar to the larvæ of _Gastropacha_ with which they were
formerly united; and on the other side there are the naked, humped,
and flat-headed larvæ of the genus _Harpyia_, Ochs., with their two
long forked appendages replacing the hindmost pair of legs, and the
grotesquely formed caterpillars of the genera _Stauropus_, Germ.,
_Hybocampa_, Linn., and _Notodonta_, Ochs.

The morphological congruence between larvæ and imagines declares itself
most sharply in genera, where it is the rule almost without exception.
In this case we can indeed be sure that a genus or sub-genus founded
on the imagines only will, in accordance with correct principles,
present a corresponding difference in the larvæ. Had the latter been
known first we should have been led to construct the same genera as
those which are now established on the structure of the imagines, and
these, through other circumstances, would have stood in the same degree
of morphological relationship as the genera founded on the imagines.
There is therefore a congruence in a double sense; in the first place
the differences between the larvæ and imagines of any two genera are
equally great, and, in the next place, the common characters possessed
by these two stages combined cause them to form precisely the same
groups defined with equal sharpness; the genera coincide completely.

So also the butterflies of the sub-family _Nymphalinæ_ can well be
separated into genera by the characters of the larvæ, and these, as
far as I am able to judge, would agree with the genera founded on the
imagines.

The genus _Melitæa_, for example, can be characterized by the
possession of 7-9 fleshy tubercles bearing hairy spines; the genus
_Argynnis_ may be distinguished by always having six hairy unbranched
spines on each segment, and the genus _Cethosia_ by two similar spines
on each segment; the genus _Vanessa_ shows sometimes as many as
seven branched spines; and the genus _Limenitis_ never more than two
branched blunt spines on each segment, and so forth. If we go further
into details it will be seen that the most closely related imagines,
as might indeed have been expected, likewise possess the most nearly
allied larvæ, whilst very small differences between the imagines are
also generally represented by corresponding differences in the larvæ.
Thus, for instance, the genus _Vanessa_ of Fabricius has been divided
into several genera by later authors. Of these sub-genera, _Grapta_,
Doubl. (containing the European _C.-album_, the American _Fabricii_,
_Interrogationis_, _Faunus_, _Comma_, &c.), is distinguished by the
fact that the larvæ not only possess branched spines on all the
segments with the exception of the prothorax, but these spines are
also present on the head; in the genus _Vanessa_ (_sensû strictiori_),
Doubl., the head and prothorax are spineless (_e.g._ _V. Urticæ_); in
the tropical genus _Junonia_, Hübn., which was also formerly (Godart,
1819[186]) united with _Vanessa_, the larvæ bear branched spines on all
the segments, the head and prothorax included.

It is possible to go still further and to separate two species of
_Vanessa_ as two new genera, although they have hitherto been preserved
from this fate even by the systematists most given to “splitting.”
This decision is certainly justifiable, simply because these species
at present stand quite alone, and the practical necessity of forming a
distinct genus does not make itself felt, and this practical necessity
moreover frequently comes into conflict with scientific claims: science
erects a new genus based on the amount of morphological difference, it
being quite immaterial whether one or many species make up this genus;
such an excessive subdivision is, however, a hindrance to practical
requirements, as the cumbrous array of names thereby becomes still
further augmented.

The two species which I might separate from _Vanessa_ on the ground of
their greater divergence, are the very common and widely distributed
_V. Io_ and _Antiopa_, the Peacock Butterfly and the Camberwell Beauty.
In the very remarkable pattern of their wings, both show most marked
characteristics; _Io_ possesses a large ocellus on each wing, and
_Antiopa_ has a broad light yellow border which is not found in any
other species of _Vanessa_. There can be no doubt but that each of
these would have been long ago raised into a genus if similarly marked
species of _Vanessa_ occurred in other parts of the world, as is the
case with the other species of the genus. Thus, it is well known that
there is a whole series of species resembling our _V. Cardui_, and
another series resembling our _V. C.-album_, the two series possessing
the same respective types of marking; indeed on these grounds the
sub-genera _Pyrameis_ and _Grapta_ have been erected.[187]

I should not have considered it worth while to have made these remarks
if it had not been for the fact that the caterpillars of _V. Io_ and
_V. Antiopa_ differ in small particulars from one another and from the
other species of the genus. These differences relate to the number and
position of the spines, as can be seen from the following table:--

SPECIES OF THE GENUS VANESSA, FABR.

  +-------------------+-----------------------------------------------+
  |                   |   Number of Spines on the head and segments   |
  |                   |                 of the larva.                 |
  |                   +-----+-----+-----+-----+-----+-----+-----+-----+
  |                   |Head.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.|Segm.|
  |                   |     |  I. | II. | III.| IV. |  V. |VI.- | XII.|
  |                   |     |     |     |     |     |     | XI. |     |
  |                   +-----+-----+-----+-----+-----+-----+-----+-----+
  |V. Io              |  0  |  0  |  2  |  2  |  4  |  6  |  6  |  4  |
  |V. Antiopa         |  0  |  0  |  4  |  4  |  6  |  6  |  7  |  4  |
  |V. Urticæ          |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Polychloros     |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Ichnusa         |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Atalanta        |  0  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. C.-album        |  2  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Interrogationis |  2  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  |V. Levana          |  2  |  0  |  4  |  4  |  7  |  7  |  7  |  4  |
  +-------------------+-----+-----+-----+-----+-----+-----+-----+-----+

This character of the number of spines will not be considered as too
unimportant when we observe how perfectly constant it remains in the
nearly allied species. This is the case in the three consecutive
forms, _Urticæ_, _Polychloros_, and _Ichnusa_. Now when we see
that two species which differ in their imaginal characters present
correspondingly small differences in their larvæ, this exact systematic
congruence indicates a completely parallel phyletic development.

Exceptions are, however, to be met with here. Thus, Hübner has united
one group of the species of _Vanessa_ into the genus _Pyrameis_ just
mentioned, on account of certain characteristic distinctions of the
butterflies. I do not know, however, how this genus admits of being
grounded on the structure of the larvæ; the latter, as appears from the
above table, agree exactly in the number and position of the spines
with the caterpillars of _Vanessa_ (_sensû strictiori_), nor can any
common form of marking be detected which would enable them to be
separated from _Vanessa_.

Still more striking is the incongruence in the genus _Araschnia_,
Hübn. (_A. Prorsa-Levana_), which, like the genus _Pyrameis_, is
entirely based on imaginal characters. This is distinguished from all
the other sub-genera of the old genus _Vanessa_ by a small difference
in the venation of the wings (the discoidal cell of the hind-wings is
open instead of closed). Now it is well-known that in butterflies the
wing-venation, as most correctly shown by Herrich-Schäffer, is the
safest criterion of “relationship.” It thus happens that this genus,
typified by the common _Levana_, is in Kirby’s Catalogue separated
from _Vanessa_ by two genera, and according to Herrich-Schäffer[188]
by forty genera! Nevertheless, the larvæ agree so exactly in their
spinal formula with _Grapta_ that we should have no hesitation in
regarding them as a species of this sub-genus. It appears to me very
probable that in this case the form-relationship of the caterpillar
gives more correct information as to the blood-relationship of the
species than that of the imago--in any case the larvæ show a different
form-relationship to the imagines.

Just as in the case of butterflies there are many genera of _Sphingidæ_
which can be based on the structure of the larvæ, and which agree with
those founded on the imagines.

Thus, the genus _Macroglossa_ is characterized by a straight anal horn,
a spherical head, and by a marking composed of longitudinal stripes,
these characters not occurring elsewhere in this combination. The
nearly allied genus _Pterogon_, on the other hand, cannot be based
on the larvæ only, since not only is the marking of the adult larva
very distinct in the different species, but the anal horn is present
in two species, whilst in a third (_P. Œnotheræ_) it is replaced by
a knob-like eye-spot. The genus _Sphinx_ (_sensû strictiori_) is
distinguished by the simple, curved caudal horn, the smooth, egg-shaped
head and smooth skin, and by a marking mainly composed of seven oblique
stripes. The genus _Deilephila_ is distinguished from the preceding by
a dorsal plate, situated on the prothorax and interrupting the marking,
as well as by the pattern, which here consists of a subdorsal line
with ring-spots more or less numerous and developed; the skin also
is rough, “shagreened,” although it must be admitted that there are
exceptions (_Vespertilio_). The genus _Chærocampa_ admits also of being
based on the form-relationship of its caterpillars, although this is
certainly only possible by disregarding the marking and taking alone
into consideration the peculiar pig-like form of the larvæ. The genus
_Acherontia_, so nearly related to _Sphinx_, possesses in the doubly
curved caudal horn a character common to the genus (three species
known[189]). Finally may be mentioned the genus _Smerinthus_, of which
the larvæ, by their anteriorly tapering form, their shagreened skin and
almost triangular head with the apex upwards, their simply curved anal
horn, and by their seven oblique stripes on each side, constitute a
genus as sharply defined as that formed by the moths.

Although in all the systematic divisions hitherto treated of there are
cases where the form-relationship of the larva does not completely
coincide with that of the imago, such incongruences are of far more
frequent occurrence in the smallest systematic group, viz. species.

The larvæ of two species have very frequently a much nearer
form-relationship than their imagines. Thus, the caterpillars of
_Smerinthus Ocellatus_ and _S. Populi_ are closely allied in
structure, marking, and colouring, whilst the moths in these two last
characters and in the form of the wings are widely separated.[190]
Judging from the larvæ we should expect to obtain two very similar
moths, but in fact both _Populi_ and _Ocellatus_ have many near allies,
and these closely related species sometimes possess larvæ which differ
more widely than those of more distantly related species of imagines.

Thus, in Amur-land and North America there occur species of
_Smerinthus_ which closely resemble our _Ocellatus_ in colour, marking,
and form of wing, and which possess the characteristic large blue
ocellus on the hind-wings. _S. Excæcatus_ is quite correctly regarded
as the representative American form of our _Ocellatus_, but its
caterpillar, instead of being leaf-green, is of a chrome-yellow, and
possesses dark green instead of white oblique stripes, and has moreover
a number of red spots, and a red band on the head--in brief, in the
very characters (colour and certain of the markings) in which the
imagines completely agree it is widely different from _Ocellatus_. It
appears also to be covered with short bristles, judging from Abbot and
Smith’s figure.[191]

Just in the same way that the species having the nearest conceivable
form-relationship to _Ocellatus_ possesses a relatively strongly
diverging larva, so does the nearest form-relation of _Populi_ (imago)
offer a parallel case. This species, which is also North American,
lives on _Juglans Alba_. The imago of _Smerinthus Juglandis_ differs
considerably from _S. Populi_ in the form of the wings, but it
resembles the European species so closely in marking and colouring that
no doubt can exist as to the near relationship of the two forms. The
caterpillar of _S. Juglandis_,[192] however, differs to a great extent
from that of _Populi_ in colour--it is not possible to confound these
two larvæ; but those of _Populi_ and _Ocellatus_ are not only easily
mistaken for one another, but are distinguished with difficulty even by
experts.

In this same family of the _Sphingidæ_ cases are not wanting in which,
on the other hand, the moths are far more closely allied than the
larvæ. This is especially striking in the genus _Deilephila_, eight
species of which are allied in the imaginal state in a remarkable
degree, whilst the larvæ differ greatly from one another in colour,
and to as great an extent in marking. These eight species are _D.
Nicæa_, _Euphorbiæ_, _Dahlii_, _Galii_, _Livornica_, _Lineata_,
_Zygophylli_, and _Hippophaës_. Of these, _Nicæa_, _Euphorbiæ_,
_Dahlii_, _Zygophylli_, and _Hippophaës_ are so much alike in their
whole structure, in the form of the wings, and in marking, that
few entomologists can correctly identify them off-hand without
comparison. The larvæ of these four species, however, are of very
different appearances. Those of _Euphorbiæ_ and _Dahlii_ are most
alike, both being distinguished by the possession of a double row of
large ring-spots. _Zygophylli_ (see Fig. 50, Pl. VI.) possesses only
faint indications of ring-spots on a white subdorsal line; and in
_Hippophaës_ there is only an orange-red spot on the eleventh segment,
the entire marking consisting of a subdorsal line on which, in some
individuals, there are situated more or less developed ring-spots (see
Figs. 59 and 60, Pl. VII.). If we only compare the larvæ and imagines
of _D. Euphorbiæ_ and _Hippophaës_, we cannot but be struck with
astonishment at the great difference of form-relationship in the two
stages of development.

In the case of _D. Euphorbiæ_ and _Nicæa_ this difference is almost
greater. Whilst these larvæ show great differences in colour, marking,
and in the roughness or smoothness of the skin (compare Fig. 51, Pl.
VI. with Figs. 43 and 44, Pl. V.), the moths cannot be distinguished
with certainty. As has already been stated, the imago of the rare
_D. Nicæa_ is for this reason wanting in most collections; it cannot
be detected whether a specimen is genuine, _i.e._ whether it may not
perhaps be a somewhat large example of _D. Euphorbiæ_.

An especially striking instance of incongruence is offered by the
two species of _Chærocampa_ most common with us, viz., _Elpenor_ and
_Porcellus_, the large and small Elephant Hawk-moths. The larvæ are
so similar, even in the smallest details of marking, that they could
scarcely be identified with certainty were it not that one species
(_Elpenor_) is considerably larger and possesses a less curved caudal
horn than the other. The moths of these two species much resemble
one another in their dull green and red colours, but differ in the
arrangement of these colours, _i.e._ in marking, and also in the form
of their wings, to such an extent that _Porcellus_ has been referred to
the genus _Pergesa_[193] of Walker. If systemy, as is admitted on many
sides, has only to indicate the morphological relationship, this author
is not to blame--but in this case a special larval classification must
likewise be admitted, in a manner somewhat similar to that at present
adopted provisionally in text-books of zoology for the Hydroid Polypes
and inferior Medusæ. This case of _Porcellus_, however, shows that
those are correct who maintain that systemy claims to express, although
incompletely, the blood-relationship, and that systematists have always
unconsciously formed their groups as though they intended to express
the genetic connection of the forms. Only on this supposition can it
appear incorrect to us to thus separate two species of which the larvæ
agree so completely.

I cannot conclude this review of the various systematic groups without
taking a glance at the groups comprised within species, viz. varieties.
Whilst in species incongruence is of frequent occurrence, in varieties
this is the rule, for which reason it admits in this case of being more
sharply defined, since we are not concerned with a double difference
but only with the question whether in the one stage a difference or an
absolute similarity is observable. By far the majority of varieties are
either simply imaginal or merely larval varieties--only the one stage
diverges, the other is quite constant.

Thus, as has already been shown, in all the seasonally dimorphic
butterflies known to me the caterpillars of the two generations of
imagines, which are often so widely different, are exactly alike; and
the same obtains for the majority of purely climatic varieties of
butterflies. Unfortunately there are as yet no connected observations
on this point. The only certain instance that I can here mention is
that of the Alpine and Polar form of _Pteris Napi_. This variety,
_Bryoniæ_, the female of which differs so greatly in marking and
colouring, possesses larvæ which cannot be distinguished from those of
the ordinary form of _Napi_.(See part I. appendix I. p. 124.)

That caterpillars can also vary locally without thereby affecting the
imagines is shown by the frequently mentioned and closely investigated
cases of di- and polymorphism in the larvæ of a number of _Sphingidæ_
(_M. Stellatarum_, _A. Atropos_, _S. Convolvuli_, _C. Elpenor_, and
_Porcellus_, &c.). The same thing is still more clearly shown by those
instances in which there are not several but only one distinct larval
form occurring in each of two different localities.

To this class belongs the above-mentioned case of _Chærocampa Celerio_
(p. 197), supposing our information concerning this species to be
correct; likewise the recently-mentioned case of the Ligurian variety
of the caterpillar of _Saturnia Carpini_; and finally the case of
_Eriogaster Lanestris_, so well known to lepidopterists. This insect
inhabits the plains of Germany, and in the Alps extends to an elevation
of 7000 feet, where it possesses a larva differently marked and
coloured (_E. Arbusculæ_) to those of the lowlands whilst the moths are
smaller, but do not differ in other respects from those of the plains.

Among the Alpine species many other such cases may occur, but these
could only be discovered by making investigations having special
reference to this point. Of the Alpine butterflies, for example,
not a single species can have been reared from the caterpillar;
for this reason but few observations have on the whole been given
by entomologists respecting the Alpine larvæ, which are not known
sufficiently well to enable such a question to be decided.

The investigation of the form-relationships existing between larvæ on
the one hand and imagines on the other has thus led to the following
results:--

We learn on comparison that incongruences or inequalities of
form-relationship occur in all systematic groups from varieties to
families. These incongruences are of two kinds, in some cases being
disclosed by the fact that the larvæ of two systematic groups, _e.g._
two species, are more closely related in form than their imagines (or
inversely), whilst in other cases the larvæ form different systematic
groups to those formed by the imagines.

The results of the investigation into the occurrence of incongruences
among the various systematic groups may be thus briefly summarised:--

Incongruences appear to occur most frequently among varieties, since
it very frequently happens that it is only the larva or only the
imago which has diverged into a variety, the other stage remaining
monomorphic. The systematic division of varieties is thus very often
one-sided.

Among species also incongruences are of frequent occurrence. Sometimes
the imagines are much more nearly related in form than the larvæ, and
at others the reverse happens; whilst again the case appears also
to occur in which only the one stage (larva) diverges to the extent
of specific difference, the other stage remaining monomorphic (_D.
Euphorbiæ_ and _Nicæa_).

The agreement in form-relationship appears to be most complete in
genera. In the greater number of cases the larval and imaginal genera
coincide, not only in the sharpness of their limits, but also--as far
as one can judge--in the weight of their distinctive characters, and
therefore in the amount of their divergence. Of all the systematic
groups, genera show the greatest congruence.

In families there is again an increase of irregularity. Although larval
and imaginal families generally agree, there are so many exceptions
that the groups would be smaller if they were based exclusively
on the larval structure than if founded entirely on the imagines
(_Nymphalidæ_, _Bombycidæ_).

If we turn to the groups of families we find a considerably increased
incongruence; complete agreement is here again rather the exception,
and it further happens in these cases that it is always the larvæ
which, to a certain extent, remain at a lower grade, and which form
well defined families; but these can seldom be associated into groups
of a higher order having a common character, as in the case of the
imagines (_Rhopalocera_).

After having thus collected (so far as I am able) the facts, we have
now to attempt their interpretation, and from the observed congruence
and incongruence of form-relationship of the two stages to endeavour to
draw a conclusion as to the underlying causes of the transformations.

It is clear at starting that all cases of incongruence can only be the
expression or the consequence of a phyletic development which has not
been exactly parallel in the two stages of larva and imago--that one
stage must have changed either more rapidly or more slowly than the
other. An “unequal phyletic development” is thus the immediate cause of
incongruence.

Thus, the occurrence of different larvæ in species of which the
imagines have remained alike may be simply understood as cases in
which the imago only has experienced a change--has taken a forward
step in phyletic development, whilst the larvæ have remained behind.
If we conceive this one-sided development to be repeated several
times, there would arise two larval forms as widely different as those
of _Deilephila Nicæa_, and _Euphorbiæ_, whilst the imagines, as is
actually the case in these species, would remain the same.

The more commonly occurring case in which one stage has a greater
form-divergence than the other, is explicable by the one stage having
changed more frequently or more strongly than the other.

The explanation of the phenomena thus far lies on the surface, and it
is scarcely possible to advance any other; but why should one stage
become changed more frequently or to a greater extent than the other?
why should one portion be induced to change more frequently or more
strongly than another? whence come these inducements to change? These
questions bring us to the main point of inquiry:--Are the causes which
give rise to these changes internal or external? Are the latter the
result of a phyletic vital force, or are they only due to the action of
the external conditions of life?

Although an answer to this question will be found in the preceding
essay, I will not support myself on the results there obtained, but
will endeavour to give another solution of the problem on fresh
grounds. The answer will indeed be the same as before:--A phyletic
force must be discountenanced, since in the first place it does not
explain the phenomena, and in the second place the phenomena can be
well explained without its assumption.

The admission of a phyletic vital force does not explain the phenomena.
The assumption that there is a transforming power innate in the
organism indeed agrees quite well with the phenomenon of congruence,
but not with that of incongruence. Since a large number of cases of
the latter depend upon the fact that the larvæ are more frequently
influenced by causes of change than their imagines, or _vice versâ_,
how can this be reconciled with such an internal force? On this
assumption would not each stage of a species be compelled to change, if
not contemporaneously at least successively, with the same frequency
and intensity, by the action of an innate force? and how by means of
the latter can there ever result a greater form-divergence in the larvæ
than in the imagines?

It is delusive to believe that these unequal deviations can be
explained by assuming that the phyletic force acts periodically.
Granting that it does so, and that the internal power successively
compels the imago, pupa, and finally the larva to change, there would
then pass a kind of wave of transformation over the different stages
of the species, as was actually shown above to be the case in the
single larval stages. The only possible way of explaining the unequal
distances between larvæ and imagines would therefore be to assume that
two allied groups, _e.g._ species, were not contemporaneously affected
by the wave, so that at a certain period of time the imago alone of one
species had become changed, whilst in the other species the wave of
transformation had also reached the larva. In this case the imagines of
the two species would thus appear to be more nearly related than their
larvæ.

Now this strained explanation is eminently inapplicable to varieties,
still less to species, and least of all to higher systematic groups,
for the simple reason that every wave of transformation may be
assumed to be at the most of such strength as to produce a deviation
of form equal to that of a variety. Were the change resulting from
a single disturbance greater, we should not only find one-sided
varieties, _i.e._ those belonging to _one_ stage, but we should
also meet as frequently with one-sided species. If, however, a wave
of transformation can only produce a variety even in the case of
greatest form-divergence, the above hypothetical uncontemporaneous
action of such a wave in two species could only give rise to such
small differences in the two stages that we could but designate them
as varieties. An accumulation of the results of the action of several
successive waves passing over the same species could not happen,
because the distance from a neighbouring species would always become
the same in two stages as soon as _one_ wave had ended its course. In
this manner there could therefore only arise divergences of the value
of varieties, and incongruences in systematic groups of a higher rank
could not thus be explained.

All explanations of the second form of incongruence from the point of
view of a phyletic force can also be shown to be absurd. How can the
fact be explained that larval and imaginal families by no means always
coincide; or that the larvæ can only be formed into families whilst
the imagines partly form sharply defined groups of a higher order? How
can an internal directive force within the same organism urge in two
quite distinct directions? If the evolution of a definite system were
designed, and the admission of such a continually acting power rendered
necessary, why such an incomplete, uncertain, and confused performance?

I must leave others to answer these questions; to me a vital force
appears to be inadmissible, not only because we cannot understand the
phenomena by its aid, but above all because it is superfluous for their
explanation. In accordance with general principles the assumption of an
unknown force can, however, only be made when it is indispensable to
the comprehension of the phenomena.

I believe that the phenomena can be quite well understood without any
such assumption--both the phenomena of congruence and incongruence, in
their two forms of unequal divergence and unequal group-formation.

Let us in the first place admit that there is no directive force in
the organism inciting periodic change, but that every change is always
the consequence of external conditions, being ultimately nothing but
the reaction--the response of the organism to some of the influences
proceeding from the environment; every living form would in this case
remain constant so long as it was not compelled to change by inciting
causes. Such transforming factors can act directly or indirectly,
_i.e._ they can produce new changes immediately, or can bring about
a remodelling by the combination, accumulation, or suppression
of individual variations already present (adaptation by natural
selection). Both forms of this action of external influences have long
been shown to be in actual operation, so that no new assumption will be
made, but only an attempt to explain the phenomena in question by the
sole action of these known factors of species formation.

If, in the first instance, we fix our attention upon that form of
incongruence which manifests itself through unequal divergence of
form-relationship, it will appear prominently that this bears precise
relations to the different systematic groups. This form of incongruence
constitutes the rule in varieties of the order Lepidoptera, it is of
very frequent occurrence in species, but disappears almost completely
in genera, and entirely in the case of families and the higher groups.
On the whole, therefore, as we turn to more and more comprehensive
groups, the incongruence diminishes whilst the congruence increases,
until finally the latter becomes the rule.

Now if congruence presupposes an equal number of transforming
impulses, we perceive that the number of the impulses which have
affected larvæ and imagines agree with one another the more closely the
larger the systematic groups which are compared together. How can this
be otherwise? The larger the systematic group the longer the period of
time which must have been necessary for its formation, and the more
numerous the transforming impulses which must have acted upon it before
its formation was completed.

But if the supposition that the impulse to change always comes from
the environment in no way favours the idea that such impulses always
affect both stages contemporaneously, and are equal in number during
the same period of time, there is not, on the other hand, the least
ground for assuming that throughout long periods the larvæ or the
imagines only would have been affected by such transforming influences.
This could have been inferred from the fact that varieties frequently
depend only upon one stage, whilst specific differences in larvæ only
also occur occasionally, the imagines remaining alike; but no single
genus is known of which all the species possess similar larvæ. Within
the period of time during which genera can be formed the transforming
impulses therefore never actually affect the one stage only, but always
influence both.

But if this is the case--if within the period of time which is
sufficient for the production of species, the one stage only is but
seldom and quite exceptionally influenced by transforming impulses,
whilst both stages are as a rule affected, although not with the same
frequency, it must necessarily follow that on the whole, as the period
of time increases, the difference in the number of these impulses which
affect the larva and of those which affect the imago must continually
decrease, and with this difference the magnitude of the morphological
differences resulting from the transforming influences must at the same
time also diminish. With the number of the successively increasing
changes the difference in the magnitude of the change in the two stages
would always relatively diminish until it had quite vanished from our
perception; just in the same manner as we can distinguish a group of
three grains of corn from one composed of six, but not a heap of 103
grains from one containing 106 grains.

That the small systematic groups must have required a short period and
the large groups a long period of time for their formation requires no
special proof, but results immediately from the theory of descent.

All the foregoing considerations would, however, only hold good if
the transforming impulses were equal in strength, or, not to speak
figuratively, if the changes only occurred in equivalent portions of
the body, _i.e._ in such portions as those in which the changes are
of the same physiological and morphological importance to the whole
organism.

Now in the lower systematic groups this is always the case. Varieties,
species, and genera are always distinguished by only relatively small
differences; deep-seated distinctions do not here occur, as is implied
in the conception of these categories. The true cause of this is, I
believe, to be found in the circumstance that all changes take place
only by the smallest steps, so that greater differences can only arise
in the course of longer periods of time, within which a great number of
types (species) can, however, come into existence, and these would be
related by blood and in form in different degrees, and would therefore
form a systematic group of a higher rank.

The short periods necessary for the production of inferior groups, such
as genera, would not result in incongruences if only _un_typical parts
of the larvæ, such as marking or spines, underwent change, whilst in
the imagines typical parts--wings and legs--became transformed. The
changes which could have occurred in the wings, &c., during this period
of time would have been much too small to produce any considerable
influence on the other parts of the body by correlation; and two
species of which the larvæ and imagines, had changed with the same
frequency, would show a similar amount of divergence between the larvæ
and between the imagines, although on the one side only _un_typical
parts--_i.e._ those of no importance to the whole organization--and
on the other side typical parts, were affected. The _number_ of the
changes would here alone determine whether congruence or incongruence
occurred between the two stages.

The case would be quite different if, throughout a long period of time,
in the one stage only typical and in the other only untypical parts
were subjected to change. In the first case a complete transformation
of the whole structure would occur, since not only would the typical
parts, such as the wings, undergo a much further and increasing
transformation in the same direction, but these changes would also lead
to secondary alterations.

In this manner, I believe, must be explained the fact that in the
higher groups still greater form-divergences of the two stages occur;
and if this explanation is correct, the cause of this striking
phenomenon, viz., that incongruence diminishes from varieties to
genera, in which latter it occurs but exceptionally, whilst in families
and in the higher groups it again continually increases, is likewise
revealed. Up to genera the incongruence depends entirely upon the one
stage having become changed _more frequently_ than the other; but
in families and groups of families, and in the orders Diptera and
Hymenoptera, as will be shown subsequently, in sub-orders and tribes,
it depends upon the _importance of the part of the body_ affected by
the predominant change. In the latter case the number of changes is
of no importance, because these are so numerous that the difference
vanishes from our perception; but an equal number of changes, even
when very great, may now produce a much greater or a much smaller
transformation in the entire bodily structure according as they affect
typical or untypical portions, or according as they keep in the same
direction throughout a long period of time, or change their direction
frequently.

Those unequal form-divergences which occur in the higher systematic
groups a re always associated with a different formation of groups--the
larvæ form different systematic groups to the imagines, so that one
of these stages constitutes a higher or a lower group; or else the
groups are of equal importance in the two stages, but are of unequal
magnitude--they do not coincide, but the one overlaps the other.

Incongruences of this last kind appear in certain cases within families
(_Nymphalidæ_), but I will not now subject these to closer analysis,
because their causes will appear more clearly when subsequently
considering the orders Hymenoptera and Diptera. Incongruences of the
first kind, however, admit of a clear explanation in the case of
butterflies. They appear most distinctly in the groups composed of
families.

Nobody has as yet been able to establish the group _Rhopalocera_ by
means of any single character common to the larvæ; nevertheless, this
group in the imagines is the sharpest and best defined of the whole
order. If we inform the merest tyro that clubbed antennæ are the chief
character of the butterflies, he will never hesitate in assigning
one of these insects to its correct group. Such a typical character,
common to all families, is, however, absent in the larvæ; and it might
be correctly said that there were no Rhopalocerous larvæ, or rather
that there were only larvæ of _Equites_, _Nymphales_, and _Heliconii_.
The larvæ of the various families can be readily separated by means
of characteristic distinctions, and it would not be difficult for an
adept to distinguish to this extent in single cases a Rhopalocerous
caterpillar as such; but these larvæ possess only _family_ characters,
and not those of a higher order.

This incongruence partly depends upon the circumstance that the
form-divergence between a Rhopalocerous and a Heterocerous family is
much greater on the side of the imagines than on that of the larvæ.
Were there but a single family of butterflies in existence, such as
the _Equites_, we should be obliged to elevate this to the rank of
a sub-order on the side of the imagines, but not on that of the
larvæ. Such cases actually occur, and an instance of this kind will be
mentioned later in connection with the Diptera. But this alone does not
explain why, on the side of the imagines, a whole series of families
show the same amount of morphological divergence from the families
of other groups. There are two things, therefore, which must here be
explained:--First, why is the form-divergence between the imagines of
the Rhopalocera and Heterocera greater than that between their larvæ?
and, secondly, why can the imagines of the Rhopalocera be formed into
one large group by means of common characters whilst the larvæ cannot?

The answers to both these questions can easily be given from our
present standpoint. As far as the first question is concerned, this
finds its solution in the fact that the form-divergence always
corresponds exactly with the divergence of function, _i.e._ with the
divergence in the mode of life.

If we compare a butterfly with a moth there can be no doubt that the
difference in the conditions of life is far greater on the side of the
imagines than on that of the larvæ. The differences in the mode of life
of the larvæ are on the whole but very small. They are all vegetable
feeders, requiring large quantities of food, and can only cease
feeding during a short time, for which reason they never leave their
food-plants for long, and it is of more importance for them to remain
firmly attached than to be able to run rapidly. It is unnecessary for
them to seek long for their food, as they generally find themselves
amidst an abundance, and upon this depends the small development of
their eyes and other organs of sense. On the whole caterpillars live
under very uniform conditions, although these may vary in manifold
details.

The greatest difference in the mode of life which occurs amongst
Lepidopterous larvæ is shown by wood feeders. But even these, which
by their constant exclusion from light, the hardness of their food,
their confinement within narrow hard-walled galleries, and by the
peculiar kind of movement necessitated by these galleries, are so
differently situated in many particulars to those larvæ which live
openly on plants, have not experienced any general change in the
typical conformation of the body by adaptation to these conditions of
life. These larvæ, which, as has already been mentioned, belong to
the most diverse families, are more or less colourless and flattened,
and have very strong jaws and small feet; but in none of them do we
find a smaller number of segments, or any disappearance, or important
transformation of the typical limbs; they all without exception possess
sixteen legs, like the other larvæ excepting the _Geometræ_.

Now if even under the most widely diverging conditions of life
adaptation of form is produced by relatively small, and to a
certain extent superficial, changes, we should expect less typical
transformations in the great majority of caterpillars which live
on the exterior of plants or in their softer parts (most of the
Micro-lepidoptera). The great diversity in the forms of caterpillars
depends essentially upon a different formation of the skin and its
underlying portions. The skin is sometimes naked, and can then acquire
the most diverse colours, either protective or conspicuous, or it may
develop offensive or defensive markings; in other cases it may be
covered with hairs which sting, or with spines which prick; certain
of its glands may develop to an enormous size, and acquire brilliant
colours and the power of emitting stinking secretions (the tentacles of
the _Papilionidæ_ and Cuspidate larvæ); by the development of warts,
angles, humps, &c., any species of caterpillar may be invested with the
most grotesque shape, the significance of which with respect to the
life of the insect is as yet in most cases by no means clear: _typical
portions_ are not, however, essentially influenced by these manifold
variations. At most only the form of the individual segments of the
body, and with these the shape of the whole insect, become changed
(onisciform larvæ of _Lycænidæ_), but a segment is never suppressed,
and even any considerable lengthening of the legs occurs but very
seldom (_Stauropus Fagi_).[194]

We may therefore fairly assert that the structure of larvæ is on the
whole remarkably uniform, in consequence of the uniformity in the
conditions of life. Notwithstanding the great variety of external
aspects, the general structure of caterpillars does not become
changed--it is only their outward garb which varies, sometimes in one
direction, and sometimes in another, and which, starting from inherited
characters, becomes adapted to the various special conditions of life
in the best possible manner.

All this is quite different in the case of the imagines, where we
meet with very important differences in the conditions of life. The
butterflies, which live under the influence of direct sunlight and a
much higher temperature, and which are on the wing for a much longer
period during the day, must evidently be differently equipped to the
moths in their motor organs (wings), degree of hairiness, and in the
development of their eyes and other organs of sense. It is true that we
are not at present in a condition to furnish special proofs that the
individual organs of butterflies are exactly adapted to a diurnal life,
but we may safely draw this general conclusion from the circumstance
that no butterfly is of nocturnal habits.[195] It cannot be stated
in objection that there are many moths which fly by day. It certainly
appears that no great structural change is necessary to confer upon a
Lepidopteron organized for nocturnal life the power of also flying by
day; but this proves nothing against the view that the structure of
the butterflies depends upon adaptation to a diurnal life. Analogous
cases are known to occur in many other groups of animals. Thus, the
decapodous Crustacea are obviously organized for an aquatic life; but
there are some crabs which take long journeys by land. Fish appear
no less to be exclusively adapted to live in water; nevertheless the
“climbing-perch” (_Anabas_) can live for hours on land.

It is not the circumstance that some of the moths fly by day which is
extraordinary and demands a special explanation, but the reverse fact
just mentioned, that no known butterfly flies by night. We may conclude
from this that the organization of the latter is not adapted to a
nocturnal life.

If we assume[196] that the Lepidopterous family adapted to a diurnal
life gives rise in the course of time to a nocturnal family, there
can be no doubt but that the transformation of structure would be far
greater on the part of the imagines than on that of the larvæ. The
latter would not remain quite unchanged--not because their imagines
had taken to a nocturnal life which for the larva would be quite
immaterial, but because this change could only occur very gradually
in the course of a large number of generations, and during this long
period the conditions of life would necessarily often change with
respect to the larvæ. It has been shown above that within the period of
time necessary for the formation of a new species impulses to change
occur on both sides; how much more numerous therefore must these be
in the case of a group of much higher rank, for the establishment of
which a considerably longer period is required. In the case assumed,
therefore, the larvæ would also change, but they would suffer much
smaller transformations than the imagines. Whilst in the latter almost
all the typical portions of the body would undergo deep changes in
consequence of the entirely different conditions of life, the larvæ
would perhaps only change in marking, hairs, bristles, or other
external characters, the typical parts experiencing only unimportant
modifications.

In this manner it can easily be understood why the larvæ of a family
of _Noctuæ_ do not differ to a greater extent from those of a family of
butterflies than do the latter from some other Rhopalocerous family, or
why the imagines of a Rhopalocerous and a Heterocerous family present
much greater form-divergences than their larvæ. At the same time is
therefore explained the unequal value that must be attributed to any
single family of butterflies in its larvæ and in its imagines. The
unequal form-divergences coincide exactly with the inequalities in the
conditions of life.

When whole families of butterflies show the same structure in their
typical parts (antennæ, wings, &c.), and, what is of more importance,
can be separated as a systematic group of a higher order (_i.e._ as a
section or sub-order) from the other Lepidoptera whilst their larval
families do not appear to be connected by any common character, the
cause of this incongruence lies simply in the circumstance that the
imagines live under some peculiar conditions which are common to them
all, but which do not recur in other Lepidopterous groups. Their larvæ
live in precisely the same manner as those of all the other families of
Lepidoptera--they do not differ in their mode of life from those of the
Heterocerous families to a greater extent than they do from one another.

We therefore see here a community of form within the same compass as
that in which there is community in the conditions of life. In all
butterflies such community is found in their diurnal habits, and in
accordance with this we find that these only, and not their larvæ, can
be formed into a group having common characters.

In the larvæ also we only find agreement in the conditions of life
within a much wider compass, viz. within the whole order. Between
the limits of the order Lepidoptera the conditions of life in the
caterpillars are, as has just been shown, on the whole very uniform,
and the structure of the larvæ accordingly agrees almost exactly in all
Lepidopterous families in every essential, _i.e._ typical, part.

In this way is explained the hitherto incomprehensible phenomenon that
the sub-ordinal group _Rhopalocera_ cannot be based on the larvæ, but
that Lepidopterous caterpillars can as a whole be associated into a
higher group (order); they constitute altogether families and an order,
but not the intermediate group of a sub-order. By this means we at the
same time reply to an objection that may be raised, viz. that larval
forms cannot be formed into high systematic groups because of their
“low and undeveloped” organization.

To this form of incongruence, viz. to the formation of systematic
groups of unequal value and magnitude, I must attach the greatest
weight with respect to theoretical considerations. I maintain that
this, as I have already briefly indicated above, is wholly incompatible
with the admission of a phyletic force. How is it conceivable that such
a power could work in the same organism in two entirely different
directions--that it should in the same species lead to the constitution
of quite different systems for the larvæ and for the imagines, or that
it should lead only to the formation of families in the larvæ and to
sub-orders in the imagines? If an internal force existed which had
a tendency to call into existence certain groups of animal forms of
such a nature that these constituted one harmonious whole of which
the components bore to one another fixed morphological relationships,
it would certainly have been an easy matter for such a power to have
given to the larvæ of butterflies some small character which would
have distinguished them as such, and which would in some measure have
impressed them with the stamp of “_Rhopalocera_.” Of such a character
we find no trace however; on the contrary, everything goes to show that
the transformations of the organic world result entirely from external
influences.



III.

INCONGRUENCES IN OTHER ORDERS OF INSECTS.


Although the order Lepidoptera is for many reasons especially
favourable for an investigation such as that undertaken in the previous
section, it will nevertheless be advantageous to inquire into the
form-relationships of the two chief stages in some other orders of
metamorphic insects, and to investigate whether in these cases the
formation of systematic groups also coincides with common conditions of
life.


HYMENOPTERA.

In this order there cannot be the least doubt as to the
form-relationship of the imagines. The characteristic combination of
the pro- and meso-thorax, the number and venation of the wings, and the
mouth-organs formed for biting and licking, are found throughout the
whole order, and leave no doubt that the Hymenoptera are well based on
their imaginal characters.

But it is quite different with the larvæ. It may be boldly
asserted that the order would never have been founded if the larvæ
only had been known. Two distinct larval types here occur, the
one--caterpillar-like--possessing a distinct horny head provided with
the typical masticatory organs of insects, and a body having thirteen
segments, to which, in addition to a variable number of abdominal legs,
there are always attached three pairs of horny thoracic legs: the
other type is maggot-shaped, without the horny head, and is entirely
destitute of mouth-organs, or at least of the three pairs of typical
insect jaws, and is also without abdominal and thoracic legs. The
number of segments is extremely variable; the larvæ of the saw-flies
have thirteen besides the head, the maggot-shaped larvæ of bees possess
fourteen segments altogether, and the gall-flies and ichneumons only
twelve or ten. We should be much mistaken also if we expected to find
connecting characters in the internal organs. The intestine is quite
different in the two types of larvæ, the posterior opening being absent
in the maggot-like grubs; at most only the tracheal and nervous systems
show a certain agreement, but this is not complete.

The order Hymenoptera, precisely speaking and conceived only
morphologically, exists therefore but in the imagines; in the larvæ
there exist only the caterpillar- and maggot-formed groups. The former
shows a great resemblance to Lepidopterous larvæ, and in the absence
of all knowledge of the further development it might be attempted to
unite them with these into one group. The two certainly differ in
certain details of structure in the mouth-organs and in the number of
segments, abdominal legs, &c., to a sufficient extent to warrant their
being considered as two sub-orders of one larval order; but they would
in any case be regarded as much more nearly related in form than the
caterpillar- and maggot-like types of the Hymenopterous larvæ.

Is it not conceivable, however, that the imagines of the
Hymenoptera--that ichneumons and wasps may be only accidentally alike,
and that they have in fact arisen from quite distinct ancestral forms,
the one having proceeded with the Lepidopterous caterpillars from one
root, and the other with the grub-like Dipterous larvæ from another
root?

This is certainly not the case; the common characters are too
deep-seated to allow the supposition that the resemblance is here
only superficial. From the structure of the imagines alone the common
origin of all the Hymenoptera may be inferred with great probability.
This would be raised into a certainty if we could demonstrate the
phyletic development of the maggot-formed out of the caterpillar-formed
Hymenopterous larvæ by means of the ontogeny of the former. From the
beautiful investigations of Bütschli on the embryonic development of
bees[197] we know that the embryo of the grub possesses a complete
head, consisting of four segments and provided with the three typical
pairs of jaws. These head segments do not subsequently become formed
into a true horny head, but shrivel up; whilst the jaws disappear with
the exception of the first pair, which are retained in the form of soft
processes with small horny points. We know also that from the three
foremost segments of the embryo the three typical pairs of legs are
developed in the form of round buds, just as they first appear in all
insects.[198] These rudimentary limbs undergo complete degeneration
before the birth of the larva, as also do those of the whole[199] of
the remaining segments, which, even in this primitive condition, show a
small difference to the three foremost rudimentary legs.

The grub-like larvæ of the Hymenoptera have therefore descended from
forms which possessed a horny head with antennæ and three pairs
of gnathites and a 13-segmented body, of which the three foremost
segments were provided with legs differing somewhat from those of the
other segments; that is to say, they have descended from larvæ which
possessed a structure generally similar to that of the existing saw-fly
larvæ. The common derivation of all the Hymenoptera from one source is
thus established with certainty.[200]

But upon what does this great inequality in the form-relationship of
the larvæ and imagines depend? The existing maggot-like grubs are
without doubt much further removed from the active caterpillar-like
larvæ than are the saw-flies from the Aculeate Hymenoptera. Whilst
these two groups differ only through various modifications of the
typical parts (limbs, &c.), their larvæ are separable by much
deeper-seated distinctions; limbs of typical importance entirely
vanish in the one group, but in the other attain to complete
development.

In the Hymenoptera there exists therefore a very considerable
incongruence in the systems based morphologically, _i.e._ on the pure
form-relationships of the larvæ and of the imagines. The reason of this
is not difficult to find: _the conditions of life differ much less in
the case of the imagines than in that of the larvæ_. In the former the
conditions of life are similar in their broad features. Hymenoptera
live chiefly in the air and fly by day, and in their mode of obtaining
food do not present any considerable differences. Their larvæ, on the
other hand, live under almost diametrically opposite conditions. Those
of the saw-flies live after the manner of caterpillars upon or in
plants, in both cases their peculiar locomotion being adapted for the
acquisition and their masticatory organs for the reduction of food.
The larvæ of the other Hymenoptera, however, do not as a rule require
any means of locomotion for reaching nor any organs of mastication for
swallowing their food, since they are fed in cells, like the bees and
wasps, or grow up in plant galls of which they suck the juice, or are
parasitic on other insects by whose blood they are nourished. We can
readily comprehend that in the whole of this last group the legs should
disappear, that the jaws should likewise vanish or should become
diminished to one pair retained in a much reduced condition, that the
horny casing of the head, the surface of attachment of the muscles of
the jaws, should consequently be lost, and that even the segments of
the head itself should become more or less shrivelled up as the organs
of sense therein located became suppressed.

The incongruence manifests itself however in yet another manner than
by the relatively greater morphological divergence of the larvæ: a
different grouping is possible for the larvæ and for the imagines. If
we divide the Hymenoptera simply according to the form-relationships of
the imagines, the old division into the two sub-orders _Terebrantia_
or _Ditrocha_ and _Aculeata_ or _Monotrocha_ will be the most correct.
The distinguishing characters of a sting or ovipositor and a one- or
two-jointed trochanter are still of the greatest value. But these
two sub-orders do not by any means correspond with the two types of
larvæ since, in the _Terebrantia_, there occur families with both
caterpillar-formed and maggot-formed larvæ.

The cause is to be found in that a portion of these families possess
larvæ which are parasitic in other insects or in galls, their bodily
structure having by these means become transformed in a quite different
direction. The mode of life of the imagines is, on the other hand,
essentially the same.

We have here therefore another case like that which we met with among
the Rhopalocerous Lepidoptera, in which the imagines appear to be
capable of being formed into a higher group than the larvæ, because the
former live under conditions of life which are on the whole similar
whilst the latter live under very divergent conditions.

The old division of the Hymenoptera into two sub-orders has certainly
been abandoned in the later zoological text-books; they are now divided
into three:--saw-flies, parasitic, and aculeate Hymenoptera; but even
this arrangement has been adopted with reference to the different
structure of the larvæ. Whether this system is better than the older,
_i.e._ whether it better expresses the _genealogical_ relationship, I
will not now stop to investigate.[201]


DIPTERA.

The imagines of the Diptera (_genuina_), with the exception of the
_Aphaniptera_ and _Pupipara_, agree in all their chief characters,
such as the number and structure of the wings, the number and joints
of the legs, the peculiar formation of the thorax (fusion of the three
segments);[202] and even the structure of the mouth organs varies only
within narrow limits. This is in accordance with their mode of life,
which is very uniform in its main features: all the true Diptera live
in the light, moving chiefly by means of flight, but having also the
power of running; all those which take food in the imago condition
feed upon fluids. Their larvæ, on the other hand, are formed on two
essentially different types, the one--which I shall designate as the
gnat-type--possessing a horny head with eyes, three pairs of jaws,
and long or short antennæ, together with a 12- or 13-segmented body,
which is never provided with the three typical pairs of thoracic legs,
but frequently has the so-called abdominal legs on the first and last
segments. The other Dipterous larvæ are maggot-shaped and without a
horny head, or in fact without any head, since the first segment, the
homologue of the head, can in no case be distinguished through its
being larger than the others; it is on the contrary much smaller. The
typical insect mouth-parts are entirely absent, being replaced by a
variously formed and quite peculiar arrangement of hooks situated on
the mouth and capable of protrusion. Never more than eleven segments
are present besides the first, which is destitute of eyes; neither are
abdominal legs ever developed.

The mode of life differs very considerably in the two groups of larvæ.
Although the Dipterous maggots are not as a rule quite incapable of
locomotion like the grubs of the Hymenoptera (bees, ichneumons), the
majority are nevertheless possessed of but little power of movement
in the food-substance on which they were deposited as eggs. They do
not go in search of food, either because they are parasitic in other
insects in the same manner as the ichneumons (_Tachina_), or else they
live on decaying animal or vegetable substances or amidst large swarms
of their prey, like the larvæ of the _Syrphidæ_ amongst _Aphides_.
They generally undergo pupation in the same place as that which they
inhabit as larvæ and indeed in their larval skin which hardens into an
oval pupa-case. Some few leave their feeding place and pupate after
traversing a short distance (_Eristalis_).

As in the case of the Hymenoptera the structure of the larvæ can here
also be explained by peculiarities in their mode of life. Creatures
which live in a mass of food neither require special organs of
locomotion nor specially developed organs of sense (eyes). They have
no use for the three pairs of jaws since they only feed on liquid
substances, and the hooks within the mouth do not serve for the
reduction of food but only for fastening the whole body. With the jaws
and their muscular system there likewise disappears the necessity for a
hard surface of attachment, _i.e._ a corneous head.

The mode of life of the larvæ of the gnat-type is quite different
in most points. The majority, and indeed the most typically formed
of these, have to go in search of their food, whether they are
predaceous, such as the _Culicidæ_ and many of the other _Nemocera_
(_Corethra_, _Simulium_), or whether they feed on plants, which they
in some cases weave into a protective dwelling tube (certain species
of _Chironomus_). Many live in water and move with great rapidity;
others bury in the earth or in vegetable substances; and even those
species which live on fungi sometimes wander great distances, as in
the well-known case of the “army worm” where thousands of the larvæ of
_Sciara Thomæ_ thus migrate.

Now the two types of larvæ correspond generally with the two large
groups into which, as it appears to me correctly, the Diptera
(_genuina_) are as a rule divided. In this respect there is therefore
an equality of form-relationship--the grouping is the same, and
the incongruence depends only upon the form-divergence between the
two kinds of larvæ being greater than between the two kinds of
imagines.[203]

That the form-divergence is greater in the larvæ than in the imagines
cannot be doubted; that this distant form-relationship cannot,
however, be referred to a very remote common origin, _i.e._ to a very
remote blood-relationship, not only appears from the existence of
transition-forms between the two sub-orders, but can be demonstrated
here, as in the case of the Hymenoptera, by the embryonic development
of the maggot-like larvæ.

Seventeen years ago I showed[204] that the grub-formed larvæ of the
_Muscidæ_ in the embryonic state possessed a well-developed head with
antennæ and three pairs of jaws, but that later in the course of the
embryonic development a marked reduction and transformation of these
parts takes place, so that finally the four head segments appear as a
single small ring formed from the coalesced pairs of maxillæ, whilst
the so-called “fore-head” (the first head segment), together with the
mandibles, becomes transformed into a suctorial-head armed with hooks
and lying within the body. At the time of writing I drew no conclusion
from these facts with reference to the phyletic development of these
larval forms; nor did Bütschli, six years later, in the precisely
analogous case of the larvæ of the bees. The inference is, however, so
obvious that it is astonishing that it should not have been drawn till
the present time.[205]

There can be no doubt that the maggot-like larvæ of insects are not by
any means ancient forms, but are, on the contrary, quite recent, as
first pointed out by Fritz Müller,[206] and afterwards by Packard[207]
and Brauer,[208] and as is maintained in the latest work by Paul
Mayer[209] on the phylogeny of insects.

The Dipterous maggots have evidently descended from a larval form
which possessed a horny head with antennæ and three pairs of jaws, but
which had no appendages to the abdominal segments; they are therefore
ordinary Dipterous larvæ of the gnat-type which have become modified
in a quite peculiar manner and adapted to a new mode of life, just as
the grubs of the Hymenoptera are larvæ of the saw-fly type, which have
become similarly transformed, although by no means in the same manner.
The resemblance between the two types of larva is to a great extent
purely external, and depends upon the process designated “convergence”
by Oscar Schmidt, _i.e._ upon the adaptation of heterogeneous animal
forms to similar conditions of life. By adaptation to a life within
a mass of fluid nutriment, the caterpillar-formed larvæ of the
Hymenoptera and the _Tipula_-like larvæ of the Diptera have acquired
a similar external appearance, and many similarities in internal
structure, or, in brief, have attained to a considerable degree of
form-relationship, which would certainly have tended to conceal the
wide divergence in blood-relationship did not the embryological forms
on the one side and the imagines on the other provide us with an
explanation.

It is certainly of great interest that in another order of insects--the
Coleoptera--grub-formed larvæ occur quite irregularly, and their origin
can be here traced to precisely the same conditions of life as those
which have produced the grubs of bees. I refer to the honey-devouring
larvæ of the _Meloïdæ_ (_Meloë_, _Sitaris_, _Cantharis_). The case is
the more instructive, inasmuch that the six-legged larval form is not
yet relegated to the development within the egg, but is retained in the
first larval stage. In the _second larval stage_ the maggot-form is
first assumed, although this is certainly not so well pronounced as in
the Diptera or Hymenoptera, as neither the head nor the thoracic legs
are so completely suppressed as in these orders. Nevertheless, these
parts have made a great advance in the process of transformation.

The grub-like larvæ of the Hymenoptera and Diptera appear to me
especially instructive with reference to the main question of the
causes of transformation. The reply to the questions: what gives the
impetus to change? is this impetus internal or external? can scarcely
be given with greater clearness than here. If these larvæ have
abandoned their ancestral form and have acquired a widely divergent
structure, arising not only from suppression but partly also from an
essentially new differentiation (suctorial head of the _Muscidæ_), and
if these structural changes show a close adaptation to the existing
conditions of life, from these considerations alone it is difficult
to conceive how such transformations can depend upon the action of a
phyletic force. The latter must have foreseen that at precisely this
or that fixed period of time the ancestors of these larvæ would have
been placed under conditions of life which would make it desirable
for them to be modified into the maggot-type. But if at the same
time the imagines are removed in a less degree from those of the
caterpillar-like larvæ, this divergence being in exact relation with
the deviations in the conditions of life, I at least fail to see how we
can escape the consequence that it is the external conditions of life
which produce the transformations and induce the organism to change.
It is to me incomprehensible how one and the same vital force can in
the same individual induce one stage to become transformed feebly and
the other stage strongly, these transformations corresponding in extent
with the stronger or weaker deviations in the conditions of life to
which the organism is exposed in the two stages; to say nothing of the
fact that by such unequal divergences the idea of a perfect system
(creative thought) is completely upset.

Nor can the objection be raised that we are here only concerned with
insignificant changes--with nothing more than the arrested development
of single organs and so forth, in brief, only with those changes which
can be ascribed to the action of the environment.

We are here as little concerned with a mere suppression of organs
through arrested development as in the case of the Cirripedia;
the transformation and reconstruction of the whole body goes even
much further than in these Crustacea, although not so conspicuous
externally. Where do we elsewhere find insects having the head inside
a cavity of the body (sectorial head of the _Muscidæ_), and of
which the foremost segment--the physiological representative of the
head--consists entirely of the coalesced antennæ and pairs of maxillæ?

The incongruences in the form-relationships are, however, exceedingly
numerous in the case of the Diptera, and a special treatise would
be necessary to discuss them thoroughly. I may here mention only one
case, because the inequality shows itself in this instance in a quite
opposite sense.

Gerstäcker, who is certainly a competent entomologist, divides the
Diptera into three tribes, viz. the _Diptera genuina_, the _Pupipara_,
and the _Aphaniptera_. The latter, the fleas, possess in their divided
thoracic segments and in their jointed labial appendages characters so
widely divergent from those of the true Diptera and of the _Pupipara_
that Latreille and the English zoologists have separated them entirely
from the Diptera and have raised them into a separate order.[210] Those
who do not agree in this arrangement, but with Gerstäcker include the
fleas under the Diptera, will nevertheless admit that the morphological
divergence between the _Aphaniptera_ and the two other tribes is far
greater than that which exists between the latter. Now the larvæ of the
fleas are completely similar in structure to those of the gnat-type,
since they possess a corneous head with the typical mouth parts and
antennæ and a 13-segmented body devoid of legs. Were we only acquainted
with the larvæ of the fleas we should rank them with the true Diptera
under the sub-order _Nemocera_. On first finding such a larva we should
expect to see emerge from the pupa a small gnat.

While the imagines of the _Nemocera_ and _Aphaniptera_ thus show but a
very remote form-relationship their larvæ are very closely allied. Can
any one doubt that in this case it is not the larva but the imago which
has diverged to the greatest extent? Have not the fleas moreover become
adapted to conditions of life widely different from those of all other
Diptera, whilst their larvæ do not differ in this respect from many
other Dipterous larvæ?

We have here, therefore, another case of unequal phyletic development,
which manifests itself in the entirely different form-relationship
of the larvæ and the imagines. Thus in this case, as in that of the
Lepidoptera, it is sometimes the larval and at other times the imaginal
stage which has experienced the greatest transformation, and, as in the
order mentioned, the objection that a phyletic vital force produces
greater and more important differentiations in the higher imaginal
stage than in the lower or less developed larval stage, is equally
ineffectual.

If, however, it be asked whether the unequal phyletic development
depends in this case upon an unequal number of transforming impulses
which the two stages may have experienced during an equal period of
time, this must be decidedly answered in the negative. The unequal
development obviously depends in this case, as in the higher systematic
groups of the Lepidoptera, upon the unequal value of the parts affected
by the changes. These parts are on the one side of small importance,
and on the other side of great importance, to the whole structure of
the insect. This is shown in the last-mentioned case of the fleas,
where, of the typical parts of the body, only the wings have become
rudimentary, whilst the antennæ, mouth-parts, and legs, and even the
form and mode of segmentation (free thoracic segments), must have
suffered most important modifications; their larvæ, on the other hand,
can have experienced only unimportant changes, since they still agree
in all typical parts with those of the gnat-type.

Although therefore in this and in similar cases a greater number of
transforming impulses may well have occurred on the one side than on
the other--and it is indeed highly probable that this number has not
been absolutely the same--nevertheless the chief cause of the striking
incongruence is not to be found therein, but rather in the _strength_
of the transforming impulses, if I may be permitted to employ this
figure, or, more precisely expressed, in the importance of the parts
which become changed and at the same time in the amount of change.

In this conclusion there is implied as it appears to me an important
theoretical result which tells further against the efficacy of a
phyletic force.

If the so-called “typical parts” of an animal disappear completely
through the action of the environment only, and still further, if these
parts can become so entirely modified as to give rise to quite new and
again typical structures (suctorial head of the _Muscidæ_) without
the typical parts of the other stage of the same individual being
thereby modified and transformed into a new type of structure, how can
we maintain a distinction between typical and non-typical parts with
respect to their origin? But if a difference exists with respect only
to the physiological importance of such parts, _i.e._ their importance
for the equilibrium of the whole organization, while, with reference to
transformation and suppression, exactly the same influences appear to
be effective as those which bring about a change in or a disappearance
of the so-called adventitious parts, where is there left any scope
for the operation of the supposed phyletic force? What right have we
to assume that the typical structures arise by the action of a vital
force? Nevertheless this is the final refuge of those who are bound
to admit that a great number of parts or characters of an animal can
become changed, suppressed, or even produced by the action of the
environment.



IV.

SUMMARY AND CONCLUSION.


The question heading the second section of this essay must at the
conclusion of the investigation be answered in the negative. The
form-relationship of the larvæ does not always coincide with that
of the imagines, or, in other words, a system based entirely on the
morphology of the larvæ does not always coincide with that founded
entirely on the morphology of the imagines.

Two kinds of incongruence here present themselves. The first arises
from the different amount of divergence between two systematic groups
in the larvæ and in the imagines, these groups being of equal extent.
The second form of incongruence consists essentially in that the
two stages form systematic groups of different extents, either the
one stage constituting a group of a higher order than the other and
therefore forming a group of unequal value, or else the two stages form
groups of equal systematic value, these groups, however, not coinciding
in extent, but the one overlapping the other.

This second form of incongruence is very frequently connected with the
first kind, and is mostly the direct consequence of the latter.

The cause of the incongruences is to be found in unequal phyletic
development, either the one stage within the same period of time having
been influenced by a greater number of transforming impulses than the
other, or else these impulses have been different in strength, _i.e._
have affected parts of greater or less physiological value, or have
influenced parts of equal value with unequal strength.

In all these cases in which there are deep-rooted form-differences,
it can be shown that these correspond exactly with inequalities in
the conditions of life, this correspondence being in two directions,
viz. in strength and in extent: the former determines the _degree_ of
form-difference, the latter its _extent_ throughout a larger or smaller
group of species.

The different forms of incongruence are manifested in the following
manner:--

(1.) Different amount of form-divergence between the larvæ on the one
side and the imagines on the other. Among the Lepidoptera this is found
most frequently in varieties and species, and there is evidence to
show that in this case the one stage has been affected by transforming
influences, either alone (varieties), or at any rate to a greater
extent (species). In the last case it can be shown in many ways that
one stage (the larva) has actually remained at an older phyletic grade
(_Deilephila_ species). Incongruences of this kind depending entirely
upon the more frequent action of transforming impulses can only become
observable in the smaller systematic groups, in the larger they elude
comparative examination. In the higher groups unequal form-divergence
may be produced by the transforming impulses affecting parts of unequal
physiological and morphological value, or by their influencing parts
of equal value in different degrees. All effects of this kind can,
however, only become manifest after a long-continued accumulation of
single changes, _i.e._ only in those systematic groups which require
a long period of time for their formation. By this means we can
completely explain why the incongruences of form-divergence continually
diminish from varieties to genera, and then increase again from genera
upwards through families, tribes, and sub-orders: the first diminishing
incongruence depends upon an _unequal number_ of transforming impulses,
the latter increasing incongruence depends upon the _unequal power_ of
these impulses.

Cases of the second kind are found among the Lepidopterous families,
and especially in the higher groups (_Rhopalocera_ and _Heterocera_),
and appear still more striking in the higher groups of the Hymenoptera
and Diptera. Thus the caterpillar shaped and maggot-formed larvæ of
the Hymenoptera differ from one another to a much greater extent
than their imagines, since the latter have experienced a complete
transformation of typical parts; whilst in the caterpillar-formed larvæ
these parts vary only within moderate limits. Similarly in the case
of the Diptera, of which the gnat-like larvæ diverge more widely from
those of the grub type than do the gnats from the true flies. On the
other hand the divergence between the imagines of the fleas and gnats
is considerably greater than that between their larvæ--indeed the larvæ
of the fleas would have to be ranked as a family of the sub-order of
the gnat-like larvæ if we wished to carry out a larval classification.
By this it is also made evident that these unequal divergences, when
they occur in the higher systematic groups, always induce at the same
time the second form of incongruence--that of the formation of unequal
systematic groups.

In general whenever such unequal divergences occur in the higher groups
they run parallel with a strong deviation in the conditions of life.
If these differ more strongly on the side of the larvæ, we find that
the structure of the latter likewise diverges the more widely, and that
their form-relationship is in consequence made more remote (saw-flies
and ichneumons, gnats and flies); if, on the other hand, the difference
in the conditions of life is greater on the side of the imagines,
we find among the latter the greater morphological divergence
(butterflies and moths, gnats and fleas).

(2.) The second chief form of incongruence consists in the formation
of different systematic groups by the larvæ and the imagines, if the
latter are grouped simply according to their form-relationship without
reference to their genetic affinities. This incongruence again shows
itself in two forms--in the formation of groups of unequal value, and
the formation of groups equal in value but unequal in extent, _i.e._ of
overlapping instead of coinciding groups.

Of these two forms the first arises as the direct result of a different
amount of divergence. Thus the larvæ of the fleas, on account of their
small divergence from those of the gnats, could only lay claim to the
rank of a _family_, whilst their imagines are separated from the gnats
by such a wide form-divergence that they are correctly ranked as a
distinct _tribe_ or _sub-order_.

The inequalities in the lowest groups, varieties, can be regarded in a
precisely similar manner. If the larva of a species has become split up
into two local forms, but not the imago, each of the two larval forms
possesses only the rank of a _variety_, whilst the imaginal form has
the value of a _species_.

Less simple are the causes of the phenomenon that in the one stage the
lower groups can be combined into one of higher rank, whilst the other
stage does not attain to this high rank. Such a condition appears
especially complicated when the two stages can again be formed into
groups of a still higher rank.

This is the case in the tribe _Rhopalocera_, which is founded on the
imagines alone, the larvæ forming only families of butterflies. Both
stages can however be again combined into the highest systematic group
of the Lepidoptera.

In this case also the difference in the value of the systematic groups
formed by the two stages corresponds precisely with the difference in
the conditions of life. This appears very distinctly when there are
several sub-groups on each side, and not when, as in the fleas, only
one family is present as a tribe on the one side and on the other as
a family. Thus in the butterflies, on the one side there are numerous
families combined into the higher rank of a sub-order (imagines),
whilst on the other side (larvæ) a group of the same extent cannot
be formed. In this instance it can be distinctly shown that the
combination of the families into a group of a higher order, as is
possible on the side of the imagines, corresponds exactly with the
limits in which the conditions of life deviate from those of other
Lepidopterous families. The group of butterflies corresponds with an
equally large circle of uniform conditions of life, whilst a similar
uniformity is wanting on the side of the larvæ.

The second kind of unequal group formation arises from the circumstance
that groups of equal value can be formed from the two stages, but these
groups do not possess the same limits--they overlap, and only coincide
in part.

This is most clearly seen in the order Hymenoptera, in which both larvæ
and imagines form two well-defined morphological sub-orders, but in
such a manner that the one larval form not only prevails throughout the
whole of the one sub-order of the imagines, but also extends beyond and
spreads over a great portion of the other imaginal sub-order.

Here again the dependence of this phenomenon upon the influence
of the environment is very distinct, since it can be demonstrated
(by the embryology of bees) that the one form of larva--the
maggot-type--although the structure now diverges so widely, has
been developed from the other form, and that it must have arisen by
adaptation to certain widely divergent conditions of life.

This form of incongruence is always connected with unequal divergence
between the two stages of the one systematic group--in this
case the _Terebrantia_. The larvæ of this imaginal group partly
possess caterpillar-like (_Phytospheces_) and partly maggot-formed
(_Entomospheces_) larvæ, and differ from one another to a considerably
greater extent than the saw-flies from the ichneumons.[211] The final
cause of the incongruence lies therefore in this case also in the fact
that one stage has suffered stronger changes than the other, so that
a deeper division of the group has occurred in the former than in the
latter.

The analogous incongruences in single families of the Lepidoptera may
have arisen in a similar manner, as has already been more clearly shown
above; only in these cases we are as yet unable to prove in detail that
the larval structure has become more strongly changed through special
external conditions of life than that of the imagines.

In the smallest systematic group--varieties, it has been possible to
furnish some proof of this. The one-sided change here depends in part
upon the _direct action_ of external influences (seasonal dimorphism,
climatic variation), and it can be shown that these influences
(temperature) acted only on the one stage, and accordingly induced
change in this alone whilst the other stage remained unaltered.

It has now been shown--not indeed in every individual case, but for
each of the different kinds of incongruence of form-relationship--that
there is an exact parallelism corresponding throughout with the
incongruence in the conditions of life. Wherever the forms diverge
more widely in one stage than in the other we also find more widely
divergent conditions of life; wherever the morphological systemy of
one stage fails to coincide with that of the other--whether in the
extent or in the value of the groups--the conditions of life in that
stage also diverge, either more widely or at the same time within
other limits; whenever a morphological group can be constructed from
one stage but not from the other, we find that this stage alone is
submitted to certain common conditions of life which fail in the other
stage.

The law that the divergence in form always corresponds exactly with
the divergence in the conditions of life[212] has accordingly received
confirmation in all cases where we have been able to pronounce
judgment. Unequal form-divergences correspond precisely with unequal
divergence in the conditions of life, and community of form appears
within exactly the same limits as community in the conditions of life.

These investigations may thus be concluded with the following law:--In
types of similar origin, _i.e._ having the same blood-relationship,
the degree of morphological relationship corresponds exactly with the
degree of difference in the conditions of life in the two stages.

With respect to the question as to the final cause of transformation
this result is certainly of the greatest importance.

The interdependence of structure and function has often been insisted
upon, but so long as this has reference only to the agreement of each
particular form with some special mode of life, this harmony could
still be regarded as the result of a directive power; but when in
metamorphic forms we not only see a double agreement between structure
and function, but also that the transformation of the form occurs in
the two chief developmental stages in successive steps at unequal
rates and with unequal strength and rhythm, we must--at least so it
appears to me--abandon the idea of an inherent transforming force; and
this becomes the more necessary when, by means of the opposite and
extremely simple assumption that transformations result entirely from
the response of the organism to the actions of the environment, all the
phenomena--so far as our knowledge of facts at present extends--can be
satisfactorily explained. A power compelling transformation, _i.e._
a phyletic vital force, must be abandoned, on the double ground that
it is incapable of explaining the phenomena (incongruence and unequal
phyletic development), and further because it is superfluous.

Against the latter half of this argument there can at most be raised
but the one objection that the phenomena of transformation are not
completely represented by the cases here analysed. In so far as this
signifies that the whole organic world, animal and vegetable, has not
been comprised within the investigation this objection is quite valid.
The question may be raised as to the limit to which we may venture to
extend the results obtained from one small group of forms. I shall
return to this question in the last essay.

But if by this objection it is meant that the restricted field of
the investigation enables us to actually analyse only a portion of
the occurring transformations,[213] and indeed only those cases,
the dependence of which upon the external conditions of life would
be generally admitted, I will not let pass the opportunity of once
more pointing out at the conclusion of the present essay that the
incongruences shown to exist by no means depend only upon those more
superficial characters the remodelling of which in accordance with
the external conditions of life may be most easily discerned and
is most difficult to deny, but that in certain cases (maggot-like
Dipterous larvæ) it is precisely the “typical” parts which become
partly suppressed and partly converted into an entirely new structure.
From the ancient typical appendages there have here arisen new
structures, which again have every right to be considered as typical.
This transformation is not to be compared with that experienced
by the swimming appendages of the _Nauplius_-like ancestor of an
_Apus_ or _Branchipus_ which have become mandibulate, nor with the
transformation which the anterior limbs must have gone through in
the reptilian ancestors of birds. The changes in question (Dipterous
larvæ) go still further and are more profound. I lay great emphasis
upon this because we have here one of the few cases which show that
typical parts are quite as dependent upon the environment as untypical
structures, and that the former are not only able to become adapted to
external conditions by small modifications--as shown in a most striking
manner by the transformations of the appendages in the Crustacea and
Vertebrata--but that these parts can become modelled on an entirely
new type which, when perfected, gives no means of divining its mode
of origin. I may here repeat a former statement:--With reference to
the causes of their origination we have no grounds for drawing a
distinction between typical and untypical structures.

It may be mentioned in concluding that quite analogous although less
sharply defined results are arrived at if, instead of fixing our
attention upon the different stages of a systematic group in their
phyletic development, we only compare the different functional parts
(organs in the wide sense) of the organisms.

A complete parallel can be drawn between the two classes of
developmental phenomena. From the very different systematic values
attached by taxonomists to this or that organ in a group of animals,
it may be concluded that the individual parts of an organism are to
a certain extent independent, and that each can vary independently,
when affected either entirely alone or in a preponderating degree by
transforming impulses, without all the other parts of the organism
likewise suffering transformation, or at least without their becoming
modified in an equal degree. Did all the parts and organs in two groups
of animals diverge from each other to the same extent, the systematic
value of such parts would be perfectly equal; we should, for example,
be able to distinguish and characterize two genera of the family of
mice by their kidneys, their liver, their salivary glands, or by the
histological structure of their hair or muscles, or even by differences
in their myology, &c. equally as well as by their teeth, length of
toes, &c. It is true that such a diagnosis has yet to be attempted; but
it may safely be predicted that it would not succeed. Judging from all
the facts at present before us, the individual parts--and especially
those connected in their physiological action, _i.e._ the system of
organs--do not keep pace with reference to the modifications which the
species undergoes in the course of time; at one period one system and
at another period some other system of organs advances while the others
remain behind.

This corresponds exactly with the result already deduced from the
unparallel development of the independent ontogenetic stages. If the
inequality in the phyletic development is more sharply pronounced in
this than in the last class of cases, this can be explained by the
greater degree of correlation which exists between the individual
systems of organs in any single organism as compared with that existing
between the ontogenetic stages, which, although developed from one
another, are nevertheless almost completely independent. We should have
expected _à priori_ that a strong correlation would have here existed,
but as a matter of fact this is not the case, or is so only in a very
small degree.

Just as in the stages of metamorphosis the inequality of phyletic
development becomes the more obliterated the more distant and
comprehensive, or, in other words, the greater the period of existence
of the groups which we compare, so does the unequal divergence of the
systems of organs become obliterated as we bring into comparison larger
and larger systematic groups.

It is not inconceivable--although a clear proof of this is certainly
as yet wanting--that a variety of the ancestral species would differ
only in one single character, such as hairiness, colour, or marking,
and such instances would thus agree precisely with the foregoing cases
in which only the caterpillar or the butterfly formed a variety.
All the more profound modifications however--such for instance as
those which determine the difference between two species--are never
limited to one character, but always affect several, this being
explicable by correlation, which, as Darwin has shown in the case of
dogs, may cause modifications in the skull of those breeds having
hanging ears in consequence of this last character alone. It must be
admitted however that one organ only would be originally affected by
a modifying influence. Thus, I am acquainted with two species of a
genus of Daphniacea which are so closely allied that they can only be
distinguished from one another by a close comparison of individual
details. But whilst most of the external and internal organs are almost
identical in the two species the sperm-cells of the males differ in a
most striking manner, in one species resembling an Australian boomerang
in form and in the other being spherical! An analogous instance is
furnished by _Daphnia Pulex_ and _D. Magna_, two species which were
for a long time confounded. Nearly all the parts of the body are here
exactly alike, but the antennæ of the males differ to a remarkable
extent, as was first correctly shown by Leydig.

Similarly in the case of genera there may be observed an incongruence
of such a kind that individual parts of the body may deviate to
a greater or to a less extent than the corresponding parts in an
allied genus. If, for instance, we compare a species of the genus
of Daphniacea, _Sida_, with a species of the nearly allied genus
_Daphnella_, we find that all the external and internal organs are in
some measure dissimilar--nevertheless certain of these parts deviate to
an especially large extent, and have without question become far more
transformed than the others. This is the case, for example, with the
antennæ and the male sexual organs. The latter, in _Daphnella_, open
out at the sides of the posterior part of the body as long, boot-shaped
generative organs, and in _Sida_ as small papillæ on the ventral side
of this region of the body. If again we compare _Daphnella_ with the
nearly allied genus _Latona_, it will be found that no part in the one
is exactly similar to the corresponding part in the other genus, whilst
certain organs differ more widely than others. This is the case for
instance with the oar-like appendages which in _Latona_ are triramous,
but in _Daphnella_, as in almost all the other Daphniacea, only
biramous.

In families the estimation of the form-divergence of the systems of
organs and parts of the body becomes difficult and uncertain: still it
may safely be asserted that the two Cladocerous families _Polyphemidæ_
and _Daphniidæ_ differ much less from one another in the structure of
their oar-like appendages than in that of their other parts, such as
the head, shell, legs, or abdominal segments. In systematic groups of
a still higher order, _i.e._ in orders, and still more in classes, we
might be inclined to consider that all the organs had become modified
to an equally great extent. Nevertheless it cannot be conclusively
said that the kidneys of a bird differ from those of a mammal to the
same extent as do the feathers from mammalian hair, since we cannot
estimate the differences between quite heterogeneous things--it can
only be stated that both differ greatly. Here also the facts are not
such as would have been expected if transformation was the result of
an internal developmental force; no uniform modification of _all_
parts takes place, but first one part varies (variety) and then others
(species), and, on the whole, as the systematic divergence increases
all parts become more and more affected by the transformation and
all tend continually to appear changed to an equal extent. This is
precisely what would be expected if the transforming impulses came
from the environment. An equalization of the differences caused by
transformation must be produced in two ways; first by correlation,
since nearly every primary transformation must entail one or more
secondary changes, and secondly because, as the period of time
increases, more numerous parts of the body must become influenced by
primary transforming factors.

A tempting theme is here also offered by attempting to trace the
inequality of phyletic development to dissimilar external influences,
and by demonstrating that individual organs have as a rule become
modified in proportion to the divergence in the conditions of life by
which they have been influenced, this action, during a given period of
time, having been more frequent in the case of one organ than in that
of the others, or, in brief, by showing the connection between the
causes and effects of transformation.

It would be quite premature, however, to undertake such a labour at
present, since it will be long before physiology is able to account
for the fine distinctions shown by morphology, and further because we
have as yet no insight into those internal adjustments of the organism
which would enable us _à priori_ to deduce definite secondary changes
from a given primary transformation. But so long as this is impossible
we have no means of distinguishing correlative changes from the primary
modifications producing them, unless they happen to arise under our
observation.



APPENDIX I.[214]

ADDITIONAL NOTES ON THE ONTOGENY, PHYLOGENY, &C., OF CATERPILLARS.


_Ontogeny of the Noctua larvæ._--References have already been given
in a previous note (67, p. 166) to observations on the number of legs
and geometer-like habits of certain _Noctua_-larvæ when newly hatched.
This interesting fact in the development of these insects furnishes
a most instructive application of the principle of ontogeny to the
determination of the true affinities, _i.e._ the blood-relationship of
certain groups of Lepidoptera. While the foregoing portions of this
work have been in course of preparation for the press, some additional
observations on this subject have been published, and I may take the
present opportunity of pointing out their systematic bearing--not,
indeed, with a view to settling definitively the positions of the
groups in question, as our knowledge is still somewhat scanty--but with
the object of stimulating further investigation.

Mr. H. T. Stainton has lately recorded the fact that the young larva
of _Triphæna Pronuba_ is a semi-looper (Ent. Mo. Mag. vol. xvii. p.
135); and in a recently published life-history of _Euclidia Glyphica_
(_Ibid._ p. 210) Mr. G. T. Porritt states that this caterpillar is
a true looper when young, but becomes a semi-looper when adult. To
these facts Mr. R. F. Logan adds (_Ibid._ p. 237) that “nearly all
the larvæ of the _Trifidæ_ are semi-loopers when first hatched.” The
_Cymatophoræ_ appear to be an exception, but Mr. Logan points out
that this genus is altogether aberrant, and seems to be allied to the
_Tortricidæ_. Summing up the results of these and the observations
previously referred to, it will be seen that this developmental
character has now been established in the case of species belonging
to the following families of the section _Genuinæ_:--_Leucaniidæ_,
_Apameidæ_, _Caradrinidæ_, _Noctuidæ_, _Orthosiidæ_, _Hadenidæ_,
and _Xylinidæ_, as well as the other _Trifidæ_ (excepting
_Cymatophora_).[215] The larvæ of the _Minores_ and _Quadrifidæ_ are
as a rule semi-loopers when adult and may be true loopers when young,
although further observations on this point are wanted. These facts
point to the conclusion that the _Noctuæ_ as a whole are phyletically
younger than the _Geometræ_, whilst the _Genuinæ_ and _Bombyciformes_
have further advanced in phyletic development than the _Minores_ and
_Quadrifidæ_. The last two sections are therefore the most closely
related to the _Geometræ_, as correctly shown by the arrangement given
in Stainton’s “Manual;” whilst that adopted in Doubleday’s “Synonymic
List,” where the _Geometræ_ precede the _Noctuæ_, is most probably
erroneous.

_Additional descriptions of Sphinx-larvæ._--In the foregoing essay on
“The Origin of the Markings of Caterpillars,” Dr. Weismann has paid
special attention to the larvæ of the _Sphingidæ_ and has utilized
for this purpose, in addition to his own studies of the ontogeny of
many European species, the figures in the chief works dealing with
this family published down to the time of appearance of his essay
(1876).[216] In order to amplify this part of the subject I have added
references to more recent descriptions and figures of Sphinx-larvæ
published by Burmeister and A. G. Butler, and I have endeavoured in
these cases to refer the caterpillars as far as possible to their
correct position in the respective groups founded on the ontogeny
and phylogeny of their allies. It is, however, obvious that for the
purposes of this work figures or descriptions of adult larvæ are of but
little value, except for the comparative morphology of the markings;
and even this branch of the subject only becomes of true biological
importance when viewed in the light of ontogeny. As our knowledge of
the latter still remains most incomplete in the case of exotic species,
it would be at present premature to attempt to draw up any genealogy
of the whole family, and I will here only extend the subject by adding
some few descriptions of species which are interesting as having been
made from the observations of field-naturalists, and which contain
remarks on the natural history of the insects.

Mr. C. V. Riley in his “Second Annual Report on the Noxious,
Beneficial, and other Insects of the State of Missouri, 1870,” gives
figures and describes the early stages and adult forms of certain
grape-vine feeding larvæ of the sub-family _Chærocampinæ_. The
full-grown larva of _Philampelus Achemon_, Drury, “measures about 3½
inches when crawling, which operation is effected by a series of sudden
jerks. The third segment is the largest, the second but half its size,
and the first still smaller, and when at rest the two last-mentioned
segments are partly withdrawn into the third.... The young larva is
green, with a long slender reddish horn rising from the eleventh
segment and curving over the back.” Mr. Riley then states that full
grown specimens are sometimes found as green as the younger ones, but
“they more generally assume a pale straw or reddish-brown colour, and
the long recurved horn is invariably replaced by a highly polished
lenticular tubercle.” The specimen figured was the pale straw variety,
this colour deepening at the sides, and finally merging into a rich
brown. The markings appear to consist of an interrupted brown dorsal
line, a continuous subdorsal line of the same colour, and six oblique
scalloped white bars along the side. Whether the colour and marking
is adapted to the vine, as is the case with the two varieties of the
dimorphic _Chærocampa Capensis_ (_q.v._), is not stated. The larva of
_Philampelus Satellitia_, Linn., when newly hatched, and for some time
afterwards is “green with a tinge of pink along the sides, and with an
immensely long straight pink horn at the tail. This horn soon begins
to shorten, and finally curls round like a dog’s tail.” The colour
of the insect changes to a reddish-brown as it grows older, and the
caudal horn is entirely lost at the third moult. The chief markings
appear to be five oblique cream-yellow patches with a black annulation
on segments 6-10, and a pale subdorsal line. The caterpillar crawls
by a series of sudden jerks, and often flings its “head savagely from
side to side when alarmed.” “When at rest, it draws back the fore
part of the body and retracts the head and first two joints into the
third.” Two points in connection with these species are of interest
with respect to the present investigations. The green colour and the
possession of a long caudal horn when young shows that these larvæ,
like those of _Chærocampa Elpenor_ (p. 178), _C. Porcellus_ (p. 184),
and _Philampelus Labruscæ_ (p. 195, note), are descended from ancestors
which possessed these characters in the adult state.[217] The next
point of interest is the attitude of alarm assumed by these larvæ,
and effected by withdrawing the head and two front segments into the
third.[218] The importance of this in connection with the similar
habit of ocellated species will be seen on reading the remarks on page
367 bearing upon the initial stages of eye-spots. The other species
figured by Mr. Riley are _Chærocampa Pampinatrix_, Smith and Abbot, and
_Thyreus Abboti_, Swains. The latter has already been referred to (p.
256).

In a paper “On a Collection of Lepidoptera from Candahar” (Proc. Zoo.
Soc., May 4th, 1880), Mr. A. G. Butler has described and figured, from
materials furnished to him by Major Howland Roberts, the larvæ of
three species of _Sphingidæ_. _Chærocampa Cretica_, Boisd., feeds on
vine; out of 100 specimens examined, there was not one black variety,
while in another closely allied species, found at Jutogh and Kashmir,
the larva is stated to be as often black as green. The general colour
of the caterpillar harmonizes with that of the underside of the vine
leaves; it possesses a thread-like dorsal, and a pale yellow subdorsal
line; also “a subdorsal row of eye-spots, each consisting of a green
patch in a yellow oval, the first spot on the fifth segment being the
largest and most distinct, those on each following segment becoming
smaller, more flattened, and less distinct, till lost on the twelfth
segment, sometimes becoming indistinct after the seventh or eighth
segment; these spots are only distinct as eye-spots on the fifth and
sixth segments, that on the sixth being flatter than that on the fifth,
those on the remaining segments appearing like dashes while the larvæ
is green, but more like eyes on its changing colour when full fed.”
The change here alluded to is the dark-brown coloration so generally
assumed by green Sphinx-larvæ previous to pupation, and which, as I
have stated elsewhere (Proc. Zoo. Soc., 1873, p. 155), is probably an
adaptation advantageous to such larvæ when crawling over the ground
in search of a suitable place of concealment. Making the necessary
correction for the different mode of counting the segments, it will be
seen that the primary ocelli of this species are in the same position
as those of the other species of this genus as described in a previous
part of this essay, and that it belongs to the second phyletic group
treated of at p. 193. The interesting fact that this species does not
display dimorphism, whilst the closely allied form from Kashmir is
dimorphic, shows that in the present species the process of double
adaptation has not taken place; and this will probably be found to be
connected with the habits of life, _i.e._ the insect being well adapted
to the colour of its food-plant may not conceal itself on the ground
by day. The caterpillar of _Deilephila Robertsi_, Butl., is found at
Candahar on a species of _Euphorbia_ growing on the rocky hills, and
is so abundant that at the end of May every plant with any leaves left
on it had several larvæ feeding upon it. “The larvæ are very beautiful
and conspicuous, and are very different in colouring according to their
different stages of growth.” The general colour is black with white
dots and spots; a subdorsal row of large roundish spots, one on each
segment, either white, yellow, orange or red; dorsal stripe variable
in colour, and sometimes only partially present or altogether absent.
“At the end of May most of the larvæ found presented a different
appearance; the black disappears more or less, and with it many of
the small white spots. In some cases the black only remains as a ring
round the larger white spots; the ground-colour therefore becomes
yellowish-green or yellow, varying very considerably.” The larva does
not change colour previous to pupation. This species, according to
the outline figure given (_loc. cit._, Pl. XXXIX., Fig. 9), appears
to belong to the first of Dr. Weismann’s groups, comprising _D.
Euphorbiæ_, _D. Dahlii_ and _D. Nicæa_ (see p. 199), and is therefore
in the seventh phyletic stage of development (p. 224). From the
recorded habits it seems most probable that the colours and markings
of this caterpillar are signals of distastefulness. It is much to
be regretted that Major Roberts has not increased the value of his
description of this species by adding some observations or experiments
bearing on this point. _Eusmerinthus Kindermanni_, Lederer, feeds on
willow. “General colour green, covered with minute white dots and seven
long pale yellow oblique lateral bands. (The ground-colour is the same
as the willow-leaves on which the larva feeds, the yellow stripes
the same as the leaf-stalks, and the head and true legs like the
younger branches).” As no subdorsal line is mentioned or figured, this
species must be regarded as belonging to the third stage of phyletic
development (see p. 242).

I have recently had an opportunity of inspecting a large number of
drawings of Sphinx-larvæ in the possession of Mr. F. Moore, and of
those species not mentioned in the previous portions of this work the
following may be noticed:--_Chærocampa Theylia_, Linn., like _Ch.
Lewisii_ (note 82, p. 194), appears to be another form connecting the
second and third phyletic groups of this genus. _Ch. Clotho_, Drury,
belongs to the third group (figured by Semper; see note 3 to this
Appendix). The larva of _Ch. Lucasii_, Walk., offers another instance
of the retention of the subdorsal line by an ocellated species. The
larva of _Ch. Lycetus_, Cram., of which Mr. Moore was so good as to
show me descriptions made at the various stages of growth, presents
many points of interest. It belongs to the third phyletic group, and
all the ocelli appear at a very early stage. The dimorphism appears
also in the young larvæ, some being green, and others black, a fact
which may be explained by the law of “backward transference” (see
p. 274). A most suggestive feature is presented by the caudal horn,
which in the young caterpillar is stated to be _freely movable_.
It is possible that this horn, which was formerly possessed by the
ancestors of the _Sphingidæ_, and which is now retained in many genera,
is a remnant of a flagellate organ having a similar function to the
head-tentacles of the _Papilio-larvæ_, or to the caudal appendages of
_Dicranura_ (see p. 289).

_Lophostethus Dumolinii_, Angas.--The larva of this species differs so
remarkably from those of all other _Sphingidæ_, that I have thought it
of sufficient interest to publish the following description, kindly
furnished by Mr. Roland Trimen, who in answer to my application sent
the following notes:--“My knowledge of the very remarkable larva
of this large and curious Smerinthine Hawk-moth is derived from a
photograph by the late Dr. J. E. Seaman, and from drawings and notes
recently furnished by Mr. W. D. Gooch. The colour is greenish-white,
inclining to grey, and in the male there is a yellow, but in the female
a bluish, tinge in this. All the segments but the second and the head
bear strong black spines, having a lustre of steel blue, and springing
from a pale yellow tubercular base. The longest of these spines are in
two dorsal rows from the fourth to the eleventh segment, the pairs on
the fourth and fifth segments being longer than the rest, very erect,
and armed with short simple prickles for three-fourths of their upper
extremity. The anal horn, which is shorter than the spines, is of the
same character as the latter, being covered with prickles, and much
inclined backwards. Two lateral rows of similar shorter spines extend
from the fourth to the 12th segment, and on each of the segments
6-11 the space between the upper and lower spines is marked with a
conspicuous pale yellow spot. Two rows of smaller similar spines extend
on each side (below the two rows of larger ones) from the second to the
thirteenth segment, one spine of the lowermost row being on the fleshy
base of each pro-leg. All the pro-legs are white close to the base, and
russet-brown beyond. Head smooth, unarmed in adult, greenish-white with
two longitudinal russet-brown stripes on face.

“The young larvæ have proportionally much longer and more erect spines
with distinct long prickles on them. There is a short pair besides,
either on the back of the head or on the second segment. Moreover,
the dorsal spines of the third and fourth segments, and the anal horn
(which is quite erect, and the longest of all), are longer than the
rest, and distinctly _forked_ at their extremity.

“Mr. Gooch notes that these young larvæ might readily be mistaken for
those of the _Acrææ_, and suggests that this may protect them. He also
states that the yellow lateral spots are only noticed after the last
moult before pupation, and that the general resemblance of the larva as
regards colour is to the faded leaves of its food-plant, a species of
_Dombeia_.”

The forked caudal horn in the young larva of this species is of
interest in connection with the similar character of this appendage in
the young caterpillar of _Hyloicus Pinastri_, p. 265.

_Retention of the Subdorsal Line by Ocellated Larvæ._--It has already
been shown with reference to the eye-spots of the _Chærocampa_-larvæ,
that these markings have been developed from the subdorsal line, and
that, in accordance with their function as a means of causing terror,
this line has in most species been eliminated in the course of the
phylogeny from those segments bearing the eye-spots in order to give
full effect to the latter (see p. 379). In accordance with the law
that a character when it has become useless gradually disappears, the
subdorsal is more or less absent in all those species in which the
ocelli are most perfectly developed; and it can be readily imagined
that in cases where adaptation to the foliage exists the suppression
of this line would under certain conditions be accelerated by natural
selection. On the other hand, it is conceivable that the subdorsal
line may under other conditions be of use to a protectively coloured
ocellated species by imitating some special part of the food-plant,
under which circumstances its retention would be secured by natural
selection.

Such an instance is offered by _Chærocampa Capensis_, Linn.; and as
this case is particularly instructive as likewise throwing light upon
the retention of the subdorsal by certain species having oblique
stripes (see p. 377, and note 166, p. 378), I will here give some details
concerning this species which have been communicated to me by Mr.
Roland Trimen, the well-known curator of the South African Museum, Cape
Town. The caterpillar of _C. Capensis_, like so many other species
of the genus, is dimorphic, one form being a bright (rather pale)
green, and the other, which is much the rarer of the two, being dull
pinkish-red. Both these forms are adapted in colour to the vine on
which they feed, the red variety according to some extent with the
faded leaves of the cultivated vines, but to a greater extent with the
young shoots and underside of the leaves of the South African native
vine (_Cissus Capensis_), on which it also feeds. There are two
eye-spots in this species in the usual positions; they are described as
being blue-grey in a white ring, and raised so as to project a little.
The subdorsal is white, and is bordered beneath by a wide shade of
bluish-green irrorated with white dots, and crossed by an indistinct
white oblique ray on each segment. These last markings are probably
remnants of an oblique striping formerly possessed by the progenitor of
this and other species of the genus (see, for instance, Fig. 25, Pl.
IV., one of the young stages of _C. Porcellus_). It is possible that
these rudimentary oblique stripes are now of service in assisting the
adaptation of the larva to its food-plant, but this cannot be decided
without seeing the insect _in situ_.

The subdorsal line extends from immediately behind the second eye-spot
to the base of the very short and much curved violet anal horn.
With reference to the protective colouring Mr. Trimen writes:--“The
difficulty of seeing these large and beautifully-coloured larvæ on
the vines is quite surprising; six or more may be well within sight,
and yet quite unnoticed. The subdorsal stripe greatly aids in their
concealment, as it well represents in its artificial light and shade
the leaf-stalks of the vine.” When this larva withdraws its front
segments the eye-spots stand out very menacingly; but in spite of this
it is greedily eaten by fowls and shrikes (_Fiscus Collaris_), and
Mr. Trimen also found that a tame suricate (_Rhyzæna Suricata_) and a
large monitor lizard (_Regenia Albogularis_) did not refuse them. The
failure of the eye-spots in causing terror in these particular cases
cannot be regarded as disproving their utility in all instances. It
must always be borne in mind that no protective character can possibly
be of service against _all_ foes; natural selection only requires that
such characters should be advantageous with respect to the majority
of the enemies of any species, and further experiments with this
caterpillar may show that in the case of smaller foes the eye-spots
are effective as a means of causing alarm. The dimorphism of the larva
of _C. Capensis_ is of special interest, although we are not yet
sufficiently acquainted with the habits of this species to offer a
complete explanation. According to Dr. Weismann’s conclusions (p. 297),
the dimorphism of the _Chærocampa_-larvæ is due to a double adaptation,
the insects first having acquired the habit of concealing themselves
by day, and the dark form having then been produced by the action of
natural selection, in order to adapt such varieties to the colour of
the soil, whilst others retained the green colour which adapts them
to the foliage of their food-plants. In accordance with this, _C.
Capensis_ may have a similar habit of concealment, or (should this be
found not to be the case) it is possible that this insect at a former
period possessed this habit and fed upon some other plant, when it
would have become dimorphic in the manner explained, and the _existing_
dimorphism may be a survival of the more ancient dimorphism, the red
form (corresponding to the older dark form) having been subsequently
modified so as to become also adapted to the new food-plant. Much light
would be thrown upon this by studying the ontogeny of the species.

_Phytophagic Variability._--A number of observations bearing on the
phytophagic variability of the Sphinx-larvæ and other caterpillars
have been recorded in a previous note (p. 305), and reference has also
been made to the food-plants of _Acherontia Atropos_ in South Africa
(note 121, p. 263). I am now enabled to add some further observations
on this species, from notes furnished to me by Mr. Roland Trimen, who
states that for many years he has noticed that at the Cape this larva
varies greatly in the depth and shade of the green ground-colour, the
variability being in strict accordance with the colour of the leaves
of the particular plant on which the individual feeds. The phenomenon
was particularly noticeable in larvæ feeding on _Buxia Grandiflora_, a
shrub in common cultivation in gardens, and of which the foliage is of
a very dull pale greyish-green. Another striking instance was noticed
in some very fine caterpillars feeding on a large shrubby _Solanum_,
which, excepting the bright yellow bands bordering the dorsal violet
bars, were generally dull ochreous-yellow, like the leaves and stalks
of the _Solanum_. On plants with bright green or deep green leaves,
the colour of the larvæ is almost in exact agreement. Mr. Trimen
adds:--“These remarks apply principally to the underside and pro-legs
and lower lateral regions, the dorsal colours of violet and yellow
varying but little. The protection afforded is very considerable, as
the larvæ almost always cling to the lower side of the twigs of their
food-plants, so that their uniformly-coloured under-surface is upwards,
and turned towards the light, and their variegated upper surface turned
downwards.”

These observations are of the highest importance, not only as adding
another instance to the recorded cases of phytophagic variation, but
likewise as showing that with this variability a protective habit
has been acquired. It is to be hoped that such a promising field
for experimental investigation as is offered by this and analogous
cases will not long remain unexplored. In attacking the problem two
chief questions have in the first place to be settled: (1) Is the
variability truly phytophagic, _i.e._ are the colour variations
actually brought about by the chemico-physiological action of the
food-plant? and (2) Are the larvæ at any period of growth susceptible
to the action of phytophagic influences? The first question could be
decided by feeding larvæ from the same batch of eggs on different
food-plants from the period of their hatching. The second question
could be settled by changing the food-plants of a series of selected
specimens at various stages of growth, and observing whether any
change of colour was produced. In accordance with the principles
advocated in a previous note (p. 305), it is conceivable _à priori_
that phytophagic variability may occur by direct chemico-physiological
action, quite irrespective of any of the changes of colour being of
protective use. In the case of brightly-coloured distasteful species
phytophagic variability might thus have full play, but in the case of
protectively-coloured edible species, phytophagic variability would be
under the control of natural selection. These considerations raise a
question of the greatest theoretical interest in connection with this
phenomenon. If phytophagic variability can have full play uncontrolled
by natural selection in brightly-coloured caterpillars, ought not this
phenomenon to be of more common occurrence in such species than in
those protectively coloured? Although our knowledge of this subject is
still very imperfect, as a matter of fact brightly coloured larvæ, so
far as I have been able to ascertain, do not appear to be susceptible
of phytophagic influences. But this apparent contradiction, instead
of opposing actually confirms the foregoing views, as will appear on
further consideration. The colours of protected species are as a whole
much inferior in brilliancy to those of inedible species, so that any
phytophagic effect would be more perceptible in the former than in the
latter, in which the highest possible standard of brilliancy appears
in most cases to have been attained. Now phytophagic variations of
colour appear to be of but small amount, or, in other words, such
variations fluctuate within comparatively restricted limits, and as
the cases at present known are mostly _adaptive_ it is legitimate
to conclude that they have been produced and brought to their
present standard by natural selection, _i.e._ that they have arisen
from phytophagic influences as a cause of variability. The initial
stages of phytophagic variations must therefore have been still less
perceptible than the now perfected final results; and this leads to the
conclusion that minute variations of this character were of sufficient
importance to protectively-coloured species to be taken advantage of
by natural selection. But minute variations in a dull-coloured larva
would, as previously pointed out, produce a comparatively much greater
effect than such variations in a brilliantly-coloured species; and as
protection is required by the former, the initial phytophagic effects
would be accumulated, and the power of adaptability conferred by the
continued action of natural selection, whilst in vividly-coloured
species where no power of adaptability is required this cause of
variation would not only produce a result which, as compared with its
effects upon dull species, may be regarded as a “vanishing quantity,”
but this result would be too insignificant to be taken advantage of
by natural selection, which is in these cases dealing only with large
“quantities,” and striving to make the caterpillars as brilliant as
possible. The fact that vividly-coloured distasteful larvæ do not show
phytophagic variation is to my mind explained proximately by these
considerations; the ultimate cause of phytophagic variability regarded
as a chemico-physiological action requires further investigation.

_Sexual Variation in Larvæ._--Since most of the markings of
caterpillars can be explained by the two factors of adaptation and
inheritance, or, in other words, by their present and past relations
to the environment, and since sexual selection can have played no
direct part in producing these colours and markings, I feel bound to
record here some few observations on the sexual differences in larvæ in
addition to the cases of _Anapæa_ and _Orgyia_ already recorded (note
i., p. 308) and of _Lophostethus Dumolinii_ (p. 527).

Mr. C. V. Riley states[219] with reference to the larva of _Thyreus
Abboti_ that the ground-colour appears to depend upon the sex, Dr.
Morris having described the insect as “reddish-brown with numerous
patches of light green,” and having expressly stated that “the _female_
is of a uniform reddish-brown with an interrupted dark-brown dorsal
line and transverse striæ.” Mr. W. D. Gooch, who has reared the South
African butterflies _Nymphalis Cithæron_ and _N. Brutus_ from their
larvæ, states[220] that these “differed sexually in both instances.”
Of _Brutus_ only a few were bred, but of _Cithæron_ many. “The sexual
difference of the latter was that the females had a large dorsal
sub-cordate cream mark, which was wanting, or only shown by a dot, in
the males, and the colour was more vivid in the edgings to the frontal
horns.”

Although such cases appear to be at present inexplicable, they are of
interest as examples of those “residual phenomena” which, as is well
known, have in many branches of science so often served as important
starting-points for new discoveries and generalizations.[221]



APPENDIX II.


The following paper by Dr. Fritz Müller[222] forms the third of
a series of communications on Brazilian butterflies published in
“Kosmos,” and as it bears upon the investigations made known in the
third essay of the present work, I will here give a translation, by
permission of the publisher, Herr Karl Alberts.


“ACRÆA AND THE MARACUJÁ BUTTERFLIES AS LARVÆ, PUPÆ, AND IMAGINES.

“In a thoughtful essay on ‘Phyletic Parallelism in Metamorphic
Species,’ Weismann has shown that in the case of Lepidoptera the
developmental stages of larva, pupa, and imago vary independently,
and that a change occurring in one stage is without influence upon
the preceding and succeeding stages, so that the course which has
been followed by the individual stages in their developmental history
has not been in all cases identical. This want of agreement may
manifest itself both by unequal divergence of form-relationship, and
by unequal group formation. With respect to unequal form-divergence
the caterpillars are sometimes more closely related in form than their
imagines, and at other times the reverse is the case. With respect
to unequal group formation again, two cases are possible; the larvæ
and imagines may form groups of unequal value, the one stage forming
higher or lower groups than the other, or they may form groups of
unequal size, _i.e._, groups which do not coincide but which overlap.
Form-relationship and blood-relationship do not therefore always agree;
the resemblances among the caterpillars would lead to a quite different
arrangement to that resulting from the resemblances among the imagines,
and it is probable that neither of these arrangements would correspond
with the actual relationships.

“Starting from this fact, which he establishes by numerous examples,
Weismann proceeds to show most convincingly that an innate power of
development or of transformation, such as has been assumed under
various names by many adherents of the development theory, has no
existence, but that every modification and advancement in species has
been called forth by external influences.

“A most beautiful illustration of the want of ‘phyletic parallelism,’
as Weismann designates the different form-relationships of the
larvæ, pupæ, and imagines, is furnished by the five genera _Acræa_,
_Heliconius_, _Eueides_, _Colænis_, and _Dione_ (= _Agraulis_). This
instance seems to me to be of especial value, because it offers the
rare case of pupæ showing greater differences than the larvæ and
imagines.

“The species of which I observed the larvæ and pupæ are _Acræa Thalia_
and _Alalia_, _Heliconius Eucrate_, _Eueides Isabella_, _Colænis Dido_
and _Julia_, _Dione Vanillæ_ and _Juno_; besides these I noticed the
pupa of _Eueides Aliphera_.

“The following remarks apply only to these species, although we may
suppose with great probability that the whole of the congeneric
forms--excepting perhaps the widely ranging species of _Acræa_--would
display similar characters to their Brazilian representatives.

“The imagines of the five genera mentioned form two sharply defined
families, the _Acræidæ_ and the butterflies of the Maracujá group.[223]
The latter comprises the three genera _Heliconius_, _Eueides_, and
_Colænis_, which differ only in very unimportant characters; _Eueides_
is distinguished from _Heliconius_ by its shorter antennæ, and
_Colænis_ differs from _Eueides_ in having the discoidal cell of the
hind-wings open. The genus _Dione_ is further removed by the different
structure of the legs, and the silvery spots on the underside of
the wings. Certain species resemble those of other genera in a most
striking manner, and much more closely both in colour and marking, and
even in the form of their wings, than they do their own congeners. This
is the case with _Acræa Thalia_ and _Eueides Pavana_, with _Heliconius
Eucrate_ and _Eueides Isabella_, and with _Eueides Aliphera_ and
_Colænis Julia_, which are deceptively alike, and the last two are
connected with _Dione Juno_, at least by the upper side of the wings.
The difficulty of judging of the relationships of the single species is
thus much aggravated; it cannot be said how much of this resemblance
is to be attributed to blood-relationship, and how much to deceptive
imitation.

“As larvæ all the Brazilian species must be placed _in one genus_, as
they agree exactly in the number and arrangement of their spines (4
spines, not in a transverse row, on segments 2 and 3; 6 spines, in a
transverse row, on segments 4-11; 4 spines, not in a transverse row,
on the last (12th) segment). They differ from one another much less
in this respect than do the German species of _Vanessa_, such, for
instance, as _V. Io_ or _Antiopa_ from _V. Polychloros_, _Urticæ_,
and _Atalanta_.[224] The larvæ of _Acræa Thalia_ are certainly
without the two spines on the head which the others possess, and, on
the other hand, they have a well-developed pair of spines on the
first segment, which, in most of the other species, are completely
absent; but this does not justify their separation, since the head
spines of _Heliconius_, _Eueides_, and _Colænis Dido_, which are of
a considerable length, are shorter than those of the next segment in
_Colænis Julia_, and _Dione Vanillæ_, and in _Dione Juno_ they dwindle
down to two minute points, this last species also bearing a short pair
on the first segment. The larva of _Dione Juno_ is thus as closely
related to that of _Acræa Thalia_ as it is to that of its congener
_Dione Vanillæ_.

“If it were desired to form two distinct larval groups this could not
be effected on the basis of their differences in form, but could only
be based on their food-plants. The larvæ of _Heliconius_, _Eueides_,
_Colænis_, and _Dione_ live on species of Maracujá (Passiflora);
those of _Acræa Thalia_ and _Alalia_ on Compositæ (_Mikania_ and
_Veronia_). These larval groups would agree with those founded on the
form-relationships of the imagines, but unlike the imaginal groups,
which can be formed into families, they would scarcely possess a
generic value.

“If we arrange the single species of caterpillars according to their
resemblances, this arrangement does not agree with that based on the
resemblances of the imagines, even if we disregard the different values
of the groups. The result is somewhat as follows:--

[Illustration:

                                         IMAGINES.
                   (Nymphalideous butterflies with tufts on wing-veins.)
                   /---------------------------------------------------\
 (Families.)                MARACUJÁ-GROUP.                     ACRÆIDÆ.
                    /---------------------------\                  |
            /-------------------\    /-------------------\         |
 (Genera.)  Heliconius.  Eueides.   Colænis.         Dione.      Acræa.
              |                    /-------\       /-------\       |
 (Species.) Eucrate.  Isabella.  Dido.  Julia.  Vanillæ.  Juno.  Thalia.
              |          |         |      |        |        |      |
              \----------+---------/      \--------/        \------/
                         \--------------------------------------/
                                         LARVÆ.”
]

       *       *       *       *       *

[Here follow the remarks on the habits of the larvæ in connection with
their colours, &c., which have already been quoted in illustration of
the use of the spiny protection (note 133, p. 293). From these facts
the author draws the conclusion that the form-relationships of the
caterpillars depend rather upon their mode of life than upon their
blood-relationships, assuming the latter to be correctly expressed by
the arrangement of the imagines at present adopted.]

[Illustration: Figs. 1-4. Pupæ of _Acræa Thalia_; _Heliconius Eucrate_;
_Eueides Isabella_, and _Colænis Dido_; life size.]

“A glance at the above figures of the pupæ of _Heliconius Eucrate_
(Fig. 2), _Eueides Isabella_ (Fig. 3), and _Colænis Dido_ (Fig. 4),
will show how great are the differences between these pupæ as compared
with the close form-relationship of all the Maracujá butterflies, and
with the no less close resemblance of their larvæ. A family which
comprised three such dissimilar pupæ would also be capable of including
that of _Acræa Thalia_ (Fig. 1).

“The pupa of this last species has nothing peculiar in its general
appearance, but possesses the ordinary pupal form; it is tolerably
rounded, without any great elevations or depressions; a minute pointed
projection is situated on the head over each eye-cover, and a similar
process projects from the roots of the wings. Its distinguishing
characters are five pairs of spines on the back of the abdominal
segments. These spines are found also in _Acræa Alalia_, but appear
to be absent in other species, _e.g._ in the Indian _A. Violæ_.
Last summer, among some batches of _Thalia_ larvæ--each batch being
the progeny from one lot of eggs--I found certain individuals which
differed from the others in having much shorter spines, and these
changed into pupæ in which the five pairs of spines were proportionally
shorter than usual, thus being an exception to the rule that changes in
one stage of development are without influence on the other stages. I
may remark, by the way, that this law, enunciated by Weismann, can only
be applied to imagines and pupæ with certain restrictions. The skin of
the pupa forms a sheath or cover for the eyes, antennæ, trunk, legs,
and wings of the imago, and if these parts undergo any considerable
modification in the latter, corresponding changes must appear in the
pupa. This is shown, for instance, by many ‘Skippers’ (_Hesperidæ_),
in which the extraordinarily long trunk necessitates a sheath of a
corresponding length. The colour of the pupa of _Acræa Thalia_ is
whitish, the wing-veins with some other markings and the spines are
black; metallic spots are absent.

“In the pupa of _Heliconius Eucrate_ the laterally compressed region of
the wings is raised into a large projection, the antennal sheaths lying
on the edges of the wings are serrated and beset with short pointed
spines; instead of the minute projections of _Acræa Thalia_, the head
bears two large humped processes; the body is raised on each side into
a foliaceous border carrying five spines of different lengths, the
foremost pair, directed towards the head, being the longest. The pupa
is brown, and ornamented with four pairs of brilliant metallic spots,
one pair close behind the antennæ, and three pairs, almost coalescent,
on the back before the longest pair of spines. A short spine projects
from the middle of each of the latter somewhat arched metallic patches.

“In the pupa of _Colænis Dido_ (which resembles that of _Colænis
Julia_, and to which may be added those of _Dione Vanillæ_ and _Juno_)
the spines are absent, the wing region is but moderately arched, and
the antennæ marked only by small elevations; instead of the leaf-like
border, there are on each side of the back five knotty or humped
processes. The metallic spots are similar in number and position to
those of _Heliconius Eucrate_; those on the back have a wart-like
process in the middle, instead of a spine.

“The pupæ of _Heliconius_ and _Colænis_ when moving their posterior
segments rapidly, as they do whenever they are disturbed, produce a
very perceptible hissing noise by the friction of these segments,
this sound, which is especially noticeable in the case of _Heliconius
Eucrate_, perhaps serving to terrify small foes. (So loud is the sound
produced in this manner by the pupæ of _Epicalia Numilia_, that my
children have named them ‘_Schreipuppen_.’)

“The pupæ of _Heliconius_ and _Colænis_ thus differ to a much greater
extent than the imagines or larvæ, and the same holds good for
_Eueides_ in a much higher degree as compared with its above-mentioned
allies. The larvæ of _Eueides_ have no distinctive characters, and
even the generic rank of the imagines is doubtful; as pupæ, on the
other hand, they are far removed (even by their mode of suspension)
not only from the remainder of the Maracujá group and from the whole
of the great Nymphalideous group (_Danainæ_, _Satyrinæ_, _Elymniinæ_,
_Brassolinæ_, _Morphinæ_, _Acræinæ_ and _Nymphalinæ_), but from almost
all other butterflies. The larva pupates on the underside of a leaf;
the pupa is fastened by the tail, but does not hang down like the pupæ
of the other _Nymphalidæ_,--its last segments are so curved that the
breast of the chrysalis is in contact with the underside of the leaf.
I am not acquainted with any other pupa among those not suspended by
a girdle which assumes such a position. Something similar occurs,
however, in the pupa of _Stalachtis_, which is without a girdle, and
according to Bates, is ‘kept in an inclined position by the fastening
of the tail.’ By this peculiarity Bates distinguishes the _Stalachtinæ_
from the _Libytheæ_ with pupæ ‘freely suspended by the tail.’

“Besides through this peculiar position of the body, the pupa of
_Eueides Isabella_ is distinguished by short hooked and long narrow
sabre-like pairs of processes on the back and head. Its colour is
whitish, yellowish, or sordid yellowish-grey; in the last variety both
the four long dorsal processes and the surrounding portions, as well as
the points of the other processes, remain white or yellowish. The pupa
_Eueides Aliphera_ is very similar, only all the processes are somewhat
shorter, the four longest (dorsal) and some other markings being black.

“Now if, as Weismann has attempted to show for larvæ and imagines,
the form-divergence always ‘corresponds exactly with the divergence
in the mode of life,’ the question arises as to what difference
in the conditions of life has brought about such a considerable
form-divergence between the pupæ of such closely allied species as the
Maracujá butterflies. In pupæ which do not eat or drink, and which
have neither to seek in courtship nor to care for progeny, it is only
protection from foes that can concern us. But in the pupæ of nearly
allied species of which the larvæ feed on kindred plants in the same
districts at the same periods of the year, can the enemies be so
different as to produce such a considerable divergence in form? One
might answer this question in the negative with some confidence, and
affirm that in this case the difference in the pupæ does not result
from the ‘divergence in the mode of life,’ or from the difference
in the external conditions, but is accidental, _i.e._ a consequence
of some fortunate variation induced by some external cause, which
variation afforded protection against common foes--to one species in
one way, and to the other species in some other way; this course,
once entered upon, having been urged on by natural selection, until
at length the wide divergence now shown is attained. How in the case
of any of the species the peculiarity in colour or form can actually
serve as a protection, I must confess myself at fault in answering.
Only in the case of the pupa of _Eueides Isabella_ will I venture to
offer a supposition. That it is not green like other pupæ which suspend
themselves among foliage (_Siderone_, _Epicalia,_ _Callidryas_, &c.),
but contrasts more or less brightly with the dark green of the leaves,
precludes the idea of concealment; on the other hand its colour is
too dull to serve as a conspicuous sign of distastefulness. In either
case the meaning of the wonderful processes of the pupa would remain
unexplained.

“We are thus compelled to seek another possibility in mimicry, by which
foes would be deceived by deceptive resemblance. But what is the object
imitated? Dead insects overgrown by fungi are often found on leaves,
the whitish or yellowish fungi growing from their bodies in various
fantastic forms. Such insects of course no longer serve as tempting
morsels. The processes of the pupa of _Eueides_ suggest such fungoid
growths, although I certainly cannot assert that to our eyes in broad
daylight the resemblance is very striking. But the pupæ hang among the
shadows of the leaves, and a less perfect imitation may deceive foes
that are not so sharp-sighted; protective resemblance must commence
moreover with an imperfect degree of imitation.”



EXPLANATION OF THE PLATES.


PLATE III.

Figs. 1-12 represent larvæ of _Macroglossa Stellatarum_, all bred from
one batch of eggs. Most of the figures are enlarged, but sometimes to a
very small extent only; the lines show the natural length.

Fig. 1. Stage I.; a caterpillar immediately after hatching. Natural
length, 0.2 centim.

Fig. 2. Stage II.; shortly after the first moult. Natural length, 0.7
centim.

Figs. 3-12. Stage V.; the chief colour-varieties.

Fig. 3. The only lilac-coloured specimen in the whole brood. Natural
length, 3.8 centim.

Fig. 4. Light-green form (rare) with subdorsal shading off beneath.

Fig. 5. Green form (rare) with strongly-pronounced dark markings
(dorsal and subdorsal lines). Natural length, 4.9 centim.

Fig. 6. Dark-brown form (common). Natural length, 4 centim. In this
figure the fine shagreening of the skin is indicated by white dots;
in the other figures these are partially or entirely omitted, being
represented only in Figs. 8 and 10.

Fig. 7. Light-green form (common). Natural length, 4 centim.

Fig. 8. Light-brown form (common). Natural length, 3.5 centim.

[Illustration:

  Plate III.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

[Illustration:

  Plate IV.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

[Illustration:

  Plate V.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

[Illustration:

  Plate VI.

  Aug. Weismann pinx.      Lith. J.A. Hofmann, Würzburg.
]

Fig. 9. Parti-coloured specimen, the only one out of the whole brood.
Natural length, 5.5 centim.

Fig. 10. Grey-brown form (rare).

Fig. 11. One of the forms intermediate between the dark-brown and green
varieties, dorsal aspect.

Fig. 12. Light-green form with very feeble dorsal line (shown too
strongly in the figure), dorsal aspect.

Figs. 13-15. _Deilephila Vespertilio._

Fig. 13. Stage III.(?); the subdorsal bearing yellow spots. Natural
length, 1.5 centim.

Fig. 14. Stage IV.; the subdorsal interrupted throughout by complete
ring-spots, the white “mirrors” of which are bordered with black, and
contain in their centres a reddish nucleus. Natural length, 3 centim.

Fig. 15. Stage V.; shortly after the fourth moult. Subdorsal line
completely vanished; ring-spots somewhat irregular, with broad black
borders; natural length, 3.5 centim.

Fig. 16. _Sphinx Convolvuli_, Stage V., brown form. Subdorsal line
retained on segments 1-3, on the other segments present only in small
remnants; at the points where the (imaginary) subdorsal crosses the
oblique stripes there are large bright spots; natural length, 7.8
centim.


PLATE IV.

Figs. 17-22. Development of the markings in _Chærocampa Elpenor_.

Fig. 17. Stage I.; larva one day after hatching. Natural length, 7.5
millim.

Fig. 18. Stage II.; larva after first moult. Length, 9 millim.

Fig. 19. Stage II.; immediately before the second moult (Fig. 30
belongs here). Length, 13 millim.

Fig. 20. Stage III.; after second moult. Length, 20 millim.

Fig. 21. Stage IV.; after third moult (Figs. 32 and 33 belong here).
Length, 4 centim.

Fig. 22. Stage V.; after fourth moult. A feeble indication of an
eye-spot can be seen on the third segment besides those on the fourth
and fifth. Ocelli absent on segments 6-10.

Fig. 23. Stage VI.; after fifth moult. The subdorsal line is feebly
present on segments 6-10, and very distinctly on segments 11 and 1-3.
Ocelli repeated as irregular black spots above and below the subdorsal
line on segments 6-11; a small light spot near the posterior border of
segments 5-10 (dorsal spots) and higher than the subdorsal line. Larva
adult.

Figs. 24-28. Development of the markings of _Chærocampa Porcellus_.

Fig. 24. Stage I.; immediately after emergence from the egg. Length,
3.5 millim.

Fig. 25. Stage II.; after first moult. Length, 10 millim.

Fig. 26. Stage III.; after second moult. Length, 2.6 centim.

Fig. 27. Eye-spots at this last stage; subdorsal much faded, especially
on segment 4. Position the same as in last Fig.; magnified.

Fig. 28. Stage IV.; after third moult; corresponds exactly with Stage
VI. of _C. Elpenor_. Dorsal view, with front segments partly retracted
(attitude of alarm). Ocelli on segment 5 less developed than in
_Elpenor_; repetitions of ocelli as diffused black spots on all the
following segments to the 11th; two light spots on each segment from
the 5th to the 11th, exactly as in _Elpenor_; subdorsal line visible
only on segments 1-3. Length, 4.3 centim.

Fig. 29. _Chærocampa Syriaca_. From a blown specimen in Lederer’s
collection, now in the possession of Dr. Staudinger. Length, 5.3
centim.

Fig. 30. First rudiments of the eye-spots of _Chærocampa Elpenor_,
Stage II. (corresponding also with Fig. 19 in position, the head of
the caterpillar being to the left). Subdorsal line slightly curved on
segments 4 and 5.

Fig. 31. Eye-spots at Stage III. of the larva Fig. 20 somewhat further
developed (larva immediately before third moult). Position as in Fig.
20.

Fig. 32. Eye-spots at Stage IV. corresponding to Fig. 21, A being the
eye-spot of the fourth and B that of the fifth segment.

Fig. 33. Eye-spot at Stage V. of the larva of _C. Elpenor_; fourth
segment.

Figs. 30-33 are free-hand drawings from magnified specimens.

Fig. 34. _Darapsa Chærilus_ from N. America. Adult larva with front
segments retracted. Copied from Abbot and Smith.

Fig. 35. _Chærocampa Tersa_, from N. America. Adult larva copied from
Abbot and Smith.


PLATE V.

Fig. 36. Sixth segment of adult _Papilio_-larvæ; A, _P. Hospiton_,
Corsica; B, _P. Alexanor_, South France; C, _P. Machaon_, Germany; D,
_P. Zolicaon_, California.

Figs. 37-44. Development of the markings of _Deilephila Euphorbiæ_.

Fig. 37. Stage I.; young caterpillar shortly after emergence. Natural
length, 5 millim.

Fig. 38. Similar to the last, more strongly magnified. Natural length,
4 millim.

Fig. 39. Stage II.; larva immediately after first moult. The row of
spots distinctly connected by a light stripe (residue of the subdorsal
line). Natural length, 17 millim.

Fig. 40. Stage III.; after second moult; magnified drawing of the last
five segments. Only one row of large white spots on a black ground
(ring-spots); subdorsal completely vanished; the shagreen-dots formerly
absent now appear in vertical rows interrupted only by the ring-spots.
Below the latter are some enlarged shagreen-dots which subsequently
become the second ring-spots. Natural length of the entire caterpillar,
21 millim.

Fig. 41. Stage IV.; the same larva after the third moult.
Transformation of the ground-colour from green to black, owing to the
spread of the black patches proceeding from the ring-spots in Fig. 40
in such a manner as to leave between them only a narrow green triangle.
The shagreen dots below the ring-spots have increased in size, but have
not yet coalesced.

Fig. 42. Stage III.; larva, same age as Fig. 40, but with _two rows_ of
ring-spots. Natural length of the whole caterpillar, 32 millim.

Fig. 43. Stage V.; larva from Kaiserstuhl. Variety with only one row of
ring-spots, and with red nuclei in the mirror-spots. Natural length, 5
centim.

Fig. 44. Stage V.; larva from Kaiserstuhl (like the three preceding).
The green triangles on the posterior edges of the segments in Fig. 42
have become changed into red. Natural length, 7.5 centim.

Fig. 45. _Deilephila Galii_; Stage IV. Subdorsal with open ring-spots.
Natural length, 3.4 centim.

Fig. 46. _D. Galii_; adult larva; Stage V. Brown variety with feeble
shagreening; subdorsal completely vanished. Natural length, 6 centim.


PLATE VI.

Fig. 47. The same species at the same stage. Black variety strongly
shagreened; similar to _Deil. Euphorbiæ_.

Fig. 48. Similar to the last. Yellow var. without any trace of
shagreening.

Fig. 49. _Deilephila Vespertilio._ Three stages in the life of the
species, representing three phyletic stages of the genus. A, life-stage
III.=phyletic stage 3 (subdorsal with open ring-spots); B, life-stage
IV.=phyletic stage 4 (subdorsal with closed ring-spots); C, life-stage
V.=phyletic stage 5 (subdorsal vanished, only _one_ row of ring-spots).

Fig. 50. _Deilephila Zygophylli_, from S. Russia; stage V. From a
blown specimen in Staudinger’s collection. In this specimen the
ring-spots are difficult to distinguish on account of the extremely
dark ground-colour; they are nevertheless present, and would probably
be more distinct in the living insect. A, open ring-spot from another
specimen of this species in the same collection.

Fig. 51. _Deilephila Nicæa_, from South France; Stage V. Copied from
Duponchel.

Fig. 52. _Sphinx Convolvuli_; Stage V., segments 10-8. Brown variety,
with distinct white spots at the points of intercrossing of the
vanished subdorsal with the oblique stripes.

Fig. 53. _Anceyrx Pinastri_; A and B, larvæ immediately after hatching.
Natural length, 6 millim.

Fig. 54. Same species; Stage II. Subdorsal, supra- and infra-spiracular
lines developed. Natural length, 15 millim.

Fig. 55. _Smerinthus Populi_; Stage I. Immediately after hatching; free
from all marking. Length, 6 millim.

Fig. 56. Same species at the end of first stage; lateral aspect.
Length, 1.3 centim.

Fig. 57. Same species; Stage II. Subdorsal indistinct; the first and
last oblique stripes more pronounced than the others. Length, 1.4
centim.

Fig. 58. _Deilephila Hippophaës_; Stage III. Subdorsal with open
ring-spot on the 11th segment. A, segment 11 somewhat enlarged. Length,
3 centim.


PLATE VII.

Fig. 59. _Deilephila Hippophaës_; Stage V. Secondary ring-spots on six
segments (10-5).

Fig. 60. Same species; Stage V. One or two red shagreen dots on
segments 10-4 in the position of the ring-spots of Fig. 59. Length, 6.5
centim.

Fig. 61. Same species; Stage V. Segments 9-6 of another specimen, more
strongly magnified. A ring-spot on segments 9 and 8 showing its origin
from two shagreen-dots; two red shagreen-dots on segment 7, on segment
6 only one.

Fig. 62. _Deilephila Livornica_ (Europe) in the last stage. Green form.
Copied from Boisduval.

Fig. 63. _Pterogon Œnotheræ_; Stage IV. Length, 3.7 centim.

Fig. 64. The same species at the same stage; dorsal view of the last
segment.

Fig. 65. The same segment in Stage V. Eye-spot completely developed.

Fig. 66. _Saturnia Carpini_, larva from Freiburg; Stage III. Natural
length, 15 millim.

Fig. 67. Same species; larva from Genoa; Stage IV. Length, 20 millim.

Fig. 68. Same species; larva from Freiburg; Stage III. Segments 8 and 9
in dorsal aspect. Length, 15 millim.

Fig. 69. The same caterpillar; lateral view of segment 8.

Fig. 70. _Smerinthus Ocellatus_; adult larva with distinct subdorsal
on the six foremost segments. The shagreening is only shown in the
contour, elsewhere omitted. Length, 7 centim.

[Illustration:

  Plate VII.

  Aug. Weismann pinx.      Lith. J. A. Hofmann, Würzburg.
]

[Illustration:

  Plate VIII.

  Aug. Weismann pinx.      Lith. J. A. Hofmann, Würzburg.
]


PLATE VIII.

Figs. 71-75 represent segments 8 and 9 of the larva of _Saturnia
Carpini_ (German form) in dorsal aspect, all at the fourth stage.
The head of the caterpillar is supposed to be above, so that the top
segment is the eighth.

Fig. 71. _Saturnia Carpini._ Darkest variety.

Fig. 72. Lighter variety.

Fig. 73. Still lighter variety.

Fig. 74. One of the lightest varieties; the black extends further on
segments 9 and 10 than on the 8th.

Fig. 75. Lightest variety.

Figs. 76-80 are only represented on a smaller scale than the remaining
Figs. in order to save space; were they enlarged to the same scale they
would be larger than the other figures.

Fig. 76. _Saturnia Carpini_ (Ligurian form); Segment 8; Stage V.

Fig. 77. Same form; same segment in stage VI.

Figs. 78, 79, and 80. _Saturnia Carpini_ (German form); dorsal aspect
of 8th segment in Stage V. (the last of this form).

Fig. 78. Darkest variety.

Fig. 79. Lighter variety.

Fig. 80. Lightest variety.

Figs. 81-86. _Saturnia Carpini_ (German form); Stage IV. Side view of
the 8th segment in six different varieties. Fig. 81 shows only two
small green spots at the bases of the upper warts besides the green
spiracular stripes. Fig. 82 shows the spots enlarged and increased by a
third behind the warts; the pro-legs have also become green.

Fig. 83. Two of the three green spots, which have become still more
enlarged, are coalescent.

Fig. 84. All three spots coalescent; but here, as also in

Fig. 85, various residues of the original black colour are left as
boundary-marks.

Fig. 86. Lightest variety.


END OF PART II.



STUDIES IN THE THEORY OF DESCENT.



=Part III.=

ON THE FINAL CAUSES OF TRANSFORMATION.



=III.=

THE TRANSFORMATION OF THE MEXICAN AXOLOTL INTO AMBLYSTOMA.


INTRODUCTION.

Since the time when Duméril made known the transformation of a number
of Axolotls into the so-called Amblystoma form, this Mexican Amphibian
has been bred in many European aquaria, chiefly with the view to
establish the conditions under which this transformation occurred, so
as to be enabled to draw further conclusions as to the true causes of
this exceptional and enigmatical metamorphosis.

Although the Amphibians propagated freely, the cases in which
transformation occurred remained extremely rare, and it was not once
possible to reply to the main question, viz. whether this metamorphosis
was determined by external conditions or by purely internal causes;
to say nothing of the possibility of there perhaps being discoverable
certain definite external influences by means of which the
metamorphosis could have been induced with certainty. But while these
points are undecided all attempted theoretical interpretations of the
phenomenon must be devoid of a solid basis.

It appeared to me from the first that the history of this
transformation of the Axolotl was of special theoretical value; indeed
I believed that it might possibly furnish a special case for deciding
the truth of those ground-principles, according to which the origin of
this species is represented by the two conflicting schools as a case of
transformation or as one of heterogenesis. I therefore determined to
make some experiments with the Axolotl myself, in the hopes of being
fortunate enough to be able to throw some light upon the subject.

In the year 1872 Prof. v. Kölliker was so good as to leave with me
five specimens of his Axolotls, bred in Würzburg, and these furnished
a numerous progeny in the following year. With these I carried out the
idea, the theoretical bearing of which will be shown subsequently,
whether it would not be possible to force all the larvæ, or at any
rate, the greater majority, to undergo transformation by exposing them
to conditions of life which made the use of gills difficult, and that
of lungs more easy; in other words, by compelling them to live partly
on land at a certain stage of life.

During that year indeed I obtained no results, most of the larvæ
perishing before the time for such an experiment had arrived, and the
few survivors did not undergo transformation, but lived on to the
following spring and then also died one after the other. Through long
absence from Freiburg, necessitated by other labours, I had evidently
left them without sufficient care and attention. I was thus led to
the conviction, which was more fully confirmed subsequently, that no
results can be obtained without the greatest care and attention in
rearing, towards which single object all one’s interest should be
concentrated, and it must not be considered irksome to have to devote
daily for many months a large amount of time to this experiment. As it
was evident that I could not afford this time without calling in other
aid, I hailed with pleasure an opportunity of witnessing the experiment
performed by other hands.

A lady living here (Freiburg), Fräulein v. Chauvin, undertook to rear
a number of my larvæ of the following year which had just hatched,
and in accordance with my idea to make the experiment of forcibly
compelling them to adopt the Amblystoma form. How completely this was
accomplished will be seen from the following notes by the lady herself,
and it will no less appear that these results were only obtained by
that care in treatment and delicacy of observation which she devoted to
the experiments.


EXPERIMENTS.

“I began the experiments on June 12th, 1874, with five larvæ about
eight days old, these being the only survivors out of twelve. Owing
to the extraordinary delicacy of these creatures, the quality and
temperature of the water, and the nature and quantity of their food
exerts the greatest influence, especially in early life, and one cannot
be too cautious in their treatment.

“The specimens were kept in a glass globe of about thirty centimeters
in diameter, the temperature of the water being regulated; as food at
first Daphnids, and afterwards larger aquatic animals were introduced
in large numbers. By this means all the five larvæ throve excellently.
At the end of June the rudiments of the front legs appeared in the most
vigorous specimens, and on the 9th of July the hind legs also became
visible. At the end of November I noticed that one Axolotl remained
constantly at the surface of the water, and this led me to suppose that
the right period had now arrived for effecting the transformation
into Amblystoma. For brevity I shall designate this as No. I., and the
succeeding specimens by corresponding Roman numerals.

“In order to bring about this metamorphosis, on December 1st, 1874, No.
I. was placed in a large-sized glass vessel containing earth arranged
in such a manner that, when the vessel was filled with water, only one
portion of the surface of the earth was entirely covered by the liquid,
and the creature in the course of its frequent perigrinations was thus
more or less exposed to the air. The water was gradually diminished on
the following days, during which period the first changes made their
appearance in the Amphibian--_the gills commenced to shrivel up_, and
at the same time the creature showed a tendency to seek the shallowest
spots. On December 4th, it took entirely to the land, and concealed
itself among some damp moss which I had placed on a heap of sand on the
highest portion of the earth in the glass vessel. At this period the
first ecdysis occurred. Within the four days from the 1st to the 4th of
December, a striking change took place in the external appearance of
No. I., the gill-tufts shrivelled up almost entirely, the dorsal crest
completely disappeared, and the tail, which had hitherto been broad,
became rounded and similarly formed to that of a land salamander. The
grey-brown colour of the body changed gradually into a blackish hue;
isolated spots, at first of a dull white, made their appearance and
these in time increased in intensity.

“When the Axolotl left the water on December 4th the gill-clefts were
still open, but these closed gradually, and after about eight days were
overgrown with skin and no longer to be seen.

“Of the other larvæ three appeared at the end of November (_i.e._ at
the same time when No. I. came to the surface of the water) to have
kept pace in development with No. I., an indication that for these
also the right period had arrived for accelerating the developmental
processes. They were therefore submitted to the same treatment as No.
I. No. II. became transformed at the same time and exactly in the same
manner as the latter; its gill-tufts were complete when it was first
placed in the shallow water, but after four days these had almost
entirely disappeared; in the course of about ten days after it took to
the land, the overgrowth of skin on the gill-clefts and the complete
assumption of the salamander form occurred. During this last period the
creature took food, but only when urged to do so.

“In Nos. III. and IV. the development proceeded more slowly. Neither of
these so frequently sought the shallow spots, nor did they as a rule
remain so long exposed to the air, so that the greater part of January
had expired before they took entirely to the land. Nevertheless the
dessication of the gill-tufts did not take a longer time than in Nos.
I. and II. as the first ecdysis occurred as soon as they took to the
land.

“No. V. showed still more striking deviations in its transformation
than Nos. III. and IV., but as this specimen appeared much weaker
than the others from the beginning and was retarded in growth to a
most notable extent, this is by no means surprising. It took fourteen
instead of four days before the transformation had advanced far enough
to enable it to leave the water. It was especially interesting to
observe the behaviour of this specimen during this period. Its weak
and delicate constitution evidently made it much more susceptible to
all external influences than the others. If exposed to the air for too
long a time it acquired a light colour, and when annoyed or alarmed it
emitted a peculiar odour, similar to that of a salamander. As soon as
these phenomena were observed it was at once placed in deeper water,
into which it immediately plunged and gradually recovered itself,
the gills always becoming again expanded. The same experiment was
repeated several times and always led to the same result, from which
we may venture to conclude that by accelerating the transformation
too energetically, the process may come to a standstill, and even by
continued compulsion may end in death.

“It yet remains to be mentioned with respect to Axolotl No. V. that
this specimen, unlike all the others, did not emerge from the water at
the first ecdysis, but at the time of the fourth.

“All the Axolotls are now (July, 1875) living, and are healthy and
vigorous, so that with respect to their state of nourishment there is
nothing to prevent their propagating. Of the first four the largest is
fifteen centim. long; Axolotl No. V. measures twelve centim.

“The preceding statements appear to demonstrate the correctness of the
views advanced in the Introduction:--Axolotl larvæ generally but not
always complete their metamorphosis if, in the first place, they emerge
sound from the egg and are properly fed; and if, in the next place,
they are submitted to the necessary treatment for changing aquatic
into aërial respiration. It is obvious that this treatment must only
be applied very gradually, and in such a manner as not to overtax the
vital energy of the Amphibian.”

       *       *       *       *       *

To the foregoing remarks of Fräulein v. Chauvin I may add that in all
five cases the transformation was complete, and not to be confounded
with that change which occurs more or less in all Axolotls in the
course of time when confined in small glass vessels. In this last case
there frequently appear changes in the direction of the Amblystoma form
without the latter being actually reached. In the five adult Axolotls
which I possessed for a short time, and of which two were at least four
years old, the gills were much shrivelled, but the aquatic tail and
dorsal crest were unchanged. The crest may, however, also disappear,
and the tail become shortened without these changes being due to a
transformation into Amblystoma, as will be shown further on.

With respect to the duration of the transformation, this amounted
in Axolotls Nos. I. to IV. altogether to twelve or fourteen days.
Of these, four days were taken by the first changes which occurred
while the creature was still in the water; the remaining time, to the
completion of the metamorphosis, was passed on land. Duméril gives the
duration of the metamorphosis as sixteen days.

The following results of the experiments just described appear to me
to be especially noteworthy:--The five Axolotl larvæ which can alone
be taken into consideration, the others having soon perished, all
experienced metamorphosis, and without an exception became Amblystomas.
Only one of them, No. I., by persistently swimming at the surface, as
was observed at the end of six months, showed a decided tendency to
undergo metamorphosis and to adopt aërial respiration. With respect to
this specimen it may therefore be confidently assumed that it would
have taken to the land, and that metamorphosis would have occurred
without artificial aid, just as was the case in the thirty specimens
which Duméril altogether observed.

Respecting Nos. II., III., and IV., on the other hand, such a
supposition is but little probable. These three larvæ endeavoured
to keep in deep water and avoided as long as possible the shallow
places which would have enforced them to take entirely to lung
breathing. Metamorphosis thus occurred more than a month later in these
individuals.

Finally, there can scarcely be any doubt that No. V. would not have
become transformed without forcible adaptation to an aërial life.

From these results we may venture to conclude that most Axolotl larvæ
change into the Amblystoma form when, at the age of six to nine
months, they are placed in such shallow water that they are compelled
to respire chiefly by their lungs. The experiments before us are
certainly at present but very few in number, but such a conclusion
cannot be termed premature if we consider that out of several hundred
Axolotls (the exact number is not given) Duméril obtained only about
thirty Amblystomas, while v. Kölliker bred only one Amblystoma out of a
hundred Axolotls.

It now only remains questionable whether _each larva_ could have been
forced to undergo metamorphosis, but this could only be decided by
new experiments. It was originally my intention to have delayed the
publication of the experiments till Fräulein v. Chauvin had repeated
them in larger numbers, but as my Axolotls have not bred this year
(1875) I must abandon my scheme, and this can be done the more readily
because, for the theoretical consideration of the facts, it is
immaterial whether _all_ or only _nearly all_ the Axolotls could have
been compelled to undergo transformation. I must not, however, omit to
mention that Herr Gehrig, the curator of our Zoological Museum, bred
a considerable number of larvæ from the same brood as that with which
Fräulein v. Chauvin experimented, and that of these larvæ six lived
over the winter _without undergoing metamorphosis_. They were always
kept in deep water and thus furnished the converse experiment to those
recorded above; they further prove that this whole brood did not have a
previous tendency to undergo metamorphosis.

If these new facts are to be made use of to explain the nature of this
extraordinary process of transformation in accordance with our present
conception, the data already known must in the first place be called to
our aid.

It has first to be established that _Siredon Mexicanus_ never, as
far as we know, undergoes metamorphosis in its native country. This
Amphibian is there only known in the _Siredon_ form, a statement which
I have taken from De Saussure,[225] who has himself observed the
Axolotl in the Mexican lakes. This naturalist never found a single
Amblystoma in the neighbourhood of the lakes, “nevertheless the larva
(Axolotl) is so common there that it is brought into the market by
thousands.” De Saussure believes that in Mexico the Axolotl does not
undergo transformation.[226] The same statement is distinctly made by
Cope,[227] whose specimens of _Siredon Mexicanus_ bred in America,
even in captivity showed “no tendency to become metamorphosed.” On the
other hand Tegetmeier observed[228] that one out of five specimens
obtained from the Lake of Mexico underwent metamorphosis, and this
accordingly establishes the second fact, viz. that the true Axolotl
becomes transformed under certain conditions into an Amblystoma when in
captivity.

This last remark would be superfluous if, as was for a long time
believed, the Paris Axolotls, of which the metamorphosis was first
observed and which at the time made such a sensation, were actually
_Siredon Mexicanus_, _i.e._ the _Siredon_ which alone in its native
country bears the name of Axolotl. In his first communication Duméril
was himself of this opinion; he then termed the animal “_Siredon
Mexicanus vel Humboldtii_,”[229] but subsequently, in his amplified
work[230] on the transformation of the Axolotl observed in the Jardin
des Plantes, he retracted this view, and after a critical comparison of
the five described species of _Siredon_, he came to the conclusion that
the species in the possession of the Paris Museum was probably _Siredon
Lichenoides_ (Baird). All the transformations of Axolotls observed
in Europe must consequently be referred to this species, since they
were--at least as far as I know--all derived from the Paris colony. My
own experimental specimens were also indirectly descended from these.

Now it must be admitted that this does not coincide with the fact that
the Amblystoma form which Duméril first obtained from his Axolotls
agreed with Cope’s species, _A. Tigrinum_, while on the other hand we
learn from Marsh[231] that _Siredon Lichenoides_ (Baird), when it does
undergo metamorphosis, becomes transformed into _Amblystoma Mavortium_
(Baird).

Marsh found _Siredon Lichenoides_ in mountain lakes (7000 feet above
the sea) in the southwest of the United States (Wyoming Territory),
and obtained from them, by breeding in aquaria, _Amblystoma Mavortium_
(Baird). He considers it indeed doubtful whether the Amphibian
undergoes this transformation in its native habitat, although he
certainly states this opinion without rigorous proof on purely
theoretical considerations, because, according to his view, “the low
temperature is there less favourable.”[232]

If I throw doubt upon this last statement it is simply because
_Amblystoma Mavortium_ is found native in many parts of the United
States, viz:--in California, New Mexico, Texas, Kansas, Nebraska, and
Minnesota. It is indeed by no means inconceivable that in the mountain
lakes where Marsh obtained this species, it may behave differently
with respect to metamorphosis than in other habitats, and this appears
probable from certain observations upon _Triton_ which will be
subsequently referred to.

Meanwhile, in the absence of further observations, we must admit that
the Paris Axolotls were not _Siredon Lichenoides_, but some nearly
allied and probably new species. But little information is furnished
by observing the course of the transformation, although it is at least
established that this Axolotl in its native habitat does not undergo
metamorphosis or does so as exceptionally as in Europe. Unfortunately
in his papers Duméril gives no precise statement respecting the
locality of this species imported from “Mexico”--it is probable that
he was himself unacquainted with it, so that I can only state on the
authority of Cope that Amblystoma has never been brought from south of
the provinces of Tamaulipas and Chilhuahua, _i.e._ south of the Tropic
of Cancer.[233]

This last statement, however, gives no certainty to the matter. Of much
more importance is the above-mentioned fact, that the true Axolotl
of the lakes about the city of Mexico does not, as a rule, become
transformed into an Amblystoma in that locality, although this species
in certain cases undergoes metamorphosis when in confinement. From
this circumstance and from the fact that the Paris Axolotl likewise
experienced but a very small percentage of metamorphosis in captivity,
we may venture to conclude that this species also, in its native
habitat, either does not become transformed at all or does so only
exceptionally.

But there is yet another body of facts which come prominently into
view on considering the history of the transformations. I refer to the
existence of numerous species of _Amblystoma_ in a natural state. In
the “Revision of the genera of _Salamandridæ_,” published some years
ago by Strauch,[234] this author, following Cope,[235] gives twenty
species of _Amblystoma_ as inhabiting North America. Although some
of these species are based on single examples, and consequently, as
Strauch justly remarks, “may well have to be reduced in the course
of time,” there must nevertheless always remain a large number of
species which live and propagate as true Amblystomas, and of which the
habitat extends from the latitude of New York to that of New Mexico.
There are therefore true species of _Siredon_ which regularly assume
the Amblystoma form under their natural conditions of life, and which
propagate in this form, while, on the other hand, there are at least
two species which, under their existing natural conditions of life,
always propagate as _Siredon_. It is but another mode of expression
for the same facts if we say that the Mexican Axolotl and the Paris
_Siredon_--whether this is _Lichenoides_ or some other species--stand
at a lower grade of phyletic development than those species of
_Amblystoma_ which propagate in the salamander form. No one can raise
any objection to this statement, while the alternative view maintained
by all authors contains a theory either expressed or implied which is,
as I believe, incorrect, viz. that the Mexican Axolotl _has remained_
at an inferior stage of phyletic development.

All zoologists[236] who have expressed an opinion upon the
transformation of the Axolotl, and who are not, like the first observer
of this fact, embarrassed by Cuvier’s views as to the immutability of
species, regard the phenomenon as though a species, which owing to some
special conditions had hitherto remained at a low stage of development,
had now through some other influences been compelled to advance to a
higher stage.

I believed for a long time that the phenomenon could not otherwise be
comprehended, so little was I then in a position to bring all the facts
into harmony with this view. Thus in the year 1872 I expressed myself
as follows[237]:--“Why should not a sudden change in all the conditions
of life (transference from Mexico to Paris) have a direct action on
the organization of the Axolotl, causing it suddenly to reach a higher
stage of development, such as many of its allies have already attained,
and which obviously lies in the nature of its organization--a stage
which it would perhaps itself have reached, although later, in its
native country? Or is it inconceivable that the sudden removal from
8000 feet above the sea (Mexican plateau) to the altitude of Paris, may
have given the respiratory organs an impetus in the direction of the
transformation imminent? In all probability we have here to do with the
direct action of changed conditions of life.”

That the substance of this last statement must still hold good is
obvious from the experiments previously described, which show that
by the application of definite external influences, we have it to
a certain extent in our power to produce the transformation. It is
precisely in this last point that there lies the new feature furnished
by these experiments.

But are we also compelled to interpret the phenomenon in the above
manner? _i.e._ as a sudden advance in the phyletic development of the
species occurring, so to speak, at one stroke? I believe not.

What first made this view appear to me erroneous, was the appearance
of the living Amblystomas bred from my Axolotl larvæ. These creatures
by no means differed from the Axolotls merely in single characters,
but they were distinct from the latter in their entire aspect; they
differed in some measure in all their parts, in some but slightly and
in other parts strongly--in brief, they had become quite different
animals. In accordance with this, their mode of life had become
completely modified; they no longer lived in the water, but remained
concealed by day among the damp moss of the vivarium, coming forth only
by night in search of food in dry places.

I had been able to perceive the great difference between the two
stages of development from the anatomical data with which I had long
been familiar, and which Duméril had made known with respect to the
structure of his Amblystomas. But the collecting of numerous details
gives no very vivid picture of the metamorphosis; it was the appearance
of the living animal that first made me conscious how deep-seated was
the transformation which we have here before us, and that this change
not merely affected those parts which would be directly influenced
by the change in the conditions of life, such as the gills, but that
most if not all the parts of the animal underwent a transformation,
which could in part be well explained as morphological adaptation to
new conditions of life, and partly as a consequence of this adaptation
(correlative changes), but could not possibly be regarded as the sudden
action of these changed conditions.

Such at least is my view of the case, according to which a _per saltum_
development of the species of such a kind as must here have taken
place, is quite inconceivable.

I may venture to assume that most observers of the metamorphosis of
Axolotl have, like myself, not been hitherto aware of the extent of
the transformation, and it may thus be explained why the theoretical
bearings of the case have on all sides been incorrectly conceived. We
have here obviously a quite extraordinary case of the first order of
importance. I believe that it can easily be shown that the explanation
of the history of the metamorphosis of the Paris Axolotl which has
hitherto been pretty generally accepted, necessarily comprises a very
far-reaching principle. If this interpretation is correct, then in my
opinion must also hold good the ideas of those who, like Kölliker,
Askenasy, Nägeli, and, among the philosophers, Hartmann and Hübner,
would refer the transformation of species in the first instance to a
power innate in the organism, to an active, _i.e._ a self-urging “law
of development”--a phyletic vital force.

Thus, if the Axolotls transformed into Amblystomas are regarded as
individuals which, impelled by external influences, have anticipated
the phyletic development of the others, then this advance can only
be ascribed to a phyletic vital force, since the transformation is
sudden, and leaves no time for gradual adaptation in the course of
generations. The _indirect_ influence of the external conditions of
life, _i.e._ natural selection, is thus excluded from the beginning.
But the _direct_ action of the changed conditions of life by no means
furnishes an explanation of the complete transformation of the whole
structure, such as I have already alluded to, and which I will now
enter into more closely.

The differences between the Paris Axolotl and its Amblystoma according
to Duméril, Kölliker, and my own observations are as follow:--

1. The gills disappear; the gill-clefts close up, and of the branchial
arches only the foremost remain, the posterior ones disappearing. At
the same time the os _hyoideum_ becomes changed (Duméril).

2. The dorsal crest completely disappears (Duméril).

3. The aquatic tail becomes transformed into one like that of the
salamanders (Duméril), which, however, is not as in the salamander
cylindrical, but somewhat compressed laterally (Weismann).

4. The skin becomes yellowish white, irregularly spotted on the sides
and back (Duméril), whilst at the same time its former grey-black
ground-colour changes into a shining greenish black (Weismann); it
loses, moreover, the slimy secretion of the skin, and the cutaneous
glands become insignificant (Kölliker).

5. The eyes become prominent and the pupils narrow (Kölliker), and
eyelids capable of completely covering the eyes are formed; in Axolotl
only a narrow annular fold surrounds the eyes, so that these cannot be
closed (Weismann).

6. The toes become narrowed and lose their skin-like appendages
(Kölliker), or more precisely, the half webs which connect the proximal
extremities of the toes on all the feet (Weismann).

7. The teeth are disposed in this species, as in all other
_Amblystomæ_, in transverse series; whilst in Axolotl, as in _Triton_
larvæ, they are arranged at the sides of the jaw in the form of a
curved arch-like band beset with several rows of teeth.[238] (Duméril.
See his fig., _loc. cit._ p. 279).

8. In Axolotl the lower jaw, in addition to the teeth on the upper edge
of the bone, also bears “de très petites dents disposées sur plusieurs
rangs;” these last disappear after metamorphosis (Duméril). I will add
that the persistent teeth belong to the _os dentale_ of the lower jaw,
and those that are shed to the _os operculare_.[239]

9. The surface of the posterior moveable part of the body is slightly
concave both before and after transformation; the anterior part is,
however, less concave in _Amblystoma_ than in _Siredon_ (Duméril).

I have not yet been able to verify Duméril’s 7th and 9th statements,
as I did not want to kill any of my living Amblystomas,[240] simply
in order to confirm the observations of a naturalist in whom one may
certainly place complete confidence. Neither have I as yet observed the
transformation of the branchial arches, but all the other statements of
Kölliker and Duméril I can entirely corroborate.

The structural differences between Axolotl and Amblystoma are
considerably greater and of more importance than those between allied
genera, or indeed than between the families of the Urodela. The genus
_Siredon_ undoubtedly belongs to a different sub-order to the genus
_Amblystoma_ into which it occasionally becomes transformed. Strauch,
the most recent systematic worker at this group, distinguishes
the sub-order _Salamandrida_ from that of the _Ichthyodea_ by the
possession of eyelids, and by the situation of the palatine teeth in
single rows on the posterior edge of the palatal bone: in _Ichthyodea_
the eyelids are wanting and the palatine teeth are either “situated on
the anterior edge of the palatal bone,” or “cover the whole surface of
the palatal plates in brush-like tufts.”

How is it possible to regard such widely divergent anatomical
characters as changes suddenly produced by the action (but once
exerted) of deviating conditions of life? Hand in hand with the
shedding of the old and the appearance of new palatine teeth, there
occurs a change in the anatomical structure of the vertebral column,
and also--as we may fairly conclude from Kölliker’s correct observation
of the cessation of the slimy secretion--in the histological structure
of the skin. Who would undertake to explain all these profound
modifications as the direct and sudden action of certain external
influences? And if any one were inclined to explain such changes as a
consequence of the disappearance of the gills, _i.e._ as correlative
changes, what else is such a correlation than the phyletic vital force
under another name?

If from one change arising from the direct action of external
agencies, the whole body can in two days become transformed in all
its parts, in the precise manner which appears best adapted for the
new conditions of life under which it is henceforward to exist, then
the word “correlation” is only a phrase which explains nothing, but
which prevents any attempt at a better explanation, and it would be
preferable to profess simply the belief in a phyletic vital force.

Moreover, it is hardly permissible to seek such an explanation, since
Urodela are known which have no gills in the adult state, and which
nevertheless possess all the other characters of the _Ichthyodea_,
viz. want of eyelids, characteristic palatine teeth, and the tongue
bone. This is the case with the genera _Amphiuma_ (Linn.), _Menopoma_
(Harl.), and _Cryptobranchus_ (v. d. Hoev.). The two first genera, as
is known, still possess gill-clefts, but _Cryptobranchus_ has even
lost these clefts, which, as in _Amblystoma_, are overgrown by skin;
nevertheless _Cryptobranchus_ is, according to the concurrent testimony
of all systematists, a true salamander in habits, tongue bone, palatine
teeth,[241] &c. It must further be added that the Axolotl itself can
lose the gills without thereby becoming transformed into an Amblystoma.
I have previously mentioned that in Axolotls which were kept in shallow
water the gills frequently became diminutive, and it also sometimes
happens that they completely shrivel up. I possess an Axolotl preserved
in alcohol in which the gills have shrivelled up into small irregular
bunches, and the dorsal crest is also so completely absent that its
place is occupied by a long furrow, and even on the tail the crest
has entirely disappeared from the lower edge and about half from the
upper edge. Notwithstanding this, the creature is widely removed from
Amblystoma in structure; it possesses the arched branchial apparatus,
the palatine teeth, the skin, &c., of the Axolotl.

These facts prove, therefore, that the shedding of the gills by no
means always entails all the other modifications which we observe in
the metamorphosis of Axolotl, so that these modifications are thus
not by any means the necessary and immediate consequence of such gill
shedding.

Whether these modifications will occur after a long series of
generations--whether the successors of _Cryptobranchus_ will also one
day acquire the salamandriform structure is another question, and one
which I could not exactly answer in the negative. But this question
does not here come into consideration, as we are now only concerned
with the _immediate result_ of the shedding of the gills.

The problem appears therefore to be as follows:--Either the hitherto
received interpretation of the transformational history of the Axolotl
as a further development of the species is incorrect, or else the case
of Axolotl incontestably proves the existence of a phyletic vital force.

We have now to ask whether the facts of this transformational history
are not capable of another explanation.

I believe that this is certainly possible, and that another
interpretation can be shown to be correct with some degree of
probability.

I am of opinion that those Amblystomas which have been developed
in captivity in certain instances from _Siredon Mexicanus_ (_S.
Pisciformis_), as well as from the Paris Axolotls, are not progressive,
but reversion forms; I believe that the Axolotls which now inhabit
the Mexican lakes were Amblystomas at a former geological (or better,
zoological) epoch, but that owing to changes in their conditions of
life, they have reverted to the earlier perennibranchiate stage.

I was undoubtedly first led to this conception by the results which
arose from my studies on the seasonal dimorphism of butterflies.[242]
In this case we were also concerned with the two different forms under
which one and the same species appears, and of which it was shown to
be probable that the one is phyletically older than the other. The
younger summer form, according to my view, has arisen, through the
gradual amelioration of the climate, from the winter form, which at an
earlier zoological epoch was the only one in existence; but the latter,
the primary form, has not for this reason ceased to exist, but now
alternates in each year as a winter form with the secondary summer form.

Now with seasonally dimorphic butterflies, it was easily possible to
induce the summer brood to assume the winter form by exposing their
pupæ for a long time to a low temperature; and it was shown to be
highly probable that this abrupt and often very extensive change or
transformation, only apparently takes place suddenly, and is but the
apparent result of the action of cold upon this generation, whilst in
fact it depends upon reversion to the primary form of the species, so
that the low temperature, which is only once applied, gives but the
impetus to reversion, and is not the true cause of the transformation.
This cause must rather be sought in the long continued action of the
cold to which the ancestors of our existing butterflies were subjected
for thousands of generations, and of which the final result is the
winter form.

If we assume for an instant that my interpretation of the
transformation of Axolotl as just offered is correct, we should have
conditions in many respects analagous to those of seasonal dimorphism.
It is true that in this case the two forms no longer alternate
regularly with each other, but the primary form may occasionally
appear instead of the secondary form, owing to the action of external
conditions.

Just as in the case of seasonal dimorphism it is possible to compel
the summer generation to abandon the summer form, and to assume the
winter guise by the action of cold; so in the present case we are able
to induce the Axolotl to adopt the Amblystoma form by making aërial
respiration compulsory at a certain stage of life; and further, just as
in seasonal dimorphism it can be shown that this artificially produced
change is only apparently an abrupt transformation, and is actually a
reversion to the much older winter form; so here we have not an actual,
but only an apparent remodelling of the species--a reversion to the
phyletically older form.

This certainly appears a paradox, inasmuch as a form here arises by
reversion which must yet undoubtedly rank as the more highly developed.
I believe, however, that much which seems paradoxical in this statement
will disappear on further examination.

It must in the first place be taken into consideration that the
phyletic development of species need not by any means always take place
by advancement. We have indeed many cases of retrogressive development,
although in a somewhat different sense, as with parasites and those
forms which have degenerated from free locomotion to a sedentary mode
of life.[243] I do not confuse this kind of retrogressive development,
arising from the arrest of certain organs and systems of organs, with
true reversion. The latter is a return to a form which has already
been once in existence; but in the former case, in spite of all
simplification of the organization, some entirely new feature always
comes into existence. But I am not able to see any absurdity in the
assumption that even true reversion, whether of a whole species or of
the individuals of a certain district, may be regarded as possible,
and I require no further concession. Why, for example, should it be
inconceivable that at a very remote period the Axolotl was adapted to
a life on land; that through the direct and indirect action of changed
conditions of life it gradually acquired the salamander form, but that
subsequently, through new and unfavourable changes in the conditions of
life, it again relapsed to the older form, or at least to one nearly
related thereto?

At any rate such an assumption contains nothing opposed to known facts,
but can be supported in many ways, and finally it commends itself, at
least in my opinion, as offering the only admissible explanation of the
facts before us.

The existence of a whole series of species of _Amblystoma_, as already
mentioned, at once shows that species of _Siredon_ can become elevated
into the salamander form, and can propagate regularly in this state,
and further, that this phyletic advance has already actually taken
place in many species.

That degeneration may also occur from this high stage to a lower stage
of development, is shown by many observations on our water-salamanders.
It is known that under certain circumstances Tritons, as it is
generally expressed, become “sexually mature in the larval condition.”

In the year 1864 De Filippi[244] found fifty Tritons in a pool at
Andermatten, in the neighbourhood of Puneigen, and of these only two
showed the structure of the adult water-salamander; all the others
still possessed gills, but notwithstanding this, they agreed in both
sexes, in size and in the development of the sexual organs, with mature
animals. De Filippi established that these “sexually mature larvæ” not
only resembled larvæ externally through the possession of gills, but
that they also possessed all the other anatomical characters of the
larvæ, _i.e._ the characteristic bunches of palatine teeth situated
on both sides in the position of the subsequent single rows, and a
vertebral column represented throughout its whole length by the _chorda
dorsalis_.

According to my view this would be a case of the reversion of the
Triton to the immediately anterior phyletic stage, _i.e._ to the
perennibranchiate stage, and in the present instance the majority
of zoologists who take their stand by the theory of descent, would
certainly concur in this view. I should at least consider it to be a
useless play upon words did we here speak of larval reproduction, and
thereby believe that we had explained something. The animal certainly
becomes sexually mature in the same condition as that in which it
first appears as a larva, but we first get an insight into the nature
of this process by considering that this so-called “sexually mature
larva” has the precise structure which must have been possessed by
the preceding phyletic stage of the species, and that an individual
reversion to the older phyletic stage of the species is consequently
before us. I maintain that Duméril is in error in regarding this case
of the Triton as parallel with the true larval reproduction of Wagner’s
_Cecidomyia_ larva. In this last case it is certainly not reversion
to an older phyletic stage that confers the power of reproduction
upon the larvæ, since the latter do not represent an older phyletic
stage of the species, but must have arisen contemporaneously with this
last stage. The enormous structural difference between the larvæ and
the imagines is not explained by the latter having arisen from the
former supplementarily as a finished production, but by both having
been contemporaneously adapted to continually diverging conditions
of life.[245] Considered phyletically, these larvæ are by no means
necessarily transitional to the origination of the flies. They could
have been quite different without the form of the imagines having
been thereby modified, since the stages of insect metamorphosis vary
independently of each other in accordance with the conditions of life
to which they are subjected, and exert scarcely any, or only a very
small form-determining influence upon each other, as has been amply
proved in the preceding essay. In any case the power of these larvæ
(the _Cecidomyiæ_) to propagate themselves asexually was first acquired
as a secondary character, as appears from the fact that there exist
numerous species of the same genus which do not “nurse.” In the form
which they now possess they could never have played the part of the
final stage of the ontogeny, nor could they formerly have possessed
the power of sexual reproduction.[246] In brief, we are here concerned
with true larval reproduction, whilst in Triton we have reversion to an
older phyletic stage.[247]

I cannot agree with my friend Professor Haeckel when he occasionally
designates the reversion of the Tritons as an “adaptation” to a purely
aqueous existence.[248] We could here only speak of “adaptation” if
we took the word in a quite different sense to that in which it was
first introduced into science by Darwin and Wallace. These naturalists
thereby designate a gradual bodily transformation appearing in the
course of generations in correspondence with the new requirements
of altered conditions of life or, in other words, the action of
natural selection, and not the result of a suddenly and direct acting
transforming cause exerted but once on a generation.

Just because the word “adaptation” can be used in ordinary language in
many senses, it is desirable that it should have only _one_ precise
signification, and above all that we should not speak of adaptation
where scarcely any morphological change occurs, but only a kind of
functional change in the sense used by Dohrn.[249] This is the case
for example, when Forel[250] shows that fresh water Pulmonifera,
the organization of which is attributed to the direct respiration
of air, can nevertheless become settled in the greatest depths of
mountain lakes through their lungs being again employed as gills.
That not the least change in the lungs hereby takes place is shown
by the observations of Von Siebold,[251] who saw the shallow water
Pulmonifera using their lungs alternately for direct aërial and
aquatic respiration, according to the amount of air contained in the
water. If with Von Siebold we merely apply the word “adaptation” to
such cases, this expression would lose the special sense which it
originally conveyed, and the word would have to be abandoned as a
_terminus technicus_; still, such cases may perhaps be spoken of as
_physiological_ adaptation.

In any case the reproductive “larvæ” of the Tritons as little present
a case of true adaptation as the Axolotl, which occasionally becomes
transformed into an Amblystoma. In both cases the transformation
referred to is by no means indispensable to the life of the individual.
Mature Tritons (devoid of gills) can exist, as I have myself seen,
for many months, and probably also for a year in deep water, although
adapted for purely pulmonary respiration; whilst Axolotls, as I have
already mentioned, can live well for a year in shallow water poor in
air. If their gills by this means become shrivelled up or completely
disappear, even this is not adaptation in the Darwinian sense, but
the effect of directly acting external influences, and chiefly of
diminished use.

A case entirely analagous to that of Filippi’s was observed by
Jullien in 1869. Four female larvæ of _Lissotriton Punctatus_
(Bell)--(synonymous with _Triton Tæniatus_, Schnd.), taken from a
pool, proved to be sexually mature. They contained mature eggs in
their ovaria ready for laying, and two of them actually deposited
eggs. Four male larvæ found in the same pool, appeared to be equally
developed with respect to size, but their testicles contained no free
spermatozoa, but only sperm-cells.[252]

I have met with a third case of a similar kind mentioned by Leydig in
his memoir, rich in interesting details, “on the tailed Amphibians
of the Wurtemburg fauna.”[253] Schreibers, the former director of
the Vienna Museum, also found “larvæ” of Tritons with well-developed
gills, but of the size of the “adult male individuals,” and, as shown
by anatomical investigation, with well “developed sexual organs,” the
ovaria especially being distended with eggs.

It is thus established that species which long ago reached the
salamander stage in phyletic development, may occasionally degenerate
to the perennibranchiate stage. This fact obviously makes my
conception of the Axolotl as a reversion form appear much less
paradoxical--indeed, the cases of reversion in Triton are precisely
analagous to the process which I suppose to have taken place in the
Axolotl. We have only to substitute Amblystomas for Tritons, to
imagine the pool in which De Filippi found his “sexually mature Triton
larvæ” enlarged to the size of the Lake of Mexico, and to conceive the
unknown, and perhaps here transitory, causes of the reversion to be
permanent, and we have all that is necessary, so far as we at present
know, for the restoration of the Axolotl; we obtain a perennibranchiate
population of the lake.

It has not yet been determined whether the perennibranchiate form of
the Triton actually prevailed permanently in De Filippi’s pool, since,
so far as I know, this has not since been examined.

Let us, however, assume for an instant that this is really the
case, and that there exists at that spot a colony of sexually
reproductive perennibranchiate Tritons: should we wonder if a true
Triton occasionally appeared among their progeny, or if we were able
to induce the majority of the individuals of this brood to become
metamorphosed into Tritons by keeping them in shallow water? According
to my view this is precisely the case of the Mexican Axolotl.

I need not, however, restrict myself to this in order to support my
hypothesis, but must also directly combat the view hitherto received,
since the latter is in contradiction with facts.

Did there really exist in the Axolotl a tendency to sudden phyletic
advancement, then one fact would remain quite incomprehensible, viz.
the sterility of the Amblystomas.

Out of about thirty Amblystomas obtained by Duméril down to the
year 1870, there was not one in a state of sexual maturity; neither
copulation nor deposition of eggs took place, and the anatomical
investigation of single specimens showed that the eggs were immature,
and that the spermatozoa, although present, were without the undulating
membrane characteristic of the salamanders, but were not devoid of
all power of movement, only, as established by Quatrefages, were
“incompletely motile.”[254]

So also the five Amblystomas about which I have been writing, show up
to the present time no appearance of reproduction.

The objection raised by Sacc,[255] that the sterility of the
Amblystomas bred from Axolotls is attributable to “bad nourishment,” is
obviously of but little avail. How is it that the Axolotls, which are
fed in a precisely similar manner, propagate so readily? Moreover, I am
able to expressly assert that my Amblystomas were very well fed. It is
true that they have as yet scarcely reached the age of two years, but
the Axolotl propagates freely in the second year, and some of Duméril’s
Amblystomas were five years old in 1870.

This fact of the sterility is strongly opposed to the idea that these
Amblystomas are the regular precursors of the phyletically advancing
genus _Siredon_.[256] I will by no means assert that my theory of
reversion actually explains the sterility, but it is at least not
directly opposed to it. Mere reversion forms may die off without
propagating themselves; but a _new form_ called forth by the action
of a phyletic vital force should not be sterile, because this is the
precise “aim” which the vital force had in view. The conception of a
vital force comprises that of teleology.

The sterility of Amblystoma moreover, although not completely
explicable from our standpoint, can be shown to be a phenomenon
not entirely isolated. In the above mentioned case of _Lissotriton
Punctatus_, the female “larvæ” were certainly sexually mature and laid
eggs, but the males of the same period contained in their testicles no
fully developed spermatozoa.

Other cases of this kind are unknown to me; at the time when I made the
experiments with butterflies already recorded (see the first essay),
this point of view was remote, and I therefore neglected to examine
the artificially bred reversion forms with respect to their organs of
reproduction. But general considerations lead to the supposition that
atavistic forms may easily remain sterile.

Darwin[257] finds the proximate causes of sterility in the first place
in the action of widely diverging conditions of life, and in the next
place in the crossing of individuals widely different in constitution.
Now it is certainly deviating conditions of life which lead to the
metamorphosis of the Axolotl, and from this point of view it cannot be
surprising if we find those individuals sterile which show themselves
so especially affected by these changed conditions as to revert to the
salamander form.

By this it is not in any way meant to be asserted that reversion
is invariably accompanied by sterility, and one cannot raise as an
objection to my interpretation of the metamorphosis of the Axolotl,
that a reproductive colony of Axolotls could never have arisen by
reversion. On the contrary, Jullien’s egg-depositing female Triton
larvæ show that also with reversion the power of reproduction may be
completely preserved.[258] From the above-mentioned general causes
of sterility, it may even be inferred that fertility can be lost in
different degrees, and it can be further understood to a certain
extent why this fertility is more completely lost by reversion
to the Amblystoma, than by the reversion of the Triton to the
perennibranchiate form.

If in these cases the reversion is brought about by a change in the
conditions of life, we may perhaps suppose that the magnitude of this
change would determine the degree of fertility, and the preservation
of the reversion form. Still more, however, would the fertility be
influenced by the extent of the morphological difference resulting
from the reversion. We know that the blending of very different
constitutions (_e.g._ the crossing of different species) produces
sterility. Something similar results from the sudden reversion to a
stage of development widely different in its whole structure. Here
also we have in a certain sense the union of two very different
constitutions in one individual--a kind of crossing.

From this point of view it can in some measure be comprehended why
sterility may be a result of reversion; on the other hand, we thereby
obtain no explanation why, with the same amount of morphological
difference, in one case complete sterility, and in another relative
fertility occurs. The morphological difference between Axolotl and
Amblystoma is exactly the same as between Triton and its “sexually
mature larva;” the difference between the two cases of reversion
depends entirely upon the direction of the leap, that taken in the
former case being precisely opposite in direction to that taken in the
latter.

Herein might be sought the explanation of the different strength with
which the reproductive power is affected; not indeed in the direction
of the leap itself, but in the differences in the ontogeny which
are determined by the differences in the direction of the leap. The
reversion of the Triton to an older phyletic stage coincides with
the arrest at a younger ontogenetic stage; or, in other words, the
older stage of the phylogeny to which reversion takes place is still
entirely comprised in the ontogeny of each individual. Each Triton is
perennibranchiate throughout a long period of its life; the reverting
individual simply reverts to the older phyletic stage by remaining at
the larval stage of its individual development.

But it is quite different with the reversion of the Axolotl to the
formerly acquired, but long since abandoned Amblystoma form. This is
not retained in the ontogeny of Axolotl, but has been completely lost;
for a long series of generations--so must we suppose--the ontogeny
has always only attained to the perennibranchiate form. Now if at the
present time certain individuals were compelled to revert to the
Amblystoma form, certainly no greater leap would have been made from
a morphological point of view, than in the reversion of Triton to the
perennibranchiate form, but at the same time the leap would be in
another direction, viz. over a long series of generations back to a
form which the species had not produced for a long period, and which
had to a certain extent become foreign to it. We should thus have here
also the grafting of a widely different constitution upon that of the
Axolotl, or, if one prefers it, the commingling of two widely different
constitutions.

Of course I am far from wishing to pretend that this “explanation” is
exact; it is nothing more than an attempt to point out the direction
in which the causes affecting the reproductive powers in different
degrees are to be looked for. A deeper penetration into and special
demonstration of the manner in which these causes bring about such
results, must be reserved for a future period. For the present it
must suffice to have indicated that there is an essential distinction
between the two kinds of reversion, and to have made it to some
extent comprehensible that this distinction may be the determining
impulse with respect to the question of sterility. Perhaps the law
here concealed from us may one day be thus formulated:--Atavistic
individuals lose the power of reproduction the more completely, the
greater the number of generations of their ancestors whose ontogeny no
longer comprises the phyletically older stage to which the reversion
takes place.

The hypothesis which interprets the transformation of the Axolotl as
a case of reversion, thus holds out the possibility of our being able
to comprehend the sterility of the Amblystomas arising in this manner,
whilst, on the other hand, for the adherents of a phyletic vital
force, not only is this observed sterility as Duméril expresses it “un
véritable énigme scientifique,” but an absolute paradox. We should
expect such a directive and inciting principle to call into existence
new forms having vitality and not destined to perish, the more so when
it is concerned with a combination of structural characters which, when
originating in another manner (viz. from other species of _Siredon_),
have long since shown themselves to have vitality and reproductive
power. We are indeed acquainted with species of _Amblystoma_ which
propagate as such, and each of which arises from an Axolotl-like larva.
Thus we cannot regard the sterile Amblystomas produced by the Paris
Axolotls as abortive attempts of a vital force--an interpretation which
is certainly in itself already sufficiently rash.

Now if it be asked what change in the conditions of life could have led
to the reversion in the Lake of Mexico[259] of the Amblystoma to the
Siredon form, I must admit that I can only offer a conjectural reply,
having but a conditional value so long as it is not supported by a
precise knowledge of the conditions there obtaining, and of the habits
both of the Axolotl and of the Amblystoma.

It may be supposed generally that reversion is brought about by
the same external conditions as those which formerly produced the
perennibranchiate stage. This supposition is in the first place
supported by the experiments here recorded, since it is evidently the
inducement to aërial respiration which causes the young Axolotl to
revert to the Amblystoma form, _i.e._ the inciting cause under whose
domineering influence the Amblystoma form must have arisen.

Here again the case is quite similar to that of seasonally dimorphic
butterflies. Reversion of the summer brood to the winter form is there
most easily caused by the action of cold, _i.e._ by the same influence
as that under whose sway the winter form was developed.

We know indeed that reversion may also arise by the crossing of
races and species, and I have attempted to show that reversion in
butterflies may also be brought about by other influences than cold;
but still the most probable supposition obviously is, that reversion
would be caused by the persistent action of the same influences as
those which in a certain sense created the perennibranchiate form.
That the latter was produced under the influence of an aquatic life
there can be no doubt, and thus, in accordance with my supposition, the
hypothetical _Amblystoma Mexicanum_, the supposed ancestral form of
the Axolotl of the Mexican Lake, might have been caused to revert to
the perennibranchiate form by a reduction in the possibilities of its
living upon land, and by its being compelled to frequent the water.

I will not here return to the consideration of every other opinion
_ab initio_. It is very advisable to distinguish between the mere
impulses which are able to produce sudden reversion, and between
actual transforming causes which result directly or indirectly in the
remodelling of a species. Thus, it is conceivable _à priori_ that
reversion may occur by the action of an inciting cause having nothing
to do with the origin of the phyletically older form. Temperature
can certainly have played no part, or only a very small part, in the
formation of the perennibranchiate form; nevertheless cold may well
have been one of the inciting causes which induced the Amblystoma at
one time to revert to the _Siredon_ form, and we cannot at present
consider De Saussure to be incorrect when he maintains that the low
temperature of the Mexican winter might prevent that transformation
(of the Axolotl into the Amblystoma) which would occur “in the warm
reptile-house” of the Jardin des Plantes. He supports this view by
stating that “Tschudi has found the Amblystoma” (of course another
species) “in the hottest parts of the United States.” “On the Mexican
plateau, however, it snows every winter, and if the lake does not
actually freeze, its temperature must fall very considerably in the
shallowest parts.”

But although this view is not opposed by any theoretical
considerations, I still hold it to be incorrect. I doubt whether it
is temperature that has brought about the reverse transformation of
the Amblystoma into the Axolotl, or which, according to De Saussure’s
conception, at the present time prevents the transformation of the
Axolotl in the Lake of Mexico. I doubt this because Amblystomas are now
known from all parts of the United States as far north as New York, a
proof that a winter cold considerably greater than that of the Mexican
plateau is no hindrance to the metamorphosis of the Axolotl, and that
the genus does not show itself to be in this respect more sensitive
than our native genera of _Salamandridæ_.

The following observations of De Saussure, in which he calls attention
to the nature of the Mexican Lake, appear to me to be more worthy of
consideration:--“The bottom of this lake is shallow, and one passes
imperceptibly from the lake into extensive marshy regions before
reaching solid ground; perhaps this circumstance makes the Axolotl
incapable of reaching dry land, and prevents the transformation.”

In any case the Lake of Mexico offers very peculiar conditions for
Amphibian life. My esteemed friend Dr. v. Frantzius has called my
attention to the fact that this lake--as well as many other Mexican
lakes--is slightly saline. At the time of the conquest of Mexico by
Ferdinand Cortez, this circumstance led to the final surrender of the
city, as the Spaniards cut off the supply of water to the besieged, and
the water of the lake is undrinkable. The ancient Mexicans had laid
down water-conduits from the distant mountains, and the city is still
supplied with water brought through conduits.

Now this saltness cannot in itself be the cause of the degeneration
to the perennibranchiate form, but it may well be so in combination
with other pecularities of the lake. The narrowest part of the lake
is the eastern, and it is only in this part that the Axolotl lives.
Now in winter, violent easterly gales rush down from the mountains and
blow continuously, driving the water before them to such an extent
that it becomes heaped up in the western portion of the lake, where it
frequently causes floods, whilst 2000 feet of the shallow eastern shore
are often laid completely dry.[260]

Now if we consider these two peculiarities, viz. salineness and
periodical drying up of a part of the bottom of the lake through
continuous gales, we certainly have for the Axolotl, conditions of
life which are only to be found in few species. One might certainly
attempt to apply these facts in a quite opposite sense, and to regard
them as unfavourable to my theory, since the retreat of the water
from a great portion of the bottom of the lake would--so one might
think--rather facilitate transition to a life upon land, and indeed
compel the adoption of such a mode of existence. But we should thus
forget that the exposed bottom of the lake is a sterile surface without
food or place of concealment, and, above all, without vegetation;
and further, that owing to the considerable salineness of the water
(specific gravity = 1.0215),[261] the whole of the exposed surface must
be incrusted with salt, a circumstance which would render it quite
impossible for the creatures to feed upon land. Sodic chloride and
carbonate are dissolved in the water in such considerable quantities,
that they are regularly deposited upon the shores of the lake as a
crust, which is collected during the dry season of the year and sent
into the market under the name of “tequisquite” (Mühlenpfordt).[262]

Thus the supposition is not wanting in support, that peculiar
conditions make it more difficult for the creature to obtain its food
upon land than in the water, and this alone may have been sufficient
to have induced it to acquire the habits of a purely aquatic existence,
and thus to revert to the perennibranchiate or Ichthyodeous form.

But enough of supposition. We must not complain that we are unable
from afar to discover with precision the causes which compelled the
Axolotl to abandon the Amblystoma stage, as long as we are not able to
explain the much nearer cases of reversion in Filippi’s and Jullien’s
Tritons; nevertheless, in these cases also, the causes affecting the
whole colony of Tritons must be general, since--at least in the case
noticed by Filippi--the greater majority of the individuals remained
in the larval condition. Experiments with Triton larvæ could throw
greater light upon this subject; it would have in the first place to be
established whether reversion could be artificially induced, and if so,
by what influences.

From the previously mentioned experiments with butterflies, as well
as from the results obtained with Axolotls, we should expect that
in Tritons, reversion to the Ichthyodeous form would take place if
we allowed the inciting cause, viz. the bathing of the gills and of
the whole body with water, to act persistently, and at the same time
withheld that influence under whose action the salamander form became
developed, viz. the bathing of the gills, the skin, and the surfaces of
the lungs with air.

Old experiments of this kind are to be met with, but they were never
carried on for a sufficient time to entirely allay the suspicion, that
the specimens concerned would perhaps have undergone the ordinary
metamorphosis if their existence had been prolonged.

Thus, Schreibers[263] relates that “by confining tadpoles of the
salamander found at large in their last stage of growth, under water by
means of an arrangement (net?), and feeding them with finely chopped
earthworms, he was able to keep them for several months--and indeed
throughout the winter--in this condition, and in this way to forcibly
defer their final change, and their transition from the tadpole stage
to that of the perfected creature during this period.” It is not stated
whether the animals finally underwent transformation, so that it cannot
be decided whether we have here a case of reversion or simply one of
retarded development. That metamorphosis may occur after a long period
of time, is shown by experiments upon the tadpole of _Pelobates_
conducted by Professor Langer in Vienna.[264] The creatures were kept
in deep water in such a manner that they were not able to land, and
by this means three out of a large number of individuals had their
metamorphosis delayed till the second summer; notwithstanding this,
transformation then occurred.

It cannot be objected to my reversion hypothesis, that it opposes on
the one side what on the other it postulates, viz. a _per saltum_
change of structure. Reversion is characterized by the sudden
acquisition of an older, _i.e._ a formerly existing phyletic stage.
That reversion occurs is a fact, whilst nobody has hitherto been able
to prove, or even to make probable, that a stage of the future (_sit
venia verbo_) has been attained at once (_per saltum_).

Now if it is possible to find influences in the present conditions
of life of the Axolotl which make it difficult or quite impossible
for it to live upon land, and which therefore appear as incentives
to the reversion to the Ichthyodeous form, the other portion of my
hypothesis--the assumption that the Axolotl had become an Amblystoma at
a former period--can also be supported by facts.

We know from Humboldt[265] that the level of the Lake of Mexico at a
comparatively recent period was considerably higher than at present. We
know further that the Mexican plateau was covered with forest, which
has now been destroyed wherever there are human, and especially Spanish
settlements. Now if we suppose that at some post-glacial period the
mountain forests extended to the borders of the lake, at that time
deep, with precipitous sides and much less saline, not only should we
thus have presented different conditions of life to those at present
existing, but also such as would be most favourable for the development
of a species of salamander.

On the whole, I believe that my attempt to explain the exceptional
metamorphosis of the Axolotl of the Mexican lake cannot be objected
to as being a too airy phantasy. In any case it is the only possible
explanation which can be opposed to that which supposes that the
occasional transformation of the Axolotl is not reversion, but an
attempt at advancement. This last assumption must, in my judgment, be
rejected on purely theoretical grounds by those who hold that a sudden
transformation of a species, when connected with adaptation to new
conditions of life, is inconceivable--by those who regard adaptation,
not as the sudden work of a magic power, but as the end result of a
long series of natural, although minute and imperceptible causes.

If my interpretation of the facts be correct, there arises certain
consequences which I may here briefly mention in conclusion.

First, with regard to more obvious results. If _Siredon Mexicanus_,
Shaw, only by occasional reversion assumes the Amblystoma form,
and never, or only exceptionally, propagates as such, but only
as _Siredon_, the more recent systematists are not justified in
striking out the genus _Siredon_ and in placing _S. Mexicanus_ as an
undeveloped form in the genus _Amblystoma_. So long as there exists
not one only, but several species of _Siredon_ which as such regularly
propagate themselves, the genus exists; and although we would not
deprive systematists of all hope of these species of _Siredon_ being
one day re-elevated to _Amblystomæ_, it nevertheless better accords
with the actually existing state of affairs if we allow the genus
_Siredon_ to remain as before among the genera of _Salamandrina_, and
to include therein all those species which, like the Paris Axolotl,
_S. Mexicanus_, Shaw, and probably also _S. Lichenoides_, Baird, only
exceptionally, or through artificial influences, assume the Amblystoma
form, but without propagating regularly in this condition. On the other
hand, we should correctly comprise under the genus _Amblystoma_ all
those species which propagate in this state regularly, and in which the
perennibranchiate stage occurs only as a larval condition.

To arrive at a decision in single cases would chiefly concern the
American naturalists, whose ever increasing activity may lead us
to hope soon for a closer investigation of the reproduction of the
numerous species of _Amblystoma_ of their native country. I should
rejoice if the facts and arguments which I have here offered should
give an impetus to such researches.

The second consequence to which I may refer, is of a purely
theoretical nature, and concerns a corollary to the “fundamental
biogenetic law” first enunciated by Fritz Müller and Haeckel. This, as
is well known, consists of the following law:--The ontogeny comprises
the phylogeny, more or less compressed and more or less modified.

Now according to this law, each step in phyletic development when
replaced by a later one, must remain preserved in the ontogeny, and
must therefore appear at the present time as an ontogenetic stage
in the development of each individual. But my interpretation of the
transformation of the Axolotl appears to stand in contradiction to
this, since the Axolotl, which at a former period was an Amblystoma,
retains nothing of the latter in its ontogeny. The contradiction is,
however, only apparent. As long as we are concerned with an actual
advance in development, and therefore with the attainment of a new step
never formerly reached, the older stages will be found in the ontogeny.
But this is not the case when the new stage is not an actual novelty,
but formerly represented the final stage of the individual development;
or, in other words, when we are concerned with the reversion, not
of single individuals, but of the species as such, to the preceding
phyletic stage, _i.e._ with a phyletic degeneration of the species.
In this case the former end-stage of the ontogeny would be simply
eliminated, and we should then only be able to recognize its former
existence by its occasional appearance in a reversion form. Thus, under
certain conditions the Triton sinks back to the perennibranchiate
stage; not in such a manner that the individual first becomes a Triton
and then undergoes perennibranchiate re-modification, but simply, as I
have already shown above, by its remaining at the Ichthyodeous stage
and no longer attaining to the Salamander form. So also, according
to my hypothesis, the salamandrine _Amblystoma Mexicanum_, formerly
inhabiting the shores of the Lake of Mexico, has degenerated to the
perennibranchiate stage, and the only trace that remains to us of its
former developmental status is the tendency, more or less retained
in each individual, to again ascend to the salamander stage under
favourable conditions.

The third and last consequence which my interpretation of the facts
entails, is the change in the part played by reversion in organic
nature. Whilst atavistic forms have hitherto been known only as
isolated and exceptional cases, interesting indeed in the highest
degree, but devoid of significance in the course of the development
of organic nature, a real importance in this last respect must now be
attached to them.

I may assume that reversion can in two ways be effectual for the
preservation or re-establishment of a living form. In the first
place, where, as in Axolotl, the new and organically higher form
becomes untenable through external influences, instead of simply
perishing--since advancement in another direction does not appear to
be possible--a reversion of the species to the older and more lowly
organized stage occurs. In the second place, the older phyletic form
may not be abandoned while a newer form is being developed therefrom,
but the former may alternate with the latter, as we see in the case
of seasonally dimorphic butterflies. It can hardly be objected if I
regard the alternation of the summer and winter form in this case as a
periodic reversion to the phyletically older (winter) form.

Although the reversion of an entire species, such as I suppose to have
been the case with the Axolotl, may be of rare occurrence, this is
certainly not the case with periodic or cyclical reversion; the latter
plays a very important part in the development of the various forms of
alternating or cyclical propagation.[266]


POSTSCRIPT.

In the previous portion of this essay it was pointed out that the
causes to which I attributed the reversion of the hypothetical
_Amblystoma Mexicanum_ to the existing Axolotl, did not appear to me
to amount to a complete explanation of the phenomenon. In the first
place these seemed to me too local, since they could only be applied
with any certainty to the Axolotl of the lake of the Mexican capital,
whilst the Paris Axolotls obtained from other parts of Mexico still
required an explanation. On the other hand, these causes did not appear
to me sufficiently cogent. Should we even learn subsequently that the
Paris Axolotl is also derived from a salt lake which is exposed to
similar winds to the Lake of Mexico, we still have in this peculiarity
of the lakes only a cause tending to make it difficult for the larva
to undergo metamorphosis, and to reach a suitable new habitat on the
land. The impossibility of doing this, or the complete absence of such
habitat, does not however follow as a necessary consequence.

It would obviously be a much more solid support for my hypothesis if it
were possible to point to some physical conditions of the land which
there precluded the possibility of the existence of Amblystomas.

For a long time I was indeed unable to discover such causes, and I
therefore concluded the previous portion of this essay and went to
press. Afterwards, when residing in one of the highest valleys of our
Alps in the Upper Engadine, an idea accidentally occurred to me, which
I do not now hesitate to regard as correct after having tested it by
known facts.

It happens that in the Upper Engadine there live only such Amphibia
as persistently, or at least frequently resort to the water. I found
frogs up to nearly 7000 feet above the sea, and Tritons at 6000 feet
(Pontresina and Upper Samaden). On the other hand, the land-living
mountain salamander, _S. Atra_,[267] was absent, although suitable
stations for this species were everywhere present, and it would have
wanted for food as little as do its allies the water-newts. Neither
would the great elevation above the sea offer any obstacle to its
occurrence, since it occasionally ascends to a height of 3000 metres
(Fatiot).[268]

Now it is well known that the atmosphere of the Upper Engadine,[269]
like that of other elevated Alpine valleys enclosed by extensive
glaciers, is often extraordinarily dry for a long period, a condition
which appears to me to explain why the black land-salamander is there
absent,[270] whilst its near water-living ally occurs in large numbers.
The skin of the naked Amphibia generally requires moisture, or else
it dries up, and the creature is deprived of a necessary breathing
apparatus, and often dies as rapidly as though some important internal
organ had been removed. Decapitated frogs hop about for a long time,
but a frog which escapes from a conservatory and wanders about for one
night in the dry air of a room, is found the following day with dry and
dusty skin half dead in some nook, and perhaps perishes in the course
of another day if left without moisture.

All that we know of the biology of the Amphibia is in accordance with
this. Thus, all the land-salamanders of southern Italy avoid the hot
and dry air of summer by burying in the ground, where they undergo
a summer sleep. This is the case with the interesting _Salamandrina
Perspicillata_,[271] and with the land-living Sardinian Triton, the
remarkable _Euproctus Rusconii_, Gené,[272] (_Triton Platycephalus_,
Schreiber). With respect to _Geotriton Fuscus_ I learn from Dr.
Wiedersheim, who has studied the life conditions of this, the lowest
European Urodelan, in its own habitat, that in Sardinia it sleeps
uninterruptedly from June till the winter; whilst on the coast of
Spezia and at Carrara, where it also occurs, it avoids the summer sleep
in a very peculiar manner. It makes use of the numerous holes in the
calcareous formation of that region, and for some months in the year
becomes a cave-dweller. As soon as the great heat occurs, often in May,
it withdraws into the holes, and again emerges in November during the
wet weather. In these lurking holes it does not fall into a sleep, but
is found quite active, and its stomach, filled chiefly with scorpions,
shows that it goes successfully in search of food; the moist air of the
holes makes it unnecessary for it to bury in the earth.

In the same sense it appears to me must be conceived the fact
that the solitary species of frog of the Upper Engadine, _Rana
Temporaria_,[273] the brown grass frog, is there much more a frequenter
of the water than in the plains. It is true that I can find no remark
to this effect in the excellent work of Fatiot, already referred to
above, and I am therefore obliged to resort to my own observations,
which, although often repeated, have always been carried on for only a
short time. I was much struck with the circumstance that the Engadine
frogs were to be found in numbers in the water long after the pairing
season, which, according to Fatiot, lasts at most to the end of June.
In the numerous pools around Samaden I found them in July and August,
whilst in the plains they only take to the water at the time of
reproduction, and seek winter quarters in the mud on the first arrival
of this season. (Fatiot, p. 321.) In the Engadine they have therefore
in some measure adopted the mode of life of the aquatic frogs, but this
of course does not prevent them from returning in damp weather to their
old habits and roving through meadows and woods.

After these considerations had made it appear to me very probable that
the dry air of the Upper Engadine accounted for the absence of the
black land-salamander, the question at once arose whether the absence
of Amblystomas from the Mexican plateau might not perhaps be due to
the same cause, _i.e._ whether such a dryness of the atmosphere might
not perhaps prevail also in that region, so that Amphibia, or at least
salamander-like Amphibia, could not long exist on the land. The height
above the sea is still greater (7000 to 8000 feet), and the tropical
sun would more rapidly dessicate everything in a country poor in water.

As I was at the time without any books that might have enlightened
me on the meteorological conditions of Mexico, I wrote to Dr. v.
Frantzius, who, by many years residence in Central America was familiar
with the climate of this region, and solicited his opinion. I received
the reply that on the high plains of Mexico an extraordinary dryness
of the atmosphere certainly prevails. “The main cause of the dryness
of the high plains is to be found in the geographical position,
the configuration of the land, and the physical structure. The
north-eastern trade-wind drives the clouds against the mountains, on
the summits of which they deposit their moisture, so that no vapour is
carried over; as long as the north-east trade-wind blows, the streams
feeding the rivers flowing into the Atlantic Ocean are abundantly fed
with water, whilst on the western slopes, and especially on the high
plains, the clouds give no precipitation. In the second half of the
year also, during our summer, the so-called rainy season brings but
little rain[274]--little in comparison with the more southern regions,
where the heavy tropical thunderstorms daily deluge the earth with
water. Mexico lies much too northerly, and does not reach the zone of
calms, within which region these tropical rains are met with.”

Thus, in the high degree of dryness of the air lasting throughout the
year, I do not doubt that we have the chief cause why no Amblystomas
occur on these elevated plains; they simply cannot exist, and would
become dried up if taken there, supposing them not to be able to change
their mode of life and to take to the water. If therefore in former
times Amblystomas inhabited Mexico, the coming on of the existing
climatic conditions left them only the alternative of becoming extinct,
or of again taking to the aquatic life of their Ichthyodeous ancestors.
That this was not _directly_ possible--that the Amblystoma form was not
able to become aquatic without a change of structure, is shown by the
fact that even in the Lake of Mexico no Amblystoma occurs. A retreat to
an aqueous existence could, as it appears, only be effected by complete
reversion to the Ichthyodeous form, which then also took place.

But my hypothesis of the transformation of the Axolotl not only
requires the proof that Amblystomas cannot exist under present
conditions in Mexico, but also the further demonstration that at a
former period other conditions prevailed there, and these of such a
nature as to make the existence of land-salamanders possible.

With respect to my question, whether we might not perhaps assume that
at some post-glacial period the conditions of atmospheric moisture on
the high plains of Mexico were essentially different from those at
present prevailing, I recollected Dr. v. Frantzius and the above-quoted
observation of Humboldt’s,[275] who discovered in the neighbourhood
of the Lake of Tezenco (Mexico) distinct evidence of a much higher
former level of the water. “All such elevated plains were certainly at
a former period so many extensive water-basins, which gradually became
filled, and are still filling up with detritus. The evaporation from
such large surfaces of water must at that time have caused a very moist
atmosphere, favourable to vegetation and adapted for the life of naked
Amphibia.”

From this side also my hypothesis thus receives support, and we may
assume with some certainty that at the beginning of the diluvial
period[276] the woods surrounding the Mexican lakes were inhabited
by Amblystomas, which, as the lakes subsequently became more and more
dried up and the air continually lost moisture, found it more difficult
to exist on the land. They would at length have completely died out,
had they not again become aquatic by reversion to the Ichthyodeous
form. It may perhaps be supposed that the above-mentioned physical
conditions--desolate, salt-incrusted shores--co-operated in the
production of the reversion, by making it difficult for the larvæ to
quit the water; but we can only judge with certainty upon this point
when, by means of experiment, we have discovered the causes which
produce reversion in the Amphibia.


ADDENDUM.

I have lately met with another interesting notice on the reproduction
of the native North American Amblystomas. Professor Spence F. Baird, of
Washington, has often observed the development from the egg of various
species, and especially of _Amblystoma Punctatum_ and _A. Fasciatum_.
His observations do not appear to be as yet published, so that I was
unable to discover any account of the development of _Amblystoma_ in
existing literature.[277] I am authorized to extract the following
brief data from a letter addressed to Dr. v. Frantzius.

In order to deposit their eggs the Amblystomas go into the water,
where the eggs are laid enclosed in a jelly-like mass, but never more
than fifteen to twenty together. The spherical eggs are very large,
perhaps a quarter of an inch in diameter. They soon develop into a
_Siredon_-like larva, which remains several months in this condition.
The gills then shrivel up, the creature begins to crawl, and gradually
passes through the different transformations to the complete Amblystoma
form.

It appears from this communication that the Amblystomas lay much larger
and much fewer eggs than the Axolotl, and that their development
throughout resembles that of our salamanders.

In concluding I may mention an anatomical fact which most strongly
supports my view that the Mexican Axolotl is a reverted Amblystoma.
I learn from Dr. Wiedersheim that the Axolotl possesses the
“intermaxillary gland” which occurs in all the land Amphibia. This
organ, lying in the intermaxillary cavity, appears, whenever it occurs,
to produce a kind of birdlime, _i.e._ a very glutinous secretion,
which serves to attach the prey to the rapidly protrusible tongue.
Although this secretion may perhaps also have another function, from
the absence of the intermaxillary gland in all exclusively aquatic
Amphibia, it follows that it must be devoid of importance for, and
inapplicable to feeding in the water. The intermaxillary gland is
absent in all _Perennibranchiata_ and _Derotremata_ which Wiedersheim
has hitherto investigated, viz. in _Menobranchus_, _Proteus_,
_Siren_, _Cryptobranchus_, _Amphiuma_, and _Menopoma_, all of which
are indeed without the cavity in which the gland is situated in the
_Salamandrina_, _i.e._ the _cavum intermaxillare_.

Now in the _Salamandrina_ the gland appears at an early stage. It is
possessed in a well-developed state by the larvæ both of species of
_Triton_ and of _Amblystoma_, where indeed the glandular structure
completely fills the _cavum intermaxillare_.

Were the Axolotl a species retarded in phyletic development,
the presence of a gland which does not occur in any other
_Perennibranchiata_, and which is only of use for life upon land, would
be quite inexplicable.

The matter becomes still more enigmatical through the fact that
the gland, although present, is quite rudimentary. Whilst in the
_Salamandrina_ the capacious intermaxillary cavity is entirely filled
by the tubes of the gland in question, in Axolotl this cavity is almost
completely filled with a closely woven connective tissue, in which
there can only be found a small number of gland-tubes--in the extreme
front, and at the base immediately over the intermaxillary teeth--these
tubes agreeing in the details of their histological structure with the
elements of the same gland in the _Salamandridæ_.

I give these anatomical details from Dr. Wiedersheim’s verbal
communication. An amplified account will subsequently appear in another
place.[278]

An explanation of this rudimentary intermaxillary gland in the Axolotl
only appears to me possible on the supposition that the latter is an
atavistic form. From this point of view it is evident that the gland
already present in all _Amblystoma_-larvæ must have been taken over
by the perennibranchiate form of the existing Axolotl, through the
reversion of the hypothetical _Amblystoma Mexicanum_ of the “diluvial
period.”[279] It can also be easily understood that this organ would
become more and more rudimentary in the course of time, since it has
no further use in the water, and the gap thus arising in the formerly
present _cavum intermaxillare_ would become filled with connective
tissue.

While the German edition of this work was going through the press
I obtained, through the kindness of my friend Dr. Emil Bessels of
Washington, the Mexican memoir upon the new Axolotl,[280] which even in
Mexico regularly, or at least in many cases, becomes developed into the
Amblystoma form.

The facts are briefly as follows:--The small Lake of Santa Isabel
is some hours’ journey from the Mexican capital. In this lake there
lives a species of Axolotl which had hitherto remained unknown, and
was described by Señor Velasco as _Siredon Tigrinus_. This species
propagates itself indeed in the Axolotl state, but in many cases it
becomes transformed into Amblystoma and takes to the land. Although
propagation in the Amblystoma condition was not observed, it can hardly
be doubted that it also propagates in this form.

At first sight these facts appear to refute my hypothesis, that
the extreme dryness of the air of the Mexican plateau precludes
the existence of land Amphibia. Nevertheless I do not abandon this
hypothesis for the former one, since a closer study of the data
furnished by Velasco confirms rather than refutes my supposition.

Velasco expressly corroborates the statement that the Axolotl hitherto
known from the great Mexican lake which never dries up (Lake of
Xochimilco and Chalco), is only met with in its native habitat in the
_Siredon_ form, _i.e._ as _Siredon Humboldtii_. According to Velasco
the cause of the frequent assumption of the Amblystoma form by the new
_Siredon Tigrinus_, is to be found in the local conditions of life of
this species. The Lake Santa Isabel is shallow, its greatest depth
amounting to three meters, and it is liable to a periodical drying up,
which is so complete that one can pass dry-shod through it in several
places. The species must therefore have long since died out had it
not been able to adapt itself periodically to a land life. Now it
could have become transformed into a land Amphibian--as Señor Velasco
observed--at various stages of growth; and indeed this author believes
that “the Creator has implanted an instinct in this creature,” which
enables it to always undergo metamorphosis at the right time.

This last assumption may or may not be taken as correct, but this much
is established, viz. that numerous individuals of this species take to
the land, and remain there during a period of many months.

But does this contain the proof that salamander-like animals are
actually able to lead a land life in Mexico--that the dry air is
advantageous, or at least supportable to them? It does not appear so
to me, but rather that all which has been reported of this Amblystoma
by Señor Velasco goes to show that the animal does not, properly
speaking, live upon land like the North American Amblystomas, or like
our land-salamanders, but that _it only experiences a summer sleep
lasting over the period of drought_. These Amblystomas were observed
as they left the dried-up lake at night in order to seek some moist
lurking-place in the neighbourhood, where they might remain concealed.
They are only known in the villages situated near the lake, and were
only seen there at large just when they were wandering from the lake
to their place of concealment. At other times they were mostly found
in the earth, buried under walls, the pavement of the market-place,
&c. When laying down a line of railway, a workman found in the earth
a whole nest of twelve Amblystomas lying close together. All these
are not mere lurking-holes which could be abandoned at any moment;
it would rather appear that we have here places of refuge for the
entire duration of the period of drought, and that these would only
be forsaken when the water of the rainy season penetrated the soil.
I am not myself in a favourable position for investigating these
suppositions more closely, but this could be done by Señor Velasco,
who lives in Mexico, and science would be much indebted to him if he
would examine as precisely as possible into the habits and conditions
of life of this, and of the other species of Mexican Axolotls.
Unfortunately this gentleman can, it would appear, have seen only the
French publications upon the transformation of the Axolotl, and could
not therefore have asked himself questions arising from my conception
of the facts; otherwise many of his observations would have led to more
definite results. The above conclusion can however be still further
supported by Señor Velasco’s data.

One might indeed insist that with us also the land-salamanders conceal
themselves in moist places during dry weather, and often lie hidden,
as in Mexico, in a hole, in a cluster of as many as ten together;
but with us they leave their lurking-place from time to time and go
in search of food. Señor Velasco mentions nothing with respect to
this. What especially struck me was the statement that the Mexican
Amblystomas _were also to be found in the water_.[281] When Lake Santa
Isabel is drained, the fishermen stretch large nets across the exit
channels, and in these they not only find ordinary Axolotls, but also
some “_sin aretes_,” which they also designate “_mochos_,” _i.e._
hornless Axolotls, because they have no gills, but have already reached
the Amblystoma stage. Our land-salamanders live in the water only
as larvæ, but they also love and require moisture. Only the female
enters the water when she wants to deposit her young (eggs with mature
larvæ), and then only at the margin of shallow pools or small brooks.
The Mexican Amblystoma thus much more resembles in its habits our
water-salamanders (Tritons), which remain in the water at least during
the whole period of reproduction. These also leave the water later,
and, like the land-salamander, seek concealment in the earth. They have
this habit also in those districts which possess a very dry atmosphere;
and especially in the Engadine, where I first conceived the idea of
taking into account the dryness of the air, I found in the pools at the
end of August and the beginning of September only larvæ of Tritons. The
older Amphibians must therefore have been on the land, presumably in
their places of winter concealment.

From what we have hitherto learnt from Señor Velasco, the mode of
life of _Amblystoma Tigrinum_ must resemble that of our Tritons,
although its structure is that of a land-salamander. I would thus
offer the following explanation of the facts at present known:--Owing
to the periodic drying up of the lake of Santa Isabel, the _Siredon
Tigrinus_ would be again compelled to undergo metamorphosis. Whether
this was formerly entirely abandoned, or whether it always occurred
in solitary individuals, is almost immaterial; in any case the habit
of metamorphosis must have been very rapidly acquired through natural
selection, and must have again become general, if the faculty was only
present in the species, although latent. Through the dryness of the
air, the Amblystomas that had taken to the land would be compelled to
bury themselves at once, and to remain asleep till the recurrence of
the rainy season, when they would hasten back into the water and would
there live as a species of Triton.

Now one might feel inclined to ask why the species of the great Mexican
lake has not also taken to this mode of life. To this it may be simply
replied that the water of this lake never dries up, and that the
Axolotls have thus never been reduced to the alternative of undergoing
metamorphosis or of perishing. If therefore the conditions of existence
in water were more favourable than on land, the tendency to abandon
metamorphosis would increase from generation to generation, and the
deportment at present observed would finally result, _i.e._ propagation
would take place exclusively in the Axolotl state. As has already been
mentioned above, the latest observations of Velasco furnish further
confirmation that the Axolotl of the great lake is never met with in
the Amblystoma condition, “although it (the Axolotl) is brought daily
from Mexico into the market throughout the whole year.” I should not
however regard it as a refutation of my view if prolonged investigation
should show that this species also (_Siredon Humboldtii_) occasionally
developed into an Amblystoma; on the contrary, it would not at all
surprise me if such cases of reversion occurred in Mexico as well as in
Europe. The fact that an immense majority of the Amphibians propagate
in the Axolotl state would not be thereby affected, and would still
require an explanation: this I am still inclined to see in the dryness
of the air of the high plains, which is so unfavourably adapted for a
life passed entirely on land.



=IV.=

ON THE MECHANICAL CONCEPTION OF NATURE.



INTRODUCTION.


In the first of the three preceding essays it was attempted to solve
the question whether the transformations of a given complex of
characters in a certain systematic group could be completely explained
by the sole aid of Darwinian principles. It was attempted to trace
the origin of the marking and colouring of the Sphinx-caterpillars to
individual variability, to the influences of the environment, and to
the laws of correlation acting within the organism. These principles
as applied to the origin of a certain well-defined, although narrowly
restricted range of forms, were tested in order to see whether they
were alone sufficient to explain the transformation of the forms.

It appeared that this was certainly the case. In all instances, or
at least where the facts necessary to obtain a complete insight were
available, the transformations could be traced to these known factors;
there remained no inexplicable residual phenomena, and we therefore had
no reason for inferring the existence of some still unknown modifying
cause lying concealed in the organism. In this region of the marking
and colouring of caterpillars, the assumption of a phyletic vital force
had to be abandoned, as being superfluous for the explanation of the
facts.

In the second essay the attempt was next made with reference to double
form-relationship, as presented for observation in metamorphic insects,
to draw conclusions as to the causes of the transformations. It
appeared here that form- and blood-relationship do not always coincide,
since the larvæ of a species, genus, or family, &c., may show quite
different form-relationships to their imagines. These facts alone told
very decisively against the existence of an internal developmental
power, so that the latter had likewise to be set aside by the method
of elimination, since the observed incongruences as well as the
congruences of form-relationship, found sufficient explanation in the
action of the environment on the organism.

This investigation thus also led to the denial of a phyletic vital
force.

In the third essay I finally sought to prove that the only case
of transformation of one species into another at present actually
observed[282], could not without further evidence be interpreted as
the result of the action of a phyletic vital force, but that more
probably we had here only an apparent case of new formation, which was
in reality but a reversion to a stage formerly in existence.

If this last investigation removes the only certain observation which
could have been adduced in favour of the hypothesis of a phyletic vital
force, so also do the two former essays show that this hypothesis, at
least in the case of insects, must be abandoned as inadequate.

The question now arises whether this conclusion, based on such a
limited range of inquiry, can also be applied to the other groups of
the organic world without further evidence.

The supporters of a principle of organic development will deny this in
each individual case, and will demand special proof for each group of
organisms; I believe this position, however, to be incorrect. Here,
if anywhere, it appears to me justifiable to apply the conclusions
inductively from special cases to general ones, since I cannot at all
see why a power of such pre-eminent and fundamental importance as a
phyletic vital force should have its activity limited to solitary
groups in the organic world. If such a power exists it must be the
inciting cause of organic development in general, and must be equally
necessary in every part of creation, as no advancement could take place
without it. In this case, however, the force would be recognizable and
demonstrable at every point; the phenomena should nowhere stand in
opposition to its admission, and should in no case be explicable or
comprehensible without it. The same laws and forces which caused the
development of one group of forms must underlie the development of the
whole organic world.

I therefore believe that we are correct in applying to the whole living
world the results furnished by the investigation of insects, and in
thus denying the existence of an innate metaphysical developmental
force.

There is, however, a quite distinct method which leads to the same
results, and to the preliminary, if not to the complete and definitive
rejection of such a principle; _the admission of this power is directly
opposed to the laws of natural science_, which forbid the assumption of
_unknown_ forces as long as it is not demonstrated that _known_ forces
are insufficient for the explanation of the phenomena. Now nobody will
assert that this has in any case been proved; the test of applying the
known factors of transformation has only just commenced, and wherever
it has been made they have proved sufficient as causal forces. Thus,
even without the foregoing special investigations we should deny a
phyletic vital force; the more so as its admission is fraught with
the greatest consequences, since it involves a renunciation of the
possibility of comprehending the organic world. We should, on this
assumption, at once cut ourselves off from all possible mechanical
explanation of organic nature, _i.e._ from all explanation conformable
to law. But this signifies no less than the renunciation of all further
inquiry; for what is investigation in natural science but the attempt
to indicate the mechanism through which the phenomena of the world
are brought about? Where this mechanism ceases science is no longer
possible, and transcendental philosophy alone has a voice.

This conception represents very precisely the well-known decision of
Kant:--“Since we cannot in any case know _à priori_ to what extent the
mechanism of Nature serves as a means to every final purpose in the
latter, or how far the mechanical explanation possible to us reaches,”
natural science must everywhere press the attempt at mechanical
explanation as far as possible. This obligation of natural science will
be conceded even by those who lay great stress upon the necessity for
assuming a designing principle. Thus, Karl Ernst von Baer states that
we have no right “to assert of the individual processes of Nature, even
when these evidently lead to a definite result, that some Mind has
originated them designedly. The naturalist must always commence with
details, and may then afterwards ask whether the totality of details
leads him to a general and final basis of intentional design.”[283]

But even if we are precluded on these grounds only from assuming
the existence of a directive power, _i.e._ a phyletic vital
force, for explaining detailed phenomena, and are at the same
time debarred from the possibility of arriving at a physical or
mechanical explanation--which amounts to no less than the abandoning
of the scientific position--it certainly cannot be asserted that
the development of the organic world is already conceived of as
a mechanical process. We rather acquiesce in the belief that the
processes both of organic and of inorganic nature depend most probably
upon purely causal powers, and that the attempt to refer these to
mechanical principles should not therefore be abandoned. There is no
ground for renouncing the possibility of a mechanical explanation, and
the naturalist _must not_ therefore resign this possibility; for this
reason he cannot be permitted to assume a phyletic power so long as it
is not demonstrated that the phenomena can never be understood without
such an assumption.

It cannot be raised as an objection that even for the explanation of
individual life a vital power was long ago admitted, as there was not
then sufficient material at hand to enable the phenomena of life to be
traced to physical forces. It is now no longer questionable that this
assumption was a useless error--a false method--at the time when made
certainly very excusable, since the aspect of the question was then,
owing to the imperfect basis of facts, very different to the present
analogous question as to the causes of derivative development. Thus,
although it is now easy to prove this assumption to be erroneous, it
was in the former sense correct, as it was in accordance with the
existing state of knowledge. At that time there was hardly one of the
numerous bridges which now connect inorganic with organic nature,
so that the supposition that life depended upon forces which had no
existence outside living beings was sufficiently near.

In any case the philosophers of that period cannot be blamed for
filling up the gaps in the existing knowledge by unknown powers,
and in this manner seeking to establish a finished system. The task
of philosophy is different to that of natural science; the former
strives at every period to set up a completely finished representation
of the universe in accordance with the existing state of knowledge.
Natural science on the other hand is only concerned in collecting
this knowledge; she need not therefore always finish off, and indeed
can never close her account, since she will never be in a position to
solve all problems.[284] But science must not for this reason pronounce
any question to be insoluble simply because it has not yet been
completely solved; this she does, however, as soon as she renounces
the possibility of a mechanical explanation by invoking the aid of a
metaphysical principle.

That this is the correct mode of scientific investigation is seen
by the abandoning of the (ontogenetic) vital force. The latter is
no longer admitted by anybody, now that we have turned from mere
speculation to the investigation of Nature’s processes; nevertheless
its non-existence has not been demonstrated, nor are we yet in a
position to prove that all the phenomena of life must be traced to
purely physico-chemical processes, to say nothing of our being actually
able to thus trace them. Von Baer also states “that the abolishment of
the vital force is an important advance; it is the reduction of the
phenomena of life to physico-chemical processes, although these indeed
still contain many gaps.” He points out how very far we are still
removed from being able to reduce to physical causes, the processes
through which the fertilized yelk of an egg becomes developed into a
chicken.

How comes it therefore that we all have a conviction that such a
complete reduction will in time become possible, or if not this, that
the development of the individual depends entirely upon the same forces
which are in operation without the organism? For what reason have we
rejected the “vital force”?

Simply because we see no reason for assuming that known forces are
insufficient for explaining the phenomena, and because we are not
justified in admitting directive forces as long as we have any hope of
one day furnishing a mechanical explanation.

But if it is not only permissible, but even necessary, to explain the
_ontogenetic_ vital power by known forces, and to commence to indicate
the mechanism which produces the individual life, why should it not
be equally necessary to abandon that assumption of a _phyletic_ vital
force which stifles any deeper inquiry, and to attempt to point out
that here also the co-operation of mechanical forces has brought about
the multitudinous and wonderful phenomena of the organic world?

The renunciation of the old vital force was certainly an immediate
consequence of the acquisition of new facts--of the knowledge that the
same compounds which compose organic bodies can be produced without the
latter. This discovery, due to Wöhler and his followers, showed that
organic products could be prepared artificially.[285] In brief, the
decline of the vital force followed from the knowledge that at least
one portion of the processes of life was governed by known forces.

But in the domain of the development of the organic world have we not
quite analogous proofs of the efficacy of known forces? Is not the
_variability_ of all types of forms a fact? and must not this under the
action of natural selection and heredity lead to permanent changes? Has
not the problem of explaining the subserviency of all organic form to
law as a _result_ without invoking its aid as a _principle_ been thus
successfully solved? It is true that we have not directly observed the
process of natural selection from beginning to end; neither has anybody
directly observed the mode in which the heat of the animal body is
generated by the processes of combustion going on in the blood and
in the tissues; nevertheless, this is believed as a certainty, and a
“vital force” is not invoked.

Now the above-mentioned Darwinian principles of transmutation are
certainly not simple forces of nature like those underlying the
development of the individual, _i.e._ chemico-physical forces, and it
cannot be said _à priori_ whether in one of these principles--perhaps
in variability or in correlation--there may not lie concealed a
metaphysical principle in addition to the physical forces. In fact it
has lately been asserted by Edward von Hartmann[286] that the theory
of selection is not a mechanical explanation, since it combines forces
which are only partly mechanical and in part directive.

It must therefore be next investigated whether this assertion is
tenable.



I.

ARE THE PRINCIPLES OF THE SELECTION THEORY MECHANICAL?


Edward von Hartmann may justly claim that his views should be
considered and tested by naturalists.[287] He would be correctly
classed with those philosophers who have approached this question with
a many-sided scientific preparation. It can nevertheless be perceived
in his case how difficult, and indeed how impossible, it is to estimate
the true value of the facts furnished by the investigation of nature,
when we attempt to take up only the results themselves, without being
practised in the methods by which these are reached, _i.e._ without
being completely at home in one of the scientific subjects concerned
through one’s own investigations. It appears to me that the denial of
the purely mechanical value of the Darwinian factors of transformation
arises in most part from an erroneous classification of the scientific
facts with which we have to deal. There can certainly be no mistake
that the entire philosophical conception of the universe, as laid down
by Von Hartmann in his “Philosophy of the Unconscious,” is unfavourable
to an unprejudiced estimate of scientific facts and to their mechanical
valuation.

Variability, heredity, and above all correlation, would not be regarded
by Von Hartmann as purely mechanical principles, but he would therein
assume a metaphysical directive principle.

In the first place, as regards variability, Von Hartmann endeavours
to show that it is only a quite unlimited variability which suffices
for the explanation of necessary and useful adaptations by means
of selection and the struggle for existence. But this does not
exist--variation rather takes place in a fixed direction only (in
Askenasy’s sense), and this can be nothing else than the expression of
an innate law of development, _i.e._ a phyletic vital force.

This deduction appears to me in two ways erroneous. In the first place
it is incorrect that a quite unlimited variability is a postulate
of the theory of selection, and in the next place the admission of
variability, which is in a certain sense “fixed in direction,” does not
necessitate the assumption of a phyletic vital force.

A mere unsettled variability, uniform in all possible directions, is,
according to Von Hartmann, necessary for the theory of selection,
because only then does the variability offer a certain guarantee “that
under given conditions of life the variations necessary for complete
adaptation will not be wanting.” But it is hereby overlooked that the
new life conditions to which the adaptation must take place are as
little fixed and unchangeable as the organism itself. In such a case of
transformation we have not to deal with a type of organization which
was before fixed and immutable, and which has to be squeezed into new
life-conditions as into a mould. The adaptation is not one-sided, but
mutual; a species in some measure selects its new conditions of life,
corresponding with those possible to its organization, _i.e._ with the
variations actually occurring. I will choose an instance which will
even be conceded by Von Hartmann as being only explicable by natural
selection, viz., a case of mimicry.

Supposing that among the South American _Heliconiidæ_ there occurred
a species of _Pieris_ which had no resemblance to these protected
butterflies, either in form, marking, or colouring; who can deny
that it would be most useful to this species to acquire the form and
colouring of a Heliconide, and thus, by taking to new conditions of
life, to avoid the persecutions of its foes? But if the physical
nature of the Pieride concerned precluded the occurrence of Heliconoid
variations, would this incapability of insinuating itself into these
new conditions necessitate the decline of the species? Could not its
existence be secured in some other manner? could not the destruction of
numerous individuals by foes be compensated for by increased fertility?
to say nothing of the numerous other means through which the number
of surviving individuals might become increased, and the existence
of the species secured. This case is not arbitrarily chosen; in the
districts where the _Heliconiidæ_ occur there are actually a large
number of Whites which do not possess the protective colours of the
former nauseous family. In the adoption of these new life conditions
we have not to deal therefore with survival or extermination, but only
with amelioration. It is not every species of “White” that can become
adapted to these conditions, because every species does not give rise
to the necessary colour variations; those that do, become in this way
modified, because they are thus better protected than before. And so
it is throughout; wherever we find protected insects enjoying immunity
from foes we see also mimickers, sometimes only single, sometimes
several, and generally from very diverse groups of insects, according
to the general resemblance which existed before the commencement of
the process of adaptation, and to the variations made possible by the
physical nature of the species concerned.

In the first essay of the second part of this work it was shown that in
certain Lepidopterous larvæ a process of adaptation is at the present
time still in progress, this depending upon the fact that while the
young caterpillar is very well protected by the leaf-green colour of
its body, this colour becomes insufficient to conceal the insect as
soon as it exceeds the leaf in size. All such caterpillars--and there
is a whole series of species--as they increase in size acquire the
habit of concealing themselves on the earth by day, and of feeding
only at night. New conditions of life are thus imposed, and these
are even compulsory, _i.e._ they could not be abandoned without
risking the existence of the species. Now in accordance with these
new conditions, some individuals in these species have lost the green
colouring of the young stages, and have acquired the brown coloration
of the dark surroundings of the insects which conceal themselves
by day. In one species this change has now occurred in almost all
individuals, in others in only a larger or smaller proportion of
them. Now supposing that among these species there occurred one, the
physical nature of which did not admit of the production of brown
shades of colour, would the species for this reason succumb? Is it not
conceivable that the want of colour adaptation might be compensated
for by better concealment, _i.e._ by burrowing into the earth, or by
a greater fertility of the species, or by the development of warning
signals--supposing the species to be unpalatable--or finally, by the
acquisition of a terrifying marking? In other words, could not the
caterpillar itself modify the new condition of life--that of being
concealed by day--in accordance with variations made possible by its
physical nature?

As a matter of fact in one of these species the green colour remains
unchanged in spite of the altered mode of life, and this species,
wherever it occurs, notwithstanding the persecution of entomologists,
is always common (_Deilephila Hippophaës_); it conceals itself better
and deeper however than those other species which, like _Sphinx
Convolvuli_, are difficult to detect on account of their brown colour.
In another species the striking yellowish green colouring is likewise
retained in the majority of individuals, but this species buries
itself by day in the loose soil (_Acherontia Atropos_).

To this it may be objected that there are also compulsory changes in
the conditions of life from which the species cannot withdraw itself,
but in which adaptation must necessarily follow, or extermination would
take place.

Such compulsory conditions of life do most assuredly occur, and there
is indeed no doubt that many living forms have perished through not
becoming transformed. I believe, however, that such conditions occur
much more rarely than one is inclined to admit at first sight. As
a rule the alternative of immediate change or of extermination is
offered only by such changes in the conditions of life as occur very
rapidly. The sudden appearance of a new and dominant enemy, such as
man, has already caused the extinction of the Dodo (_Didus ineptus_),
and of Steller’s Sea Cow (_Rhytina Stelleri_), and of other vertebrate
animals, and constantly leads to the extermination of many other
species of different classes. When in America hundreds of thousands of
acres of primeval forest are annually destroyed, the conditions of life
of a numerous fauna and flora must be thereby suddenly changed, leaving
no choice but extermination.

Such abrupt changes in the conditions of life occur, however, but
seldom in nature unless caused by man, and must therefore have very
rarely happened in former epochs of the earth’s history. Even climatic
changes, which we might at first regard as of this character, and
which produce a modification in one fixed direction, occur always so
gradually that the species has time either to adapt itself to the
conditions in this or that direction, according to the variations
possible to its physical nature, or else to emigrate.

It thus appears to me erroneous to suppose that variability must be
“merely undetermined” in order to complete its part in Darwin’s theory
of selection, and its “illimitedness” seems to me also as little
necessary for this purpose. Von Hartmann imagines that it is only
unlimited variability that furnishes a guarantee that any type, to
whatever extent diverging from its point of departure, will be reached
by the Darwinian method of gradual transmutation by means of selection
and the struggle for existence.

But who has ever asserted that _any_ type can be reached from any
point? Or if anybody has said such nonsense, who can prove that its
admission is necessary for the theory of selection? Nowhere in systemy
do we see any point of support for such an assumption. But when Von
Hartmann imagines that the “unlimited” variability which he postulates
for Darwin “is in itself unlimited, the limits of its divergence in
a given direction being found, not in itself, but only in external
obstacles,” he conceives variability to be something independent of,
and in some way added to, the animal body, and not a mere expression
for the fluctuations in the type of the organism. If, however, we
conceive variability in this latter, the true scientific sense, it
is in no way “quantitatively unlimited,” nor are its limits even
determined by _external_ influences, but essentially by _internal_
influences, _i.e._ by the underlying physical nature of the organism.
Darwin has indeed already shown this in a most beautiful manner in his
investigations upon the correlations of organs and systems of organs
of the body. To make use of a metaphor, the forces acting within the
body are in equilibrium; if one organ becomes changed this causes a
disturbance in the forces, and the equilibrium must be restored by
changes in other parts, and these again entail other modifications,
and so forth. Herein lies the reason why the primary change cannot
exceed a certain amount if the restoration of the equilibrium is
not to be quite impossible. This is but a metaphor, and I do not
wish to assert that we are at present in a position to formulate and
demonstrate mathematically for any particular case, how much an organ
can become changed in any one species before an interruption of the
internal harmony of the body takes place. But such impossibility of
demonstration does not appear to me to furnish a sufficient reason
for regarding variability as the expression of a directive power--as
an “innate tendency to variation conformable to law.”[288] On the
contrary, it is to me easily conceivable that we only learn to analyse
the processes of nature in detail very slowly, because of their
necessary complexity. It thus appears to me quite useless when in this
sense Wigand makes use of the objection, that “the gooseberry has not
undergone any enlargement since 1852, although it is inconceivable
why it should not attain the size of a pumpkin if variability was
not internally limited.” It may well be that this is for the present
“inconceivable;” nevertheless, this does not justify us in setting
up a hypothetical “force of variation” which will not admit of the
gooseberry surpassing the pumpkin in size. We are bound to maintain
that it is the action and reaction of known forces which sets a limit
to the enlargement of this fruit.

In more simple instances the causes of such limitations to growth can
be well perceived. Several decades have passed since Leuckart proved
in how exact a relation the proportion of volume and surface stood to
the degree of organization of an animal. In animals of a spherical
form the surface is quite sufficient for respiration, so long as they
are of microscopic size. But such an organism cannot become enlarged
at pleasure, because the ratio of the surface to the volume would
become quite different. The surface increases as the square, whilst
the volume increases as the cube, so that very soon the surface of
the more rapidly increasing bodily mass can no longer suffice for
respiration.[289] This sort of limitation is in no way equivalent to
that purely external kind which, for instance, manifests itself in such
a manner as to prevent the indefinite lengthening of the tail feathers
of the Bird of Paradise. In this case feathers that were too long would
hinder flight, and such individuals would accordingly be eliminated by
natural selection. The cause is in the former case purely internal,
depending upon the equilibrium of the forces governing the organism.

Von Hartmann is entirely in the right when he asserts that variability
is neither qualitatively nor quantitatively unlimited. In both
senses it is limited (in direction as well as in amount) by the
physico-chemical forces acting in some contrary way in each specific
organism--by the physical nature of each living form. He errs,
however, both in making absolute illimitability a necessary postulate
of the theory of selection, as also in inferring the existence of
a directive principle from that limitation of variability which is
certainly present. “Tendencies to variation” do however exist, not in
the sense of a directive power, but as expressions of the different
physical constitutions of species, which necessarily cause unequal
reactions to the same external actions, as will be more clearly proved
below.[290]

This is, of course, a modification of Darwin’s original assumption of
an unbounded variability not limited in direction; but Darwin himself
has later coincided in the view that the quality of the variations is
essentially determined by the nature of the organism.[291]

I now turn to the consideration of the second factor of the theory of
selection--heredity. This also, according to Von Hartmann is not a
mechanical principle. Darwin himself has now become convinced how great
is the probability against the hereditary retention of modifications
which, whether feebly or strongly pronounced, appear _only in single
individuals_, _i.e._ of those so-called “fortuitous” variations which
are not the expression of a directive developmental principle. “But as
among the numberless possible directions of an indefinite variability,
useful modifications can only occur in single cases, Darwin has by this
supplementary admission himself retracted an inadmissible assumption of
his theory of selection,” and so forth. A “regular, designed tendency
to variation, acting from within and contemporaneously affecting a
large number of individuals,” must therefore be assumed “in order to
insure the by itself improbable inheritance.”

But even from the unbounded variability laid down by the author, it
by no means follows that useful variations can only occur in single
individuals. In the whole category of _quantitative_ variations the
reverse is always the case. Is it the lengthening of some part that
is concerned; so would a large number of individuals always possess
the useful variation, since we are not dealing with an _absolute_
enlargement, but only with the fact that the part concerned is longer
than in other individuals.[292]

But if _qualitative_ variations come into consideration, it may be
asked whether Darwin’s “supplementary admission” does not go too
far. Such calculations as those quoted by Darwin from the article in
the _North British Review_ of March 1867 are extremely deceptive,
since we have no means of measuring the amount of protection afforded
by a useful variation, and we can therefore hardly compute with
any certainty, in how great a percentage of individuals a change
must contemporaneously occur in order to have a chance of becoming
transferred to the following generation. If our blue rock-pigeon could
exist in a polar climate, and if we had the power of introducing it
gradually, but not suddenly, into these regions in a wild state, who
can doubt that it would assume the white colour of all polar animals?
Nevertheless, among wild rock-pigeons white varieties do not occur
more frequently than among swallows, crows, or magpies. Or must the
white colour of polar animals, the yellow colour of desert species, and
the green colour of leaf-frequenting forms, be always referred to a
“regular, designed, fixed tendency to variation acting from within,”
and causing a “large number of individuals” to vary in a similar manner?

There is, however, a grain of truth in the foregoing; variations which
occur singly have but little chance of becoming predominant characters,
and this is obviously what Darwin concedes. But this is by no means
equivalent to the assumption that only those variations which from the
first occur in numerous individuals have a chance of being perpetuated.
Let us keep to the facts. We have not the slightest reason either
for regarding the white colour of polar animals as the direct action
of cold, or for considering that the green colour of foliage-living
caterpillars depends upon direct action arising from the habit of
resting upon the leaves;[293] both these characters are explicable only
by natural selection, and there is nothing to favour the assumption
(which Von Hartmann postulates as necessary for success) that many
individuals varied into white at the same time. We know no single
extra-polar species of a dark colour which frequently, _i.e._ in many
individuals of every generation, varies into white, but we know many
species which from time to time produce single white individuals. Now
when, on the other hand, we find that all polar animals to which the
white coloration is advantageous, and indeed none but species of which
the nearest allies vary only individually into white, possess this
colour, must we not conclude from this alone that _single_ variations
can, under favourable conditions, become predominant characters?

It appears to me that in this question one weighty factor has been too
little regarded, even by the supporters of the selection theory, viz.,
the slowness of most, and especially of climatic changes, which I have
already insisted upon. If the transformation of a temperate into an
arctic climate occurred so rapidly that the species exposed to it had
the alternative either of becoming white in ten or twenty generations
or of being unable to exist, then the hasty intervention of a directive
power could alone save them from extermination by causing hundreds of
thousands of individuals to become similarly coloured with all speed.
But it is quite different if the change of climate takes place only in
the course of several thousand generations; and this, according to the
geological evidence, must have been the true state of the case.

Let us take a definite example--the well-known one of the hare. With
us this animal remains brown in the winter and but seldom produces
white varieties, whilst its ally the Alpine hare is white during
seven months of the year, the Norwegian hare during nine months, and
the Greenland hare throughout the whole year. If our climate became
transformed into an arctic one, after a given time there would arrive
a period when the older coloration no longer possessed any advantage
over the occasional and singly-appearing white variations; the winter
days during which the ground was covered with snow would have become so
numerous, that the protection afforded to the white animals would be
equal to the protection enjoyed by the brown individuals on the equally
numerous days free from snow. From this time forth the hares that
were white in winter would not be subjected to a greater decimation
by foxes, &c., than the brown individuals. This period must however
be represented as consisting of one or more centuries, and it would
be strange if from the individual white hares, which now had an equal
chance of existing, some white families did not become established.
But the state of affairs would gradually become reversed--the brown
hares would experience greater decimation, and wherever there were
white families these would possess an advantage in the struggle for
existence. It does not follow that the dark individuals would be
forthwith extirpated; on the contrary, the advantage in favour of the
white would be but small throughout a long period of time, and these
individuals would only gradually increase to a higher percentage of
the total population; nevertheless their numbers would constantly but
very slowly augment. In the course of time this increase would become
more rapid for two reasons--first, because even a very small advantage
in favour of the increasing number of individuals would always leave a
greater number of these victorious; and secondly, because on the whole
as the climate became more arctic, the advantage of being white would
continually become more decisive in determining which should live and
which should succumb.

Thus I see no reason why individual variations which do not appear only
once, but which frequently recur in the course of generations, should
not acquire predominance under favourable conditions. All facts are
in accord with this. Even the common hare shows us that it would be
quite capable of becoming coloured in a similar manner. In the museum
of Stuttgart there are three specimens of _Lepus timidus_, killed in
Wurtemburg, which are completely white, and several others which are
silver-grey or spotted with white. In eastern Russia the common hare
possesses a light grey, almost white, winter coat, and Seidlitz[294]
makes known the interesting observation that such light specimens occur
_singly_ in Livonia, where “the common hare has become naturalized
since the commencement of the century.”

As I have already insisted upon above, from the point of view
of the conditions of life there is no reason for assuming rapid
transformations; the change of conditions is almost always extremely
slow; and indeed in numerous instances no objective change occurs,
but simply a subjective one, if we may thus designate those cases in
which the alteration in the conditions of life depends upon a change
in the animal form which is undergoing transformation, and not in that
of the environment. This is the case in the above-mentioned instances
of mimicry, where the whole change in the conditions of life arises
from one species becoming similar to another. The process of natural
selection has here as long a period of time as it requires to perfect
its results. It is quite similar in all cases of special protective
adaptations of form and colour. In all these it is always _improvement_
that is concerned, and not the question “to be or not to be” with which
we have to deal.

It is just cases of this last kind, however, which are best fitted for
exposing the improbability and insufficiency of the assumption of a
variational tendency as a distinct directive power. We have only to
fix our attention upon some particular case of sympathetic colouring,
or, still better, of mimicry. A “tendency to variation” implies that a
large number of individuals produce varieties resembling the model to
be imitated, and this--at least according to Von Hartmann--must take
place in each of the successive generations, so that by this means,
combined with heredity, the useful variation becomes increased. But how
comes it that this “tendency to variation” coincides with the existence
of the model both in time and place? Can this be due to accident if
the two have not a common cause? The upholders of a directive power
will certainly not admit this; so that there remains only Leibnitz’s
assumption of a pre-established harmony contained in the first organic
germ, which, after innumerable transformations of the organic form
and after millions of years, gave rise in the midst of the Amazonian
region to an inedible Heliconide with certain yellow, black, and white
markings on the wings, and at precisely the same time developed the
tendency in a Pieride at the same spot on the globe to imitate this
Heliconide as a model!

In addition to this assumption, which is certainly but little worthy
of consideration, there is perhaps one other remaining, viz., that all
or many Pierides and other species of butterflies possessed the same
tendency to a Heliconoid variation and were always everywhere striving
to develop this type, but succeeded only where they accidentally
coincided in time and place with the model, the “tendency” being thus
furthered by natural selection. But the facts negative this assumption,
since such imitative variations have never been observed to a
perceptible extent in other species.[295]

All variations which are demonstrably useful can be similarly dealt
with if their origin is explained by variational tendencies.

We perceive that the objection which Von Hartmann brings against
heredity is only valid on the ground that this process affords no
security for the preservation of variations which occur singly. That
heredity itself is a mechanical process is not directly disputed; it is
simply assumed that new characters can be transferred by inheritance
only when they are produced by the metaphysical “developmental
principle,” and not when they arise “accidentally.” This critic does
not therefore direct his attack against heredity, but rather against
the mechanical origin of variability.

Von Hartmann might have said here that a reference of the phenomenon
of heredity to purely mechanical causes, _i.e._ a mechanical theory
of heredity, is up to the present time wanting. That he has not done
so proves on the one hand that he despised the dialectical art, but,
on the other hand, that he himself has not overlooked the subserviency
of the total phenomenon to law, and that he grants the possibility of
finding a mechanical explanation therefor. If, in fact, the power of
inheritance does not depend upon mechanical principles, I know not
what organic processes we are entitled to regard as mechanical, since
they are all dependent in essence upon heredity, with which process
they are at one, and from which they cannot be thought of as isolated.
Haeckel correctly designates reproduction as surplus individual
growth, and accordingly refers the phenomena of heredity to those of
growth. Conversely, growth may also be designated reproduction, since
it depends upon a continuous process of multiplication of the cells
composing the organism, from the germ-cell to the innumerable congeries
of variously differentiated cells of the highly developed animal body.
Who can fail to see that these two processes, the reproduction of the
germ-cell and its offspring in the economy of the individual, and the
reproduction of individuals and species in the economy of the organic
world, show an exact and by no means simply superficial analogy?[296]
But whoso grants this must also conceive both processes to depend upon
the same cause--he cannot assume for the one a causal power and for
the other a directive principle. If nutrition and cell-multiplication
are purely mechanical processes, so also is heredity. Although it
has not yet been possible to demonstrate the mechanism of this
phenomenon, it can nevertheless be seen broadly that by means of a
minimum of living organic matter (_e.g._ the protoplasm of the sperm
and germ-cell) certain motions are transferred, and these can be
regarded as directions of development, as I have already briefly laid
down in a former work.[297] The power of organisms to transmit their
properties to their offspring appears to me to be only conceivable in
such a manner “that the germ of the organism by its chemico-physical
composition together with its molecular structure, has communicated to
it a fixed direction of development--the same direction of development
as that originally possessed by the parental organism....” (_loc.
cit._ p. 24). This is confessedly nothing more than a hint, and we do
not learn therefrom the means by which developmental direction can be
possibly transferred to another organism.

Recently Haeckel, that indefatigable pioneer to whom we are indebted
for such a rich store of new ideas, has attempted to bridge over this
gap in his essay on “The Perigenesis of the Plastidule,” Berlin, 1876.
The basic idea, that heredity depends upon the transference of motion,
and variability upon a change of this motion, completely corresponds
with the conviction gained in the province of physical science, that
“all laws must finally be merged in laws of motion” (Helmholtz[298]).
I hold this view to be the more completely justifiable--although
certainly not in the remotest degree as proved--because I formerly
designated the acquired individual variations as the “diversion of the
inherited direction of development.” Haeckel’s hypothesis in so far
accomplishes more than Darwin’s pangenesis, in which a transference
of matter, and not of a species of motion peculiar to this matter, is
assumed. But although the germ of a mechanical theory of heredity may
be contained in Haeckel’s hypothesis, this nevertheless appears to me
to be somewhat remote from completely solving the problem. It brings
well into prominence one portion of the process of inheritance; under
the image of a molecular motion of the plastidule, which motion is
modifiable by external influences, we can well understand the fact of
a change gradually taking place in the course of generations. On the
other hand, the assumption of consciousness in the plastidule,--however
admissible philosophically--although only as a formula, scarcely
furnishes any deeper knowledge. In the light of a theory, detailed
instances which were formerly obscure should become comprehensible.
I fail to see, however, how the various forms of atavism, _e.g._ the
reversions which so commonly occur by crossing different races, become
more comprehensible by assuming consciousness in the plastidule. If
in both parents the plastidule long ago acquired different molecular
motions, why, in its rencounters in the germ, does it recollect past
times and reassume the older and long abandoned motion? That it does
acquire the latter is indeed a fact if we once refer the directional
development of the individual to molecular motion of the plastidule;
the wherefore does not appear to me, however, to become clearer by
assuming consciousness in the plastidule. A mechanical theory of
heredity must rather be able to show that the plastidule movements of
the male and female germ-cells, in their rencounter in the case of the
crossing of widely divergent forms, become mutually modified in such
a manner that the motion of the common ancestral form must occur as
the resultant. To such demonstration there is however as yet a long
step. Haeckel himself moreover points out that his hypothesis is by
no means a “mechanical theory of heredity,” but only an introduction
to this theory, which he hopes “will be capable of being elevated to
the rank of a genetic molecular theory” (_loc. cit._ p. 17). But
although we must also confess with the critic of the “Philosophy of the
Unconscious,” that “the facts of heredity have hitherto defied every
scientific explanation,”[299] this furnishes us with no excuse for
flying to a metaphysical explanation, “which is here certainly least
able to satisfy the inability to understand the connection arising from
natural laws.”

It is not to be wondered at that Von Hartmann, on the ground of the
“Unconscious” on which he takes his stand, speaks of the law of
correlation as an unconscious acknowledgment of a “non-mechanical
universal principle on the side of Darwinism.” By “correlation” he
understands something quite different to the idea which we attach to
this expression. He supposes that “Darwinism sees itself compelled
to acknowledge through empirical facts the uniform correlation of
characters pertaining to the specific type; but it thereby contradicts
its mechanical principles of explanation, all of which amount to the
same thing as conceiving the type as a mosaic, chequered, superficial,
and accidental aggregate of characters, which have been singly
acquired, contemporaneously or successively, by selection or habit.”
I do not believe, however, that any such conception has ever been
admitted either by Darwin or any one else. The admission that not all,
but only every deep-seated _physiological_ detailed modification, is
or may be bound up with a system of correlated changes, indeed implies
that we on our side also acknowledge an internal harmony of parts--an
equilibrium, as I have above expressed it.

But does this include the admission of a teleological principle, or
exclude a mechanical explanation? Do we thereby acknowledge a “specific
type” in the sense of an inseparably connected complex of characters,
none of which can be taken away without all the others becoming
modified? Does such a view agree generally with the empirical facts?

Neither of these views appears to me to represent the case.

I will first answer the second question. On all possible sides the
earlier view of the absolute nature of species is contradicted; there
is no boundary between species and varieties. But when Von Hartmann
assumes that by the transformation of one species “into another” the
“whole uniformly connected complex must become changed,” he falls
back into the old doctrine of the absolute nature of species, which
is sharply contradicted by multitudes of facts. We not unfrequently
observe varieties which differ from the parent-form by only a single
character, whilst others show numerous differences, and again others
may be seen in which the differences predominate. This last deviation
would then be designated by many systematists as a new species, but not
so by others.

The “specific type” is thus indeed a kind of mosaic-work, but it is
a structure to which all the single characters--the stones of the
mosaic--belong and build up one harmonious whole, and not a meaningless
confusion. Some of the stones or groups of stones can be taken away
and replaced by others differently coloured without the structure
being thereby necessarily distorted, _i.e._ destroyed as a structure;
but the larger the stones which are exchanged the more necessary will
corrections in the other parts of the structure become, in order that
the harmony of the whole may be preserved.

Still more weighty than those insensible transitions which in various
groups of animals so frequently connect species with species, appear
to me, however, the facts made known in the second essay of the
second part of this volume, which prove that the two forms in which
one species appears can change entirely independently of one another.
The caterpillar changes and becomes a new variety or even species
(according to the form-value of the change), whilst the butterfly
remains unaltered. How could this occur if some other law than that
of physiological equilibrium linked together the parts or characters
and permitted them to become severed? Must not the two stages become
changed with and through one another, like the parts of one body, since
they first together constitute the specific type? Is not the fact of
this not happening a proof that the whole “uniformly connected complex”
of the specific type is not bound and held together by a metaphysical
principle, but simply by natural laws?

Now when Von Hartmann comprises the relations of different species
to one another under the idea of correlation, such for instance as
the relation of dependence in which orchidaceous flowers stand with
respect to the insects which visit them, he completely abandons the
scientific conception which should be associated with this expression,
and compares together two heterogeneous things which have nothing in
common excepting that they are both considered by him as a result
of the “Unconscious.” The consequence which is then deduced from
this correlation of his own construction, viz., that an organic law
of correlation is only another expression for a “law of organic
development” in the sense of a metaphysical power, obviously cannot be
admitted.

By correlation we understand nothing more than the dependence of one
part of the organism upon the others and the mutual inter-relations
of these parts, which depend entirely upon a “physiological relation
of dependence,” as Von Hartmann himself has correctly designated it.
Herein is evidently comprised the total morphology of the organism--the
structure as a whole, the length, thickness and weight of the single
parts, as well as the histological structure of the tissues, since upon
all these depends the performance of the single parts. But when, under
correlation, Von Hartmann comprises “also a morphological, systematic,
inter-action of all the elements of the organism with reference both
to the typical ground-plan of the organization as well as to the
microscopic anatomical structure of the tissues,” he drags into the
idea something foreign to it, not on the ground of facts, but actually
in opposition to them, and supported only by a supposed “innate
developmental principle” which “is not of a mechanical nature.”

The living organism has already been often compared with a crystal, and
the comparison is, _mutatis mutandis_, justifiable. As in the growing
crystal the single molecules cannot become joined together at pleasure,
but only in a fixed manner, so are the parts of an organism governed
in their respective distribution. In the crystal where nothing but
homogeneous parts become grouped together their resulting combination
is likewise homogeneous, and it is obvious that they offer but very
little possibility of modification, so that the governing laws thus
appear restricted and immutable. In the organism, whether regarded
microscopically or macroscopically, various parts become combined, and
these therefore offer numerous possibilities of modification, so that
the governing laws are more complex, and appear less restricted and
unchangeable. In neither instance do we know the final causes which
always lead to a given state of equilibrium; in the case of a crystal
it has not occurred to anybody to ascribe the harmonious disposition
of the parts to a teleological power; why then should we assume such
a force in the organism, and thus discontinue the attempt, which has
already been commenced, to refer to its natural causes that harmony of
parts which is here certainly present and equally conformable to law?

On these grounds the assertion that the theory of selection is not an
attempt at a “mechanical” explanation of organic development appears to
me to be incorrect. Variability and heredity, as well as correlation,
admit of being conceived as purely mechanical, and must be thus
regarded so long as no more cogent reasons can be adduced for believing
that some force other than physico-chemical lies concealed therein.

But we certainly cannot remain at the purely empirical conception as
laid down by Darwin in his admirable work on the “Origin of Species.”
If the theory of selection is to furnish a method of mechanical
explanation, it is essential that its factors should be formulated in a
precise mechanical sense. But as soon as we attempt to do this it is
seen that, in the first enthusiasm over the newly discovered principle
of selection, the one factor of transformation contained in this
principle itself has been unduly pushed into the background, to make
way for the other more apparent and better known factors.

I have for many years insisted that the first, and perhaps most
important, or in any case the most indispensable, factor in every
transformation, is _the physical nature of the organism itself_.[300]

It would be an error to believe that it is entirely the external
conditions which determine what changes shall appear in a given
species; the nature of these changes depends essentially upon the
physical constitution of the species itself, and a modification
actually arising can obviously be only regarded as the resultant of
this constitution and of the external influences acting thereon.

But if an essential or perhaps even a preponderating share in
determining new characters is to be undoubtedly ascribed to the
organism itself, for a mechanical representation of organic
developmental processes everything depends upon our being able
to conceive this most important factor in a definite theoretical
manner, and to comprise under one common point of view its apparently
contradictory manifestations of constancy and variability.

Now every change of considerable extent is certainly considered by
Darwin to be the direct or indirect consequence of external actions;
but indirect action always presupposes a certain small variability
(individual variability), without which larger modifications cannot
be brought about. Empirically this small amount of variability is
doubtless present, but the question arises, upon what does it depend?
Can it be conceived as arising mechanically, or is it perhaps just at
this point that the metaphysical principle steps in and offers those
minute variations which make possible that course of development which,
according to this view, is immutably pre-determined? It is certainly
the absence of a theoretical definition of variability which always
leaves open a door for smuggling in a teleological power. A mechanical
explanation of variability must form the basis of this side of the
theory of selection.

This explanation is not difficult to find. All dissimilarities of
organisms must depend upon the individuals having been affected by
dissimilar external influences during the course of the development of
organic nature. If we ascribe to the organism the power of giving rise
by multiplication only to exact copies of itself, or, more correctly,
the power of transmitting unaltered to its successors the motion of its
own course of development, each “individual variation” must depend upon
the power of the organism to react upon external influences, _i.e._ to
respond by changes of form and of function, and consequently to modify
its original (inherited) developmental direction.

It has sometimes been insisted upon, that the “individuals of the same
species” or the offspring of one mother cannot be absolutely equal,
because, from the commencement of their existence, they have been
subjected to dissimilar actions of the environment. But this implies
that by perfectly equal influences they would become equal, _i.e._
it supposes that variability is not inseparably bound up with the
essence of the organism, but is only the consequence of developmental
tendencies which are in themselves equal being unequally influenced. As
a matter of fact the first germs of an individual certainly cannot be
supposed to be perfectly equal, because the individual differences of
the ancestors must be contained therein in different degrees according
to their constitution, and we should have to go back to the primordial
organism of the earth in order to find a perfectly homogeneous root,
a _tabula rasa_ from which the descendants would commence their
development. Whether such a homogeneous root ever existed is however
doubtful; it is much more probable that _numerous_ organisms first
arose spontaneously,[301] and these cannot be presumed to have been
absolutely equal, since the conditions under which they came into
life cannot have been perfectly identical. Let us, however, for the
sake of simplicity assume a single primordial organism; the first
generation which took its rise from this by reproduction could only
have possessed such individual differences as were produced by the
action of dissimilar external influences. But the third generation,
together with self-acquired, would also have shown _inherited_,
dissimilarities, and in each succeeding generation the number of
tendencies to individual difference imparted to the germ by heredity
must have increased to a certain degree, so that it may be said that
all germs, from their first origination, bear in themselves a tendency
to show individual peculiarities, and would develop these even if
they should not be again affected by dissimilar influences. This is
obviously the case, since the youngest egg-cells in the ovary of an
animal are, as can be demonstrated, always exposed to unequal external
conditions with respect to nutrition and pressure.[302] Hence, if it
were possible that two germs were exactly equal with respect to the
direction of development imparted to them by heredity, they would
nevertheless furnish two incongruent individuals; and if, conversely,
it were possible that two individuals could be exposed to absolutely
the same external influences from the formation of the embryo, these
also could not be identical, because the individual differences of the
ancestors would entail small differences, even in asexual reproduction,
in the direction of development transmitted to the egg. The differences
between individuals of similar origin thus finally depend entirely upon
the dissimilarity of external influences--on the one side upon those
which divert the development of the progenitors, and on the other side
upon those which divert the individual itself from its course, _i.e._
from the developmental direction transmitted hereditarily. Although
I thus essentially agree with Darwin and Haeckel in so far as these
authors refer the “universal individual dissimilarity” to dissimilar
external actions, I differ from Darwin in this, that I do not see an
essential distinction between the direct and indirect production of
individual differences, if by the latter is meant only the unequal
influencing of the germ in the parental organism. Haeckel is certainly
correct in referring the “primitive differences of the germs produced
by the parents” to the inequalities of nutrition to which the single
germs must inevitably have been exposed in the parent organism;
but another dissimilarity of the germs must evidently be added--a
dissimilarity which has nothing to do with unequal nutrition, but
which depends upon unequal inheritance of the individual differences
of the ancestors, a source of dissimilarity which must arise to a
greater extent in sexual than in asexual reproduction. Just as in
sexual propagation there occurs a blending of the characters (or
more precisely, developmental directions) of two _contemporaneous
individuals_ in _one_ germ, so in every mode of reproduction there meet
together in the same germ the characters of a whole _succession of
individuals_ (the ancestral series), of which the most remote certainly
make themselves but seldom felt in a marked degree.

The fact of individual variability can in this way be well understood;
the living organism contains in itself no principle of variability--it
is the _statical element_ in the developmental processes of the
organic world, and would always reproduce exact copies of itself if the
inequality of the external influences did not affect the developmental
course of each new individual; these influences are therefore the
_dynamical elements_ of the process.

From this conception of variability two important empirically
established facts can be theoretically deduced, viz. the limitability
of variation with respect to quality, which has already been previously
mentioned, and the origination of transformations by the direct action
of external conditions of life.

If the differences in individuals of the same origin depend upon
the action of unequal influences, variation itself is nothing else
than the reaction of the organism to a definite external inciting
cause, the quality of the variation being determined by the quality
of the inciting cause and by that of the organism. In the cases of
individual variation hitherto considered, the quality of the organism
is equal but that of the inciting cause is unequal, and in this way
there arise minute differences in organisms of an equal physical
constitution--variations of a different quality.

The same result, viz., different qualities of variation, may also arise
in a reverse manner by organisms of a different physical nature being
affected by equal external influences. The response of the organism to
the cause inciting change would be different according to its nature,
or, in other words, organisms of different natures react differently
when affected by equal modifying influences. The physical nature of
the organism plays the chief part with respect to the quality of the
variations; each specific organism can thus give rise to extremely
numerous, but not to all conceivable, variations; that is, only to
such variations as are made possible by its physical composition.
From this it follows further that the possibilities of variation in
two species are more widely different, the wider they diverge in
physical constitution (including bodily morphology)--that a cycle of
variation is peculiar to every species. In this manner we are led to
the knowledge that there must certainly exist a “fixed direction of
variation,” but not in the sense of Askenasy and Von Hartmann, as the
result of an unknown internal principle of development, but as the
necessary, _i.e._ mechanical, consequence of the unequal physical
nature of the species, which must respond even to the same inciting
cause by unequal variations.

The facts, as far as we know them, agree very well with this
conclusion. Allied species vary in a similar manner, whilst species
which are more distantly related vary in a different manner, even
when acted upon by the same external influences. Thus, in the first
part of these “Studies” I have remarked that many butterflies under
the influence of a warm climate acquire an almost black coloration
(_Polyommatus Phlæas_), whilst on the other hand others become lighter
(_Papilio Podalirius_).

We can thus understand why always certain courses of development are
followed, a fact which cannot be completely explained by the nature of
the conditions of life which induce the variations. But as soon as we
clearly perceive that the quality of the changes essentially depends
upon the physical nature of the organism itself, we arrive at the
conclusion that species of widely diverging constitutions must give
rise to different variations, whilst those of allied constitutions
would produce similar variations. But definite courses of development
are thus traced out, and we perceive that from any point of the organic
developmental series, it is impossible that any other point can be
attained at pleasure. Variation in a definite direction thus by no
means necessitates the acknowledgment of a metaphysical developmental
principle, but can be well conceived as the mechanical result of the
physical constitution of the organism.

The manner in which the dissimilar physical constitution of organisms
must arise can also be easily shown, although the first commencement of
the whole developmental series, _i.e._ the oldest living forms must be
assumed to have been almost homogeneous in their physical constitution.
The quality of the variation is, as said before, not merely the
product of the physical constitution, but the resultant of this and of
the quality of the changing external conditions. Thus from the first
“species” there proceeded, through the dissimilar influence of external
conditions of life, several new “species,” and as this took place the
former physical nature of the organism at the same time became changed,
necessitating also a new mode of reacting upon external influences,
_i.e._ another direction of variation. The difference from the primary
“species” must certainly be conceived as having been very minute, but
it must have increased with each new transformation, and must have
proceeded exactly parallel with the degree of physical change connected
with each transformation. Thus, hand in hand with the modifications,
the power of modification, or mode of reaction of the organism to
changing influences, must have continually become re-modified, and we
finally obtain an endless number of differently constituted living
forms, of which the variational tendencies are different in exact
proportion to their physical divergence, so that nearly allied forms
respond similarly, and widely divergent forms very differently, to the
same inciting causes.

Individual variation arises, as I have attempted to show, by each
individual having been continually affected by different, and indeed
by constantly changing, influences. Let us, however, imagine on the
contrary, that a large group of individuals is affected by the same
influences--in fact by such influences as the remaining individuals of
the species are not exposed to: this group of individuals would then
vary in a nearly similar manner, since both factors of variation, viz.
the external influence and the physical constitution, are equal or
nearly so. Such local variations would first become prominent when the
same external influence had acted upon a series of generations, and
the minima of variation produced in the individual by the once-exerted
action of the cause inciting change had become augmented by heredity.
Transformations of some importance (up to the form-value of species)
can thus arise simply by the direct action of the environment, in the
same way as that in which individual differences are produced--only
the latter fluctuate from generation to generation, since the inciting
influences continually change; whilst, in the former, the constant
external cause inciting modification always reproduces the same
variation, so that an accumulation of the latter can take place.
Climatic varieties can be thus explained.

A more efficacious augmentation of the variations arising in the single
individual is certainly brought about by the _indirect_ action of the
environment upon the organism. It is not here my intention to explain
once more the processes of natural selection; I mention this only in
order to point out that in these cases transformation depends upon a
_double action_ of the environment, since the latter first induces
small deviations in the organism by direct action, and then accumulates
by selection the variations thus produced.

By regarding variability in this manner--by considering each variation
as the reaction of the organism to an external action, as a diversion
of the inherited developmental direction, it follows that without a
change in the environment no advance in the development of organic
forms can take place. If we imagine that from any period in the earth’s
history the conditions of life remain completely unchanged, the species
present on the earth at this period would not, according to our view,
undergo any further modification. Herein is clearly expressed the
difference of this view from that other one according to which the
inciting principle of modification is not in the environment, but lies
in the organism itself in the form of a phyletic vital force.

I cannot here refrain from once more returning to the old (ontogenetic)
vital force of the natural philosophers, since the parallel between
this and its younger sister, the “phyletic vital force” which
appears in so many disguises, is indeed striking. Were the inciting
principle of the development of the individual actually an independent
vital force acting within the organism, the birth and growth of
the individual would be able to take place without the continuous
encroachment of the environment, such as occurs in nutrition and
respiration. Now this is known to be impossible, so that those who
support the existence of such a force, if any still exist, would be
driven to the obscure idea of a co-operation between the designing
power and the influences of the environment, just in the same manner
as such a co-operation is at present postulated by the defenders of
the phyletic vital force. I shall further on take the opportunity of
pointing out that this last idea is quite untenable; with respect to
the (ontogenetic) vital force any clearer proof cannot well be adduced,
but it will be admitted that the confused notion of the co-operation
and inter-action of teleological and causal powers is, from our
point of view, opposed to those very simple and clear ideas which
are in harmony with the views on phyletic development. As in racial
development each change of the organic type is entirely dependent upon
the action of the environment upon the organism, so in the development
of the individual, the totality of the phenomena of the personal life
must depend upon similar actions. Physiology, as is known, herein
entirely supports our view, since this shows that without the continual
alternating action of the environment and of the organism there can be
no life, and that vital phenomena are nothing but the reactions of the
organism to the influences of the environment.

It will be immediately perceived how exactly the processes of phyletic
and of ontogenetic development coincide, not merely in their external
phenomena but in their nature, if we trace the consequences of the
existing knowledge of the structure of the animal body. Although we
may not entirely agree with Haeckel’s doctrine of individuality in
its details, its correctness must on the whole be conceded, since it
cannot be disputed that the notion of individuality is a relative one,
and that several categories of morphological individuals exist, which
appear not only _singly_ as _physiological individuals_, _i.e._ as
independent living beings of lowest grade, but which can also _combine_
to form beings of a higher order.

But if we admit this, we should see with Haeckel nothing but
reproduction in the origination of a high organism from a single cell,
the egg; this reproduction being at the same time combined with various
differentiations of the offspring, _i.e._ with adaptations of the
latter to various conditions of life. Not even in the fact that the
tissues and organs of a single physiological individual stand in great
dependence upon one another through physical causes,[303] is there any
striking difference between this view and the phyletic composition of
the animal (and vegetable) kingdom out of physiological individuals
(Haeckel’s “_Bionten_”), since contemporaneous animals (individuals and
species) are known to influence one another in the most active manner.

Now if we further consider that the same units (cells) which, by
their reproduction and division of labour, at present compose the
body of the highest organism, must at one time have constituted as
independent beings the beginning of the whole of organic creation, and
that consequently the same processes (division of cells) which now
lead to the formation of a mammal, at that time led only to a long
series of different independent beings, it will be admitted that both
developmental series must depend upon the same inciting powers, and
that with reference to the causes of the phenomena it is not possible
that any great gap can exist between ontogeny and phylogeny, _i.e._
between the life-phenomena of the individual and those of the type.
According to our view both depend upon that co-operation of the same
material physical forces which admits of being briefly summarized
as the reaction of organized living matter to influences of the
environment.

Our opponents either cannot boast of such harmony in their conception
of nature, or else they must, together with the phyletic vital force,
re-admit into their theory the old ontogenetic vital force. I know not
indeed why they should not do so. Whoever inclines to the view that
organic nature is governed not merely by causal, but at the same time
by teleological, forces, may admit that the latter are as effective as
inciting causes of individual, as they are of phyletic, development.
According to my idea they are even bound to admit this, since it cannot
be perceived why the adaptations of the ontogeny should not depend upon
the same metaphysical principle assumed for each individual, as the
adaptations of the phylogeny; the latter are indeed only brought about
by the former. I believe therefore that the vital force (ontogenetic)
of the ancients stands or falls with the modern (phyletic) vital force.
We must admit both or neither, since they both rest on the same basis,
and are supported or opposed by the same arguments. Whoever feels
justified in setting up a metaphysical principle where complete proof
that _known forces_ are sufficient for the explanation of the phenomena
has not yet been adduced, must do the same with respect to individual,
as he does to phyletic, development, since this proof is in both cases
very far from being complete, and still contains large and numerous
gaps.[304]

The theoretical conception of variation as the reaction of the organism
to external influences has also not yet been experimentally shown to
be correct. Our experiments are still too coarse as compared with the
fine distinctions which separate one individual from another; and
the difficulty of obtaining clear results is greatly increased by
the circumstance that a portion of the individual deviations always
depends upon heredity, so that it is frequently not only difficult,
but absolutely impossible, to separate those which are inherited from
those which are acquired. Still further are we removed from being
able to refer variation to its final mechanical causes, _i.e._ from a
mechanical theory of reproduction, which would bring within the range
of mathematical calculation both the phenomena of stability (heredity)
and of change (variability).

But although sufficient proofs of the correctness of the views
here advocated cannot at present be adduced, these views are not
contradicted by any known facts--they are, on the contrary, supported
by many facts which they in turn make comprehensible (local forms,
different cycles of variation in heterogeneous species). These views
are finally completely justified by their furnishing the only possible
theoretical formulation of variability on which a mechanical conception
of organic development can be based. That such a conception is not only
admissible, but is unavoidable, at least to the naturalist, I have
already attempted to prove.



II.

MECHANISM AND TELEOLOGY.


In the third volume of his smaller works Karl Ernst von Baer submits
the theory of selection to a most searching examination. Without
actually calling in question its scientific admissibility, he believes
that this theory is dependent upon its satisfying one condition, viz.
that it should connect the teleological with the mechanical principle.

“The Darwinian hypothesis, as stated by its supporters, always ends in
denying to the processes of nature any relation to a future, _i.e._ any
relation of aim or design. Since such relations appear to me _quite
evident_,” &c. And further:--“If the scientific correctness of the
Darwinian hypothesis is to be admitted, it must accommodate itself to
this universal striving after a purpose. If it cannot do this we should
have to deny its value.”

These words appear almost equivalent to passing a sentence of doom
upon the theory of selection and the mechanical conception of nature,
for how can one and the same process be effected simultaneously by
necessity and by designing powers? The one excludes the other, and we
must--so it appears--take our stand either on one side or the other.

Nevertheless we cannot set aside Von Baer’s proposition without
further examination simply because it is apparently incapable of being
fulfilled, since it contains a truth which should not be overlooked,
even by those who uphold the mechanical theory of nature. It is the
same truth which is also made use of by the philosophical opponents of
this theory, viz. that the universe as a whole cannot be conceived as
having arisen from blind necessity--that the endless harmony revealed
in every nook and corner by all the phenomena of organic and of
inorganic nature cannot possibly be regarded as the work of chance,
but rather as the result of a “vast designed process of development.”
It is also quite correct when, in reply to the supposed objection
that the mechanical theory of nature is not concerned with chances
but with necessities, Von Baer answers that the operations of a
series of necessities which “are not connected together” can only be
termed accidents in their opposing relations. He illustrates this by
instancing a target. If I hit the latter by a well-aimed shot, nobody
would explain this as the result of an accident, but if “a horseman is
riding along a gravelly road past this target, and one of the pebbles
thrown up by the hoof of the galloping horse hits the mark, this would
be termed an accident of extremely rare occurrence. My target was not
the mark for the pebble, therefore the hit was purely accidental,
although the projection of the stone in this precise direction with
the velocity which it had acquired, was sufficiently explained by
the kick given by the horse. But the hit was accidental because the
kick of the galloping horse, although it necessarily projected the
pebble, had no relation at all to my target. For the same reason we
must regard the universe as an immense accident if the forces which
move it are not designedly regulated--the more immense because it is
not a single motion of projection that acts here, but a large number
of heterogeneous powers, _i.e._ a large number of variously acting
necessities which are, as a whole, devoid of purpose, but which
nevertheless accomplish this purpose, not only at any single moment,
but constantly. A truly admirable series of desirable accidents!”[305]

The same idea is expressed, although in a very different manner, by
Von Hartmann, in the concluding chapter of his work already quoted.
He thinks that “design is a necessary and certain consequence of the
mechanical laws of nature.” “Were the mechanism of natural laws not
teleological there would be no mechanically regulated laws, but a weak
chaos of obstinate and capricious powers. Not until the causality of
the laws of inorganic nature had superseded the expression “dead”
nature, and had shown itself as the mainspring of life and of a
conformability to design visible on all sides, did it deserve the
name of mechanical lawfulness; just as a complication of wheels and
machinery made by man, which move in some definite manner with respect
to one another, only acquires the name of a mechanism or of a machine
when the immanent teleology of the combination and of the various
movements of the parts is revealed.”[306]

Against the correctness of the idea underlying these statements
scarcely anything can in my opinion be said. The harmony of the
universe and of that portion of it which we designate organic nature,
cannot be explained by chance, _i.e._ without a common ground for
co-operating necessities; by the side of mere mechanism it is
impossible not to acknowledge a teleological principle--the only
question is, in what manner can we conceive this as acting without
abandoning the purely mechanical conception of nature?

This is obviously effected if, with Von Baer and Von Hartmann, we
permit the metaphysical principle to interrupt the course of the
mechanism of nature, and if we consider both the former and the latter
to work together with equal power. Von Hartmann expressly makes such an
admission under the name of an “internal principle of development,” to
which he attributes such an important share that one cannot understand
why it should have any need for the employment of causal powers, and
why it does not simply do everything itself. Von Baer expresses himself
much less decisively, and even in many places insists upon the purely
mechanical connection of organic natural phenomena; but that with him
also the idea of interruption by a metaphysical principle is present,
is principally shown by his assuming, at least partly, the _per saltum_
development of species. This necessarily involves an actively internal
power of development.

Although I have already brought forward many arguments against the
existence of such a power, and although in refuting it every form of
development by directive powers is at the same time overthrown, it
nevertheless appears to me not to be superfluous in such a deeply
important question to show that a _per saltum_ development, and
especially the so-called heterogeneous generation, is inconceivable,
not only on the ground of the arguments formerly employed against the
phyletic vital force in general, but quite independently of these.

In the first place it must be said that the positive basis of this
hypothesis is insecure. Cases of sudden transformation of the whole
organism with subsequent inheritance are as yet quite unknown. It has
been shown that the occasional transformation of the Axolotl must most
probably be regarded in a different light. Another case, taken for
heterogeneous generation, viz. the budding of twelve-rayed Medusæ in
the gastric cavity of an eight-rayed species, has lately been shown by
Franz Eilhard Schulze[307] to be a kind of parasitism or commensalism.
The buds of the _Cuninæ_ do not spring, as was supposed, from the
_Geryonia_, but are developed from a _Cunina_ egg. But even if we
recall here the cases of alternation of generation and heterogenesis,
this would not be of any value by way of proof; it would only be thus
indicated how one might picture to oneself a sudden transformation.
That in alternation of generation, or generally, in every mode of
cyclical reproduction, we have not to deal with the abandonment of
one type of organization and the transition to some other, is proved
by the continual return to the type of departure--by the cyclical
character of the entire transformation. That two quite heterogeneous
types can belong to one cycle of development is, however, capable
of a far better and more correct explanation than would be given
by the supporters of _per saltum_ development. If we trace cyclical
reproduction to the adaptation of different developmental stages
or generations to deviating conditions of life, we thus not only
explain the exact and often striking agreement between form and mode
of life--we not only bridge over the gap between metamorphosis and
alternation of generation, but we can also understand how, within one
and the same family of Hydrozoa, species can occur with or without
alternation of generation, and further how other species can exist in
which the alternation of generation (the production of free Medusæ) is
limited to the one sex; we can understand in general how one continuous
series of forms may lead from the simple sexual organ of the Polypes
to the independent and free swimming sexual form of the Medusæ, and
how hand in hand with this the simple reproduction becomes gradually
cyclical. It is just these intermediate steps between the two kinds of
reproduction that make quite untenable the idea that the heterogeneous
forms in cyclical propagation arise through so-called “heterogeneous
generation,” _i.e._ through sudden _per saltum_ transformation. It
is excusable if philosophers to whom these facts are strange, or who
have to take the trouble of working them up, should adduce alternation
of generation as an instance of “heterogeneous generation,” but by
naturalists this should be once and for ever abandoned.

All other facts which have hitherto been referred to “heterogeneous
generation” are still less explicable as such, inasmuch as they always
relate to changes in single parts of an organism, such as the sudden
change of fruit or flower in cultivated plants. The notion of _per
saltum_ development, however, demands a total transformation--it
comprises (as Von Hartmann quite correctly and logically admits) the
idea of a _fixed specific type_ which can only be re-modelled _as a
whole_, and cannot become modified piecemeal. It must further be added,
that the observed variations which have arisen abruptly in single parts
are not as a rule inherited:[308] fruit-trees are only propagated by
grafting, _i.e._ by perpetuating the individual, and not by ordinary
reproduction by seeds. Now, if we nowhere see sudden variations of
large amount perpetuated by heredity, whilst we everywhere observe
small variations which can all be inherited, must it not be concluded
that _per saltum_ modification is not the means which Nature
employs in transforming species, but that an accumulation of small
variations takes place, these leading in time to large differences?
Is it logical to reject the latter conclusion because our period of
observation is too brief to enable us to directly follow long series
of accumulations, whilst _per saltum_ variation is admitted, although
unsupported by a single observation? As long as there remains any
prospect of tracing large deviations to the continually observed
phenomenon of small variations, I believe we have no right to resort to
the purely hypothetical explanation afforded by _per saltum_ variations.

But the hypothesis of “heterogeneous generation” is not only without a
basis of facts--it can also be directly shown to be untenable. Since
the operation of an internal power of transformation does not explain
adaptation to the conditions of life, the claims of natural selection
to explain these transformations must be admitted; but the co-operation
of a phyletic vital force and natural selection is inconceivable if we
imagine the modifications to occur _per saltum_.

The supposed “heterogeneous generation” is always illustrated by the
example of alternation of generation. The origination of a new animal
form is thus conceived to take place in the same manner as we now see,
in the cyclical reproduction of the Medusæ, free swimming, bell-shaped
Medusoids, produced from fixed polypites, or _Cercariæ_ from Trematode
worms by internal budding; in brief, it is imagined that one animal
form suddenly gives rise to another widely deviating form by purely
internal causes. Now on this theory it would be an unavoidable
postulate, that by such a process of _per saltum_ development there
arises not merely a new type of some species, but at the same time
individuals capable of living and of persisting under, and fitted to,
given conditions of life. But every naturalist who has attempted to
completely explain the relation between structure and mode of life
knows that even the small differences which separate one species from
another, always comprise a number of minute structural deviations which
are related to well defined conditions of life--he knows that in every
species of animal the whole structure is adapted in the most exact
manner _in every detail_ to special conditions of life. It is not an
exaggeration when I say in every detail, since the so-called “purely
morphological parts” could not be other than they are without causing
changes in other parts which exercise a definite function. I will not
indeed assert that in the most closely related species all the parts
of the body must in some manner differ from one another, if only to a
small extent; it seems to me not improbable, however, that an exact
comparison would very frequently give this result. That animals which
are so widely removed in their morphological relations as Medusæ and
Polypes, or Trematoda and their “nurses,” are differently constructed
in each of their parts can, however, be stated with certainty.

Now if this wide deviation in every part were in itself no obstacle to
the assumption of a designing and re-modelling power, it would become
so by the circumstance that all the parts of the organism must stand in
the most precise relation to the external conditions of life, if the
organism is to be capable of existing--all the parts must be exactly
adapted to certain conditions of life. How can this be brought about
by a transforming force acting spasmodically? Von Hartmann--who, in
spite of his clear perception and widely extended scientific knowledge,
cannot possibly possess a strong conviction of that harmony between
structure and life-conditions prevailing throughout the whole system
of the organism, and which personal research and contemplation are
alone able to give--simply bridges over the difficulty by permitting
natural selection to come to his aid as an “auxiliary principle” of
the re-modelling power. It would not be supposed that naturalists
would resort to the same device--nevertheless those who support the
phyletic force and _per saltum_ development generally invoke natural
selection as the principle which governs adaptation. But when does
this agency come into operation? When by germinal metamorphosis a
new form has arisen, this, from the first moment of its existence,
must be adapted to the new conditions of life or it must perish. No
time is allowed for it to continue in an unadapted state throughout
a series of generations until adaptation is luckily reached through
natural selection. Let us have either natural selection or a phyletic
force--both together are inconceivable. If there exists a phyletic
force, then it must itself bring about adaptation.

It might perhaps be here suggested that the same objection applies to
that process of modification which is effected by small steps, but that
it does so only when the change occurs suddenly. This, however, as I
have already attempted to show, but very rarely takes place; in many
cases (mimicry) the conditions even change in the first place through
the change in form and therefore, as is evident, as gradually as the
latter. It must be the same in all other cases where transformation of
the existing form and not merely extinction of the species concerned
takes place. The transmutation must always keep pace with the change
in the conditions of life, since if the latter change more rapidly the
species could not compete with rival species--it would become extinct.

The abrupt transformation of species implies sudden change in the
conditions of life, since a Medusa does not live like a Polype,
nor a Trematode like its “nurse.” For this reason it is impossible
that natural selection can be an aiding principle of “heterogeneous
generation.” If such abrupt transformation takes place it must produce
the new form instantly equipped for the struggle for existence,
and adapted in all its organs and systems of organs to the special
conditions of its new life. But would not this be “pure magic”? It
is not thereby even taken into consideration that here--as in the
cases of mimicry--time and place must agree. The requirements of a
pre-established harmony (“_prästabilirte Harmonie_”) further demand
that an animal fitted for special conditions of life should only make
its appearance at that precise period of the earth’s history when these
special conditions are all fulfilled, and so forth.

But he who has learnt to perceive the numerous and fine relations
which, in every species of animal, bring the details of structure into
harmony with function, and who keeps in view the impelling power of
these conditions, cannot possibly hold to the idea of a _per saltum_
development of animal forms. If development has taken place, it must
have occurred gradually and by minute steps--in such a manner indeed
that each modification had time to become equilibrated to the other
parts, and in this way a succession of modifications gradually brought
about the total transformation of the organism, and at the same time
secured complete adaptation to new conditions of life.

Not only abrupt modification however, but every transformation is
to be rejected when based upon the interference of a metaphysical
principle of development. Those to whom the arguments already advanced
against such a principle appear insufficient may once more be asked,
how and where should this principle properly interfere? I am of
opinion that one effect can have but one sufficient cause; if this
suffices to produce it, no second cause is required. The hand of a
watch necessarily turns once round in a circle in a given time as soon
as the spring which sets the mechanism in movement is wound up; in an
unwound watch a skilful finger can perhaps give the same movement to
the hand, but it is impossible that the latter can receive both from
the operator and from the spring at _the same time, the same motion_ as
that which it would receive through either of these two powers _alone_.
In the same manner it appears to me that the variations which lead to
transformation cannot be at the same time determined by physical and by
metaphysical causes, but must depend upon either one or the other.

On no side will it be disputed that at least one portion of the
processes of organic life depends upon the mechanical co-operation of
physical forces. How is it conceivable that sudden pauses should occur
in the course of these causal forces, and that a directive power should
be substituted therefor, the latter subsequently making way again for
the physical forces? To me this is as inconceivable as the idea that
lightning is the electric discharge of a thunder-cloud, of which the
formation and electrical tension depends upon causal forces, and of
which the time and place are purely determined by such forces, but that
Jupiter has it nevertheless in his power to direct the lightning flash
according to his will on to the head of the guilty.

Now although I deny the possibility or conceivability of the
contemporaneous co-operation of teleological and of causal forces
in producing any effect, and although I maintain that a purely
mechanical conception of the processes of nature is alone justifiable,
I nevertheless believe that there is no occasion for this reason to
renounce the existence of, or to disown, a directive power; only we
must not imagine this to interfere directly in the mechanism of the
universe, but to be rather behind the latter as the final cause of this
mechanism.

Von Baer himself points this out to us, although he does not follow
up the complete consequences of his arguments. He especially insists
in his book, which abounds in beautiful and grand ideas, that the
notions of necessity (causality) and of purpose by no means necessarily
exclude one another, but rather that they can be connected together in
a certain manner. Thus, the watchmaker attains his end, the watch, by
combining the elastic force of a spring with wheel-work, _i.e._ by
utilizing physical necessities; the farmer accomplishes his purpose,
that of obtaining a crop of corn, by sowing the seed in suitable land,
but the seed must germinate as an absolute necessity when exposed to
the influences of warmth, soil, moisture, &c. Thus, in these instances
a chain of necessities is undoubtedly connected with a teleological
force, the human will; and it directly follows from such cases that
wherever we see an aim or result attained through necessities, the
directive force does not interrupt the course of the series of
necessities which have already commenced, but is active before the
first commencement of these necessities, since it combines and sets the
latter in movement. From the moment when the mechanism of the watch
is combined harmoniously and the spring wound up, it goes without the
further interference of the watchmaker, just as the corn-seed when once
placed in the earth develops into a plant without assistance from the
farmer.

If we apply this argument to the development of the organic world,
those who defend mechanical development will not be compelled to deny a
teleological power, only they would have with Kant[309] to think of the
latter in the only way in which it can be conceived, viz. _as a Final
Cause_.

In the region of inorganic nature nobody any longer doubts the purely
mechanical connection of the phenomena. Sunshine and rain do not now
appear to us to be whims of a deity, but divine natural laws. As the
knowledge of the processes of nature advances, the point where the
divine power designedly interrupts these processes must be removed
further back; or, as the author of the criticism of the philosophy
of the Unconscious[310] expresses it, all advance in the knowledge
of natural processes depends “upon the continual elimination of the
idea of the miraculous.” We now believe that organic nature must be
conceived as mechanical. But does it thereby follow that we must
totally deny a final Universal Cause? Certainly not; it would be a
great delusion if any one were to believe that he had arrived at a
comprehension of the universe by tracing the phenomena of nature to
mechanical principles. He would thereby forget that the assumption
of eternal matter with its eternal laws by no means satisfies our
intellectual need for causality. We require before everything an
explanation of the fact that relationships everywhere exist between the
parts of the universe--that atoms everywhere act upon one another.[311]
He who can content himself with the assumption of matter may do so,
but he will not be able to show that the assumption of a Universal
Cause underlying the laws of nature is erroneous.

It will not be said that there is no advantage in assuming such a
Final Cause, because we cannot conceive it, and indeed cannot so much
as demonstrate it with certainty. It certainly lies beyond our power
of conception, in the obscure region of metaphysics, and all attempts
to approach it have never led to anything but an image or a formula.
Nevertheless there is an advance in knowledge in the assumption of this
Cause which well admits of comparison with those advances which have
been led to by certain results of the new physiology of the senses. We
now know that the images which give us our sense of the external world
are not “actual representations having any degree of resemblance,”[312]
but are only signs for certain qualities of the outer world, which
do not exist as such in the latter, but belong entirely to our
consciousness. Thus we know for certain that the world is not as we
perceive it--that we cannot perceive “things in their essence”--and
that the reality will always remain transcendental to us. But who will
deny that in this knowledge there is a considerable advance, in spite
of its being for the most part of a negative character? But just as we
must assume behind the phenomenal world of our senses an actual world
of the true nature of which we receive only an incomplete knowledge
(_i.e._ a knowledge corresponding only in reality with the relations of
time and space), so behind the co-operating forces of nature which “aim
at a purpose” must we admit a Cause, which is no less inconceivable
in its nature, and of which we can only say one thing with certainty,
viz., that it must be teleological. Just as the former first leads
us to perceive the true value of our sensual impressions, so does
the latter knowledge lead us to foresee the true significance of the
mechanism of the universe.

It is true that in neither case do we learn more than that there is
something present which we do not perceive, but in both instances this
knowledge is of the greatest value. The consciousness that behind
that mechanism of the universe which is alone comprehensible to us
there still lies an incomprehensible teleological Universal Cause,
necessitates quite a different conception of the universe--a conception
absolutely opposed to that of the materialist. Most correctly and
beautifully does Von Baer say that “a purpose cannot be otherwise
conceived by us than as proceeding from a will and consciousness; in
this would the ‘aiming at a purpose,’ which appears to us as reasonable
as it is necessary, have its deepest root.” If we conceive in this
world a divine Universal Power exercising volition as the ultimate
basis of matter and of the natural laws resident therein, we thus
reconcile the apparent contradiction between the mechanical conception
and teleology. In the same way that Von Hartmann, somewhere speaks of
the immanent teleology of a machine, we might speak of the immanent
teleology of the universe, because the single forces of matter are
so exactly adjusted that they must give rise to the projected world,
just as the wheels and levers of a machine bring forth a required
manufactured article. I admit that these are grossly anthropomorphic
ideas. But as mortals can we have any other ideas? Is not the notion
of purpose in itself an equally anthropomorphic one? and is there
any certainty that the idea of causality is less so? Do we know that
causality is unlimited, or that it is universally valid? In the
absence of this knowledge, should it not be permissible to satisfy
as far as we can the craving of the human mind for a spiritual First
Cause of the universe, by speaking of it in terms conceivable to
human understanding? We can take up such a final position and still
be conscious that we thereby form no certain conception, and indeed
come no nearer to the reality. The materialist still makes use of the
notion of “eternity,” and frequently handles it as though it were a
perfectly known quantity. We nevertheless do not seriously believe that
by the expression “eternal matter,” any true idea resulting from human
experience is gained.

If it is asked, however, how that which in ourselves and in the
remainder of the animal world is _intellectual_ and _perceptive_, which
_thinks_ and _wills_, is ascribable to a mechanical process of organic
development--whether the development of the mind can be conceived as
resulting from purely mechanical laws? I answer unhesitatingly in the
affirmative with the pure materialist, although I do not agree with
him as to the manner in which he derives these phenomena from matter,
since thinking and extension are heterogeneous things, and one cannot
be considered as a product of the other. But why should not the ancient
notion of “conscious matter” given out by Maupertuis and Robinet, not
be again entertained, as pointed out in recent times by Fechner?[313]
Would there not thus be found a useful formula for explaining phenomena
hitherto quite incomprehensible?

Von Hartmann in criticizing himself, designates the sensibility of
atoms as an “almost inevitable hypothesis” (p. 62), “inevitable
because if sensibility were not a general and original property of the
constituent elements of matter, it would be absolutely incomprehensible
how through its potentiality and integration that sensibility known
to us as being possessed by the organism could have arisen.” “It is
impossible that from purely external elements devoid of all internality
(_Innerlichkeit_) there should suddenly appear, by a certain mode
of combination, an internality which becomes more and more richly
developed. The more certainly science becomes convinced that in
the sphere of externality (_Äusserlichkeit_) the higher (organic)
phenomena are only results of combination, or are the aggregate
phenomena of the elementary atomic forces, the more surely, when she
once seriously concerns herself with this other question, will she not
fail to be convinced that the sensibility possessed by higher stages
of consciousness can be only combination-results, or the aggregate
phenomena of the elementary sensations of atoms, although these
atomic sensations as such always remain below the level of the higher
combinations of consciousness.” In confusing this double-sided nature
of the objective phenomenon “lies the main error of all materialism
and of all subjective idealism. Just as the attempt of the latter
(subjective idealism) to construct the external phenomena of existence
in space out of functions of internality and their combinations is
impossible, so is the endeavour of the former (materialism) to build up
internal sensation out of any combinations of force acting externally
in space equally impossible.”

I have no intention of going any deeper into these questions. I mention
them only in order to point out that even from this side there appears
to me no obstacle in the way of a purely mechanical conception of the
processes of the universe. The naturalist may be excused if he attempts
to penetrate into the region of philosophy; it arises from the wish to
be able to contribute a little towards the reconciliation of the latest
knowledge of the naturalist with the religious wants of the human
mind--towards the aim striven for by both sides, viz. a satisfactory
and harmonious view of the universe, according with the state of
knowledge of our time.

I believe that I have shown that the theory of selection by no means
leads--as is always assumed--to the denial of a teleological Universal
Cause and to materialism, and I thereby hope that I have cleared the
way for this doctrine, the importance of which it is scarcely possible
to over-estimate. Many, and not the most ill-informed, do not get so
far as to make an unbiassed examination into the facts, because they
are at the outset alarmed by the to them inevitable consequence of the
materialistic conception of the universe. Mechanism and teleology do
not exclude one another, they are rather in mutual agreement. Without
teleology there would be no mechanism, but only a confusion of crude
forces; and without mechanism there would be no teleology, for how
could the latter otherwise effect its purpose?[314]

Von Hartmann correctly says:--“The most complete mechanism conceivable
is likewise the most completely conceivable teleology.” We may thus
represent the phenomenal universe as such a completely conceivable
mechanism. With this conception vanish all apprehensions that the new
views would cause man to lose the best that he possesses--morality and
purely human spiritual culture. He who, with Von Baer, considers the
laws of nature as the “permanent expressions of the will of a creative
principle,” will clearly perceive that a further advance in the
knowledge of these laws need not divert man from the path of increasing
improvement, but must further him in this course--that the knowledge
of truth, whatever may be its purport, cannot possibly be considered
a backward step. Let us take our stand boldly on the ground of new
knowledge, and accept the direct consequences thereof, and we shall
not be obliged to give up either morality or the comforting conviction
of being part of an harmonious world, as a necessary member capable of
development and perfection.

Any other mode of interference by a directive teleological power in
the processes of the universe than by the appointment of the forces
producing them, is however, at least to the naturalist, inadmissible.
We are still far removed from completely understanding the mechanism by
means of which the organic world is evoked--we still find ourselves at
the very beginning of knowledge. We are, however, already convinced
that both the organic and the inorganic worlds are dependent only upon
mechanical forces, for to this conclusion we are led, not only by the
results of investigators who have restricted themselves to limited
provinces, but also by the most general considerations. But although
the force of these arguments may not be acknowledged, and although
one might maintain that the inductional proofs against the existence
of a “phyletic vital force” have been directed only against points of
detail, or have never been completely demonstrated, _i.e._ for all
points, it must nevertheless be conceded, that for the naturalist
the mechanical conception of Nature is the only one possible--that
he is not at all justified in abandoning this view so long as the
interference of teleological forces _in the course_ of the processes
of organic development has not been demonstrated to him. Thus, it will
not be immaterial whether a conception of Nature which to many seems
inevitable is consistent with the idea of universal design, or a final
directive universal principle, since the value which we may attach to
our own lives and aims, essentially depends thereon. The final and main
result of this essay will thus be found in the attempted demonstration
that the mechanical conception of Nature very well admits of being
united with a teleological conception of the Universe.


THE END.



FOOTNOTES


[1] A most minute and exact description of the newly hatched larva of
_Chionobas Aëllo_ is given by the American entomologist, Samuel H.
Scudder. Ann. Soc. Ent. de Belgique, xvi., 1873.

[2] I am aware that this certainly cannot be said of philosophers
like Lotze or Herbert Spencer; but these are at the same time both
naturalists and philosophers.

[3] “Über die Artrechte des _Polyommatus Amyntas_ und _Polysperchon_.”
Stett. ent. Zeit. 1849. Vol. x. p. 177-182. [In Kirby’s “Synonymic
Catalogue of Diurnal Lepidoptera” _Plebeius Amyntas_ is given as a
synonym and _P. Polysperchon_ as a var. of _P. Argiades_ Pall. R.M.]

[4] “Die Arten der Lepidopteren-Gattung _Ino_ Leach, nebst einigen
Vorbemerkungen über Localvarietäten.” Stett. ent. Zeit. 1862. Vol.
xxiii. p. 342.

[5] [Eng. ed. W. H. Edwards has since pointed out several beautiful
cases of seasonal dimorphism in America. Thus _Plebeius Pseudargiolus_
is the summer form of _P. Violacea_, and _Phyciodes Tharos_ the summer
form of _P. Marcia_. See Edwards’ “Butterflies of North America,”
1868-79.]

[6] [Eng. ed. I learn by a written communication from Dr. Speyer that
two Geometræ, _Selenia Tetralunaria_ and _S. Illunaria_ Hüb., are
seasonally dimorphic. In both species the winter form is much larger
and darker.] [_Selenia Lunaria_, _S. Illustraria_, and some species of
_Ephyra_ (_E. Punctaria_ and _E. Omicronaria_) are likewise seasonally
dimorphic. For remarks on the case of _S. Illustraria_ see Dr. Knaggs
in Ent. Mo. Mag., vol. iii. p. 238, and p. 256. Some observations on
_E. Punctaria_ were communicated to the Entomological Society of London
by Professor Westwood in 1877, on the authority of Mr. B. G. Cole. See
Proc. Ent. Soc. 1877, pp. vi, vii. R.M.]

[7] [In 1860 Andrew Murray directed attention to the disguising colours
of species which, like the Alpine hare, stoat, and ptarmigan, undergo
seasonal variation of colour. See a paper “On the Disguises of Nature,
being an inquiry into the laws which regulate external form and colour
in plants and animals.” Edinb. New Phil. Journ., Jan. 1860. In 1873 I
attempted to show that these and other cases of “variable protective
colouring” could be fairly attributed to natural selection. See Proc.
Zoo. Soc., Feb. 4th, 1873, pp. 153-162. R.M.]

[8] [A phenomenon somewhat analogous to seasonal change of protecting
colour does occur in some Lepidoptera, only the change, instead of
occurring in the same individual, is displayed by the successive
individuals of the same brood. See Dr. Wallace on _Bombyx Cynthia_,
Trans. Ent. Soc. Vol. v. p. 485. R.M.]

[9] “Über den Einfluss der Isolirung auf die Artbildung.” Leipzig,
1872, pp. 55-62.

[10] [Mr. A. R. Wallace maintains that the obscurely coloured females
of those butterflies which possess brightly coloured males have been
rendered inconspicuous by natural selection, owing to the greater
need of protection by the former sex. See “Contributions to the
Theory of Natural Selection,” London, 1870, pp. 112-114. It is now
generally admitted that the underside of butterflies has undergone
protectional adaptation; and many cases of local variation in the
colour of the underside of the wings, in accordance with the nature of
the soil, &c., are known. See, for instance, Mr. D. G. Rutherford on
the colour-varieties of _Aterica Meleagris_ (Proc. Ent. Soc. 1878, p.
xlii.), and Mr. J. Jenner Weir on a similar phenomenon in _Hipparchia
Semele_ (_loc. cit._ p. xlix.) R.M.]

[11] [The fact that moths which, like the Geometræ, rest by day with
the wings spread out, are protectively marked on the _upper_ side,
fully corroborates this statement. R.M.]

[12] “Über die Einwirkung verschiedener, während der
Entwicklungsperioden angewendeter Wärmegrade auf die Färbung und
Zeichnung der Schmetterlinge.” A communication to the Society of
Natural Science of Steiermark, 1864.

[13] See Exp. 9, Appendix I.

[14] See Exp. 11, Appendix I.

[15] See Exps. 4, 9, and 11, Appendix I.

[16] It seems to me very necessary to have a word expressing whether a
species produces one, two, or more generations in the year, and I have
therefore coined the expression _mono-_, _di-_, and _polygoneutic_ from
γονεύω, I produce.

[17] [Eng. ed. In the German edition, which appeared in 1874, I was
not able to support this hypothesis by geographical data, and could
then only ask the question “whether in the most northern portion of
its area of distribution, appears in two or only in one generation?”
This question is now answered by the Swedish Expedition to the Yenisei
in 1876. Herr Philipp Trybom, one of the members of this expedition,
observed _A. Levana_ at the end of June and beginning of July, in
the middle of Yenisei, in 60°-63° N. (Dagfjärilar från Yenisei in
Översigt ap k. Vertensk. Akad. Förhandlingon, 1877, No. 6.) Trybom
found _Levana_ at Yenisk on June 23rd, at Worogova (61° 5´) on July
3rd, at Asinova (61° 25´) on July 4th, at Insarowa (62° 5´) on July
7th, and at Alinskaja (63° 25´) on July 9th. The butterflies were
especially abundant at the beginning of June, and were all of the
typical _Levana_ form. Trybom expressly states, “we did not find a
single specimen which differed perceptibly from Weismann’s Figs. 1 and
2 (‘Saison-Dimorphismus’ Taf. I.).”

The Swedish expedition soon left the Yenisei, and consequently was not
able to decide by observations whether a second generation possessing
the _Prorsa_ form appeared later in the summer. Nevertheless, it may be
stated with great probability that this is not the case. The districts
in which _Levana_ occurs on the Yenisei have about the same isotherm as
Archangel or Haparanda, and therefore the same summer temperature. Dr.
Staudinger, whose views I solicited, writes to me:--“In Finnmark (about
67° N.) I observed no species with two generations; even _Polyommatus
Phlæas_, which occurs there, and which in Germany has always two, and
in the south, perhaps, three generations, in Finnmark has only one
generation. A second generation would be impossible, and this would
also be the case with _Levana_ in the middle of Yenisei. I certainly
have _Levana_ and _Prorsa_ from the middle of Amur, but _Levana_ flies
there at the end of May, and the summers are very warm.” The middle of
Amur lies, moreover, in 50° N. lat., and therefore 10°-13° south of the
districts of the Yenisei mentioned.

It must thus be certainly admitted that on the Yenisei _A. Levana_
occurs only in the _Levana_ form, and that consequently this species
is at the present time, in the northernmost portion of its area of
distribution, in the same condition as that in which I conceive
it to have been in mid Europe during the glacial period. It would
be of the greatest interest to make experiments in breeding with
this single-brooded _Levana_ from the Yenisei, i.e., to attempt to
change its offspring into the _Prorsa_ form by the action of a high
temperature. If this could not be accomplished it would furnish a
confirmation of my hypothesis than which nothing more rigorous could be
desired.]

[18] See Exp. 10, Appendix I.

[19] When Dorfmeister remarks that hibernating pupæ which, at an
early stage “were taken for development into a room, or not exposed
to any cold, gave dwarfed, weakly and crippled,” or otherwise damaged
butterflies, this is entirely attributable to the fact that this able
entomologist had neglected to supply the necessary moisture to the
warm air. By keeping pupæ over water I have always obtained very fine
butterflies.

[20] [For other remarkable cases of sexual dimorphism (not _antigeny_
in the sense used by Mr. S. H. Scudder, Proc. Amer. Acad., vol. xii.
1877, pp. 150-158) see Wallace “On the Phenomena of Variation and
Geographical Distribution, as illustrated by the Papilionidæ of the
Malayan Region,” Trans. Linn. Soc., vol. xxv. 1865, pp. 5-10. R.M.]

[21] [Eng. ed. Dimorphism of this kind has since been made known: the
North American _Limenitis Artemis_ and _L. Proserpina_ are not two
species, as was formerly believed, but only one. Edwards bred both
forms from eggs of _Proserpina_. Both are single-brooded, and both have
males and females. The two forms fly together, but _L. Artemis_ is much
more widely distributed, and more abundant than _L. Proserpina_. See
“Butterflies of North America,” vol. ii.]

[22] [Eng. ed. Edwards has since proved experimentally that by the
application of ice a large proportion of the pupæ do indeed give rise
to the var. _Telamonides_. He bred from eggs of _Telamonides_ 122 pupæ,
which, under natural conditions, would nearly all have given the var.
_Marcellus_. After two months’ exposure to the low temperature there
emerged from August 24th to October 16th, fifty butterflies, viz.
twenty-two _Telamonides_, one intermediate form between _Telamonides_
and _Walshii_, eight intermediate forms between _Telamonides_ and
_Marcellus_ more nearly related to the former, six intermediate forms
between _Telamonides_ and _Marcellus_, but more closely resembling the
latter, and thirteen _Marcellus_. Through various mishaps the action of
the ice was not complete and equal. See the “Canadian Entomologist,”
1875, p. 228. In the newly discovered case of _Phyciodes Tharos_ also,
Edwards has succeeded in causing the brood from the winter form to
revert, by the application of ice to this same form. See Appendix II.
for a _résumé_ of Edwards’ experiments upon both _Papilio Ajax_ and
_Phyciodes Tharos_. R.M.]

[23] Thus from eggs of _Walshii_, laid on April 10th, Edwards obtained,
after a pupal period of fourteen days, from the 1st to the 6th of June,
fifty-eight butterflies of the form _Marcellus_, one of _Walshii_, and
one of _Telamonides_.

[24] [The word ‘Amixie,’ from the Greek ἀμιξία, was first adopted
by the author to express the idea of the prevention of crossing by
isolation in his essay “Über den Einfluss der Isolirung auf die
Artbildung,” Leipzig, 1872, p. 49. R.M.]

[25] [Eng. ed. In 1844, Boisduval maintained this relationship of the
two forms. See Speyer’s “Geographische Verbreit. d. Schmetterl.,” i. p.
455.]

[26] According to a written communication from Dr. Staudinger, the
female _Bryoniæ_ from Lapland are never so dusky as is commonly the
case in the Alps, but they often have, on the other hand, a yellow
instead of a white ground-colour. In the Alps, yellow specimens are not
uncommon, and in the Jura are even the rule.

[27] [According to W. F. Kirby (Syn. Cat. Diurn. Lepidop.), the species
is almost cosmopolitan, occurring, as well as throughout Europe, in
Northern India (var. _Timeus_), Shanghai (var. _Chinensis_), Abyssinia
(var. _Pseudophlæas_), Massachusetts (var. _Americana_), and California
(var. _Hypophlæas_). In a long series from Northern India, in my own
collection, all the specimens are extremely dark, the males being
almost black. R.M.]

[28] [Eng. ed. From a written communication from Dr. Speyer, it appears
that also in Germany there is a small difference between the two
generations. The German summer brood has likewise more black on the
upper side, although seldom so much as the South European summer brood.]

[29] [Assuming that in all butterflies similar colours are produced by
the same chemical compounds. R.M.]

[30] [Mr. H. W. Bates mentions instances of local variation in colour
affecting many distinct species in the same district in his memoir “On
the Lepidoptera of the Amazon Valley;” Trans. Linn. Soc., vol. xxiii.
Mr. A. R. Wallace also has brought together a large number of cases
of variation in colour according to distribution, in his address to
the biological section of the British Association at Glasgow in 1876.
See “Brit. Assoc. Report,” 1876, pp. 100-110. For observations on the
change of colour in British Lepidoptera according to distribution see
papers by Mr. E. Birchall in “Ent. Mo. Mag.,” Nov., 1876, and by Dr.
F. Buchanan White, “Ent. Mo. Mag.,” Dec., 1876. The colour variations
in all these cases are of course not _protective_ as in the well-known
case of _Gnophos obscurata_, &c. R.M.]

[31] See Figs. 10 and 14, 11 and 15, Plate I.

[32] “On the Origin and Metamorphoses of Insects,” London, 1874.

[33] I at first thought of designating the two forms of cyclical or
homochronic heredity as ontogenetic- and phyletic-cyclical heredity.
The former would certainly be correct; the latter would be also
applicable to alternation of generation (in which actually two or
more phyletic stages alternate with each other) but not to all those
cases which I attribute to heterogenesis, in which, as with seasonal
dimorphism, a series of generations of _the same_ phyletic stage
constitute the point of departure.

[34] When Meyer-Dürr, who is otherwise very accurate, states in his
“Verzeichniss der Schmetterlinge der Schweiz,” (1852, p. 207), that the
winter and summer generations of _P. Ægeria_ differ to a small extent
in the contour of the wings and in marking, he has committed an error.
The characters which this author attributes to the summer form are
much more applicable to the female sex. There exists in this species a
trifling sexual dimorphism, but no seasonal dimorphism.

[35] P. C. Zeller, “Bemerkungen über die auf einer Reise nach Italien
und Sicilien gesammelten Schmetterlingsarten.” Isis, 1847, ii.-xii.

[36] “Isoporien der europäischen Tagfalter.” Stuttgart, 1873.

[37] [Trans. Linn. Soc., vol. xxv. 1865, p. 9. R.M.]

[38] It is certainly preferable to make use of the expression
“metagenesis” in this special sense instead of introducing a new one.
As a general designation, comprehending metagenesis and heterogenesis,
there will then remain the expression “alternation of generation,” if
one does not prefer to say “cyclical propagation.” The latter may be
well used in contradistinction to “metamorphosis.”

[39] _Loc. cit._ chap. iv.

[40] The idea that alternation of generation is derived from
polymorphism (not the reverse, as usually happens; i.e. polymorphism
from alternation of generation) is not new, as I find whilst correcting
the final proof. Semper has already expressed it at the conclusion of
his interesting memoir, “Über Generationswechsel bei Steinkorallen,”
&c. See “Zeitschrift f. wiss. Zool.” vol. xxii. 1872.

[41] See my essay “Über den Einfluss der Isolirung auf die Artbildung.”
Leipzig, 1872.

[42] [In the case of monogoneutic species which, by artificial
‘forcing,’ have been made to give two generations in the year, it has
generally been found that the reproductive system has been imperfectly
developed in the second brood. A minute anatomical investigation of the
sexual organs in the two broods of seasonally dimorphic insects would
be of great interest, and might lead to important results. R.M.]

[43] “Grundzüge der Zoologie.” 2nd ed. Leipzig, 1872. Introduction.

[44] With reference to this subject, see the discussion by the Belgian
Entomological Society, Brussels, 1873.

[45] P. E. Müller, “Bidrag til Cladocerners Fortplantingshistorie,”
1868.

[46] Sars, in “Förhandlinger i Videnskabs Selskabet i Christiania,”
1873, part i.

[47] [Eng. ed. Recent researches on alternation of generation in
the Daphniacea have convinced me that _direct_ action of external
conditions does not in these cases come into consideration, but only
_indirect_ action.]

[48] See my memoir, “Über Bau und Lebenserscheinungen der _Leptodora
hyalina_,” Zeitschrift f. wiss. Zool., vol. xxiv. part 3, 1874.

[49] Stettin. entom. Zeit., vol. xviii. p. 83, 1857.

[50] Compt. Rend., vol. lxxvii. p. 1164, 1873.

[51] [“Accidental” in the sense of our being in ignorance of the laws
of variation, as so frequently insisted upon by Darwin. R.M.]

[52] [Eng. ed. Since this was written I have studied the ornamental
colours of the _Daphniidæ_; and, as a result, I no longer doubt that
sexual selection plays a very important part in the marking and
colouring of butterflies. I by no means exclude both transforming
factors, however; it is quite conceivable, on the contrary, that a
change produced directly by climate may be still further increased by
sexual selection. The above given case of _Polyommatus Phlæas_ may
perhaps be explained in this manner. That sexual selection plays a
part in butterflies, is proved above all by the odoriferous scales and
tufts of the males discovered by Fritz Müller.] [For remarks on the
odours emitted by butterflies and moths, see Fritz Müller in “Jena.
Zeit. f. Naturwissen.,” vol. xi. p. 99; also “Notes on Brazilian
Entomology,” Trans. Ent. Soc. 1878, p. 211. The odoriferous organs
of the female _Heliconinæ_ are fully described in a paper in “Zeit.
f. Wissen. Zool.,” vol. xxx. p. 167. The position of the scent-tufts
in the sphinx-moths is shown in Proc. Entom. Soc. 1878, p. ii. Many
British moths, such as _Phlogophora meticulosa_, _Cosmia trapezina_,
&c. &c., have tufts in a similar position. The fans on the feet of
_Acidalia bisetata_, _Herminia barbalis_, _H. tarsipennalis_, &c., are
also probably scent organs. A large moth from Jamaica, well known to
possess a powerful odour when alive (_Erebus odorus_ Linn.), has great
scent-tufts on the hind legs. For the application of the theory of
sexual selection to butterflies, see, in addition, to Darwin’s “Descent
of Man,” Fritz Müller in “Kosmos,” vol. ii. p. 42; also for January,
1879, p. 285; and Darwin in “Nature,” vol. xxi. January 8th, 1880, p.
237. R.M.]

[53] Nägeli, “Entstehung und Begriff der naturhistorischen Art,”
Munich, 1865, p. 25. The author interprets the facts above quoted in a
quite opposite sense, but this is obviously erroneous.

[54] See my essay, “Über den Einfluss der Isolirung auf die
Artbildung.” Leipzig, 1872.

[55] [Eng. ed. In the summer of 1877, Dr. Hilgendorf again investigated
the Steinheim fossil shells, and found his former statements to be
completely confirmed. At the meeting of the German Naturalists and
Physicists at Munich, in 1877, he exhibited numerous preparations,
which left no doubt that the chief results of his first research were
correct, and that there have been deposited a series of successively
derived species together with their connecting intermediate forms.]

[56] See my essay, “Über die Berechtigung der Darwin’schen Theorie.”
Leipzig, 1868.

[57] I expressly insist upon this here, because the notice of
Askenasy’s thoughtful essay which I gave in the “Archiv für
Anthropologie” (1873) has frequently been misunderstood.

[58] The experiments upon _Papilio Ajax_ and _Phyciodes Tharos_,
described in this Appendix, were made by Mr. W. H. Edwards (see his
“Butterflies of North America;” also the “Canadian Entomologist,” vol.
vii. p. 228-240, and vol. ix. p. 1-10, 51-5, and 203-6); and I have
added them, together with some hitherto unpublished results, to Dr.
Weismann’s Essay, in order to complete the history of the subject of
seasonal dimorphism up to the present time.--R.M.

[59] This is a striking illustration of the diversity of individual
constitution so frequently insisted on by Dr. Weismann in the foregoing
portion of this work.

[60] The reader who wishes to acquire a detailed knowledge of the
different varieties of this butterfly, of which a very large number
are known, must consult the plates and descriptions in Edwards’
“Butterflies of North America,” vol. ii.

[61] Mr. Edwards has shown also that _Argynnis Myrina_ can lay fertile
eggs when but a few hours out of the chrysalis. Canad. Ent., September,
1876, vol. viii. No. 9.

[62] Mr. Edwards remarks that the habit of becoming lethargic is of
great service to a digoneutic species in a mountain region where it is
exposed to sharp changes of temperature. “If the fate of the species
depended on the last larval brood of the year, and especially if the
larvæ must reach a certain stage of growth before they were fitted to
enter upon their hibernation, it might well happen that now and then an
early frost or a tempestuous season would destroy all the larvæ of the
district.”

[63] Compare this with Weismann’s remarks, pp. 19-22, and 53.

[64] See Canad. Ent., vol. ix. p. 69.

[65] Figures of the different forms of this species are given in vol.
i. of Edward’s “Butterflies of North America.”

[66] Only the species of _Smerinthus_ can be made to lay eggs regularly
in confinement; _Macroglossa Stellatarum_ laid a number in a large
gauze-covered breeding-cage; the species of _Deilephila_ could not be
induced to lay more than single ones in such a cage. From species of
_Chærocampa_ also I never obtained but a few eggs, and from _Sphinx_
and _Acherontia_ never more than single ones.

[67] [Eng. ed. Since the appearance of the German edition of this
work, numerous descriptions of the young stages of caterpillars have
been given, but in all cases without representing the relationship of
the forms.] [In the excellent figures of larvæ at various stages of
growth, given in some of the more recent works on Lepidoptera, there
will be found much material which may be regarded as a contribution to
the field of research entered on by the author in the present essay,
_i.e._ the ontogeny and comparative morphology of larval markings,
although it is much to be regretted that the figures and descriptions
have not been given from this point of view. In his “Butterflies of
North America,” for example, W. H. Edwards figures the young as well
as the adult larvæ of species of _Apatura_, _Argynnis_, _Libythea_,
_Phyciodes_, _Limenitis_, _Colias_, _Papilio_, &c. Burmeister, in his
recently published “Lépidoptères de la République Argentine,” figures
the young stages of species of _Caligo_, _Opsiphanes_, _Callidryas_,
_Philampelus_, &c. Messrs. Hellins and Buckler have figured and
described the early stages of large numbers of the caterpillars of
British Lepidoptera, but their figures remain unpublished. The larvæ
of many of our native species belonging to the genera _Liparis_,
_Tæniocampa_, _Epunda_, _Cymatophora_, _Calocampa_, &c., are dull when
young, but become brightly coloured at the last moult. Such changes
of colour are probably associated with some change, either in the
habits or in the environment; and a careful study of the ontogenetic
development of such species in connection with their life-history would
furnish results of great value to the present inquiry. The same remarks
apply to those _Noctuæ_ larvæ which are brightly coloured in their
young stages, and become dull when adult.

Among other papers which may be considered as contributions to
the present subject, I may mention the following:--In 1864 Capt.
Hutton published a paper, “On the Reversion and Restoration of the
Silkworm, Part II.” (Trans. Ent. Soc. 1864, p. 295), in which he
describes the various stages of development of several species of
_Bombycidæ_. In 1867 G. Semper published accounts of the early stages
of several Sphinx-larvæ (“Beiträge zur Entwicklungsgeschichte einiger
ostasiatischer Schmetterlinge,” Verhandl. k.k. Zoolog.-botan. Gesell.
in Wien, vol. xvii.). The question as to the number of claspers in
young _Noctuæ_ larvæ has been raised in notes by Dr. F. Buchanan White
(“Ent. Mo. Mag.,” vol. v. p. 204) and B. Lockyer (“Entomologist,” 1871,
p. 433). A valuable paper, “On the Embryonic Larvæ of Butterflies,”
was published in 1871 by S. H. Scudder (“Ent. Mo. Mag.,” vol. viii. p.
122). For remarks on the development of the larva of _Papilio Merope_,
see J. P. Mansel Weale in Trans. Ent. Soc., 1874, p. 131, and Pl. I.;
also this author on the young stages of the larva of _Gynanisa Isis_,
Trans. Ent. Soc., 1878, p. 184. For an account of the development of
the larvæ of certain North American species of _Satyrus_, see W. H.
Edwards in the “Canadian Entom.,” vol. xii. p. 21. Mr. P. H. Gosse’s
recent description of the newly hatched caterpillar of _Papilio
Homerus_ (Proc. Ent. Soc. 1879, p. lv), furnishes a good illustration
of the value of studying the ontogeny. The natural affinities of
the _Papilionidæ_ were at one time much disputed, some systematists
placing this family at the head of the Lepidoptera, and others
regarding them as being more closely allied to the moths. Mr. Gosse’s
observation tends to confirm the latter view, now generally received by
Lepidopterists, since he states that the larva in question “suggests
one of the great _Saturniadæ_, such as _Samia Cecropia_.” Mr. Scudder,
in the paper above referred to, adopts an analogous argument to show
the close relationship between the _Papilionidæ_ and _Hesperidæ_. R.M.]

[68] [Mr. A. G. Butler has recently furnished a good illustration of
the danger of classifying Lepidoptera according to the affinities of
the perfect insects only, in his paper, “On the Natural Affinities of
the Lepidoptera hitherto referred to the Genus _Acronycta_ of authors,”
Trans. Ent. Soc. 1879, p. 313. If the author’s views are ultimately
accepted, the species at present grouped under this genus will be
distributed among the _Arctiidæ_, _Liparidæ_, _Notodontidæ_, and
_Noctuæ_. Mr. Butler’s determination of the affinities of the species
supposed to belong to the genus mentioned, is based chiefly upon a
comparative examination of the larvæ, and this is far more likely to
show the true blood-relationship of the species than a comparison of
the perfect insects only. A study of the comparative ontogeny can alone
give a final answer to this question. R.M.]

[69] [In his recent revision of the _Sphingidæ_, Mr. A. G. Butler
(Trans. Zoo. Soc., vol. ix. part x.) retains Walker’s arrangement. R.M.]

[70] The deposition of black pigment may commence immediately before
ecdysis.

[71] [Mr. Herbert Goss states (Proc. Ent. Soc. 1878, p. v.) that
according to his experience, the green and brown varieties of _C.
Porcellus_ (erroneously printed as _Elpenor_ in the passage referred
to) are about equally common, the former colour not being in any way
confined to young larvæ. Mr. Owen Wilson in his recent work, “The Larvæ
of British Lepidoptera and their food-plants,” figures (Pl. VIII.,
Figs. 3 and 3a) the two forms, both apparently in the adult state.
During the years 1878-79, my friend, Mr. J. Evershed, jun., took five
of these full-grown larvæ in Surrey, one of these being the green
variety. In order to get more statistics on this subject, I applied
this year (1880) to Messrs. Davis of Dartford, who informed me that
among 18-20 adult caterpillars of _Porcellus_ in their possession,
there was only one green specimen. R.M.]

[72] I unite the genera _Pergesa_ and _Darapsa_ of Walk. with
_Chærocampa_, Dup.; the first appears to me to be quite untenable,
since it is impossible that two species, of which the caterpillars
agree so completely as those of _C. Elpenor_ and _Porcellus_, can
be located in different genera. _Porcellus_ indeed was referred to
the genus _Pergesa_ because of its different contour of wings, an
instance which distinctly shows how dangerous it is to attempt to found
Lepidopterous genera without considering the caterpillars. The genus
_Darapsa_ also appears to me to be of very doubtful value, and in any
case requires further confirmation with respect to the larval forms.

[73] [Mr. A. G. Butler (Trans. Zoo. Soc., vol. ix., part. x., 1876)
gives a list of about eighty-four species of _Chærocampa_, and sixteen
of _Pergesa_, besides numerous other species belonging to several
genera placed between _Chærocampa_ and _Pergesa_. Of _Darapsa_, he
states “that this genus was founded upon most heterogeneous material,
the first three species being referable to Hübner’s genus _Otus_, the
fifth to Walker’s genus _Diodosida_, the sixth and eighth to the genus
_Daphnis_ of Hübner, the seventh, ninth, and tenth to _Chærocampa_ of
Duponchel; there therefore remains only the fourth species, allied to
_Chærocampa_, but apparently sufficiently distinct.” The species still
retained in the genus _Darapsa_ is _D. rhodocera_, Wlk., from Haiti.
R.M.]

[74] [_Otus Syriacus_ of Butler’s revision. R.M.]

[75] Abbot and Smith. “The Natural History of the rarer Lepidopterous
Insects of Georgia, collected from the observations of John Abbot, with
the plants on which they feed.” London, 1797, 2 vols. fol.

[76] [_Otus Chœrilus_ and _O. Myron_ of Butler’s revision. R.M.]

[77] [To this group may also be added _Ampelophaga Rubiginosa_,
Ménétriés, from China and Japan, the caterpillar of which, having the
distinct subdorsal line without any trace of eye-spots, is figured by
Butler (_loc. cit._, Pl. XCI., Fig. 4). This author also gives a figure
of another species belonging to the subfamily _Chærocampinæ_ (Pl. XC.,
Fig. 11), viz. _Acosmeryx Anceus_, Cram., from Amboina, Java, Silhet,
and S. India; the caterpillar is green, with seven oblique yellow
stripes along the sides, and a very conspicuous white subdorsal line
with a red border above. As there are no eye-spots, this species may
be referred to the present group provisionally, although its general
marking is very distinct from that of the _Chærocampa_ group. R.M.]

[78] [Eng. ed. Dr. Staudinger has since obtained the caterpillar of _C.
Alecto_ from Beyrout; it possesses “a very distinct subdorsal line,
and on the fourth segment a beautiful eye-spot, which is repeated with
gradual diminution to segments 7-8”.]

[79] Figured in “A Catalogue of Lepidopterous Insects in the Museum
of the East India Company,” by Thomas Horsfield and Frederick Moore.
London, 1857. Vol. i., Pl. XI.

[80] Figured in Trans. Ent. Soc., New Series, vol. iv., Pl. XIII.

[81] _Ibid._

[82] [The following species figured by Butler (_loc. cit._ Pls. XC.
and XCI.) appear to belong to the second group--_Chærocampa Japonica_,
Boisd., which is figured in two forms, one brown, and the other green.
The former has two distinct ocelli on the fourth and fifth segments,
and a distinct rudiment on the sixth, whilst the subdorsal line extends
from the second eye-spot to the caudal horn, and beneath this line
the oblique lateral stripes stand out conspicuously in dark brown on
a lighter ground. The ocelli are equally well developed on the fourth
and fifth segments in the green variety, the subdorsal line commencing
on the sixth segment, and extending to the caudal horn; there is no
trace of a third eye-spot, nor are there any oblique lateral stripes;
the insect is almost the exact counterpart of _C. Elpenor_ in its
fourth stage. (See Fig. 21, Pl. IV.) _Pergesa Mongoliana_, Butl., is
brown, without a trace of the subdorsal line except on the three front
segments, and with only one large eye-spot on the fourth segment.
_Chærocampa Lewisii_, Butl., from Japan, is likewise figured in two
forms. The brown variety has the subdorsal line on the three front
segments only, distinct ocelli on the fourth and fifth segments, and
gradually diminishing rudiments on the remaining segments. The green
form appears to be transitional between the present and the third
group, as it possesses a distinct, but rudimentary eye-spot on the
third segment, besides the fully developed ones on the fourth and
fifth, and very conspicuous, but gradually decreasing repetitions
of rudimentary ocelli on segments 6-10. To this group may be added
_Chærocampa Aristor_, Boisd., the caterpillar of which is figured by
Burmeister (Lép. Rép. Arg., Pl. XV., Fig. 4) in the characteristic
attitude of alarm, with the front segments retracted, and the ocelli on
the fourth segment prominently exposed. The subdorsal line is present
in this species. Burmeister also figures two of the early stages (Pl.
XV., Fig. 7, A and B), and describes the complete development of
_Philampelus Labruscæ_, another species belonging to the subfamily
_Chærocampinæ_. The earliest stage (3-4 days old) is simple green, with
no trace of any marking except a black spot on each side of the fourth
segment, the position of the future ocelli. A curved horn is present
both in this stage and the following one, during which the caterpillar
is still green, but now has seven oblique red lateral stripes. The
caudal horn is shed at the second moult, after which the colour becomes
darker, the adult larva (figured by Madame Mérian, in her work on
Surinam, pl. 34 and Sepp., pl. 32) being mottled brown. In addition to
the ocellus on the fourth segment, there is another slightly larger
on the eleventh segment, so that this species may perhaps be another
transition to the third group; but our knowledge is still too imperfect
to attempt to generalize with safety. R.M.]

[83] Cat. Lep. Ins. East Ind. Comp., Pl. XIII. [Figured also by Butler
(=_Chæerocampa Silhetensis_, Walker), _loc. cit._ Pl. XCII., Fig. 8.
R.M.]

[84] Cat. Lep. Ins. East Ind. Comp., Pl. XIII. [Figured also by Butler,
_loc. cit._ Pl. XCI., Fig. 1. R.M.]

[85] Horsfield and Moore, _loc. cit._ Pl. X.

[86] _Ibid._ [=_Pergesa Acteus_, Walker. R.M.]

[87] [Figured also by Burmeister, _loc. cit._ Pl. XV., Fig. 3. R.M.]

[88] Horsfield and Moore, _loc. cit._, Pl. XI.

[89] To be accurate this should be designated the infra-spiracular
line; but this term cannot be well applied except in cases where
there is also a supra-spiracular line, as, for instance, in _Anceryx
(Hyloicus) Pinastri_.

[90] Upon this fact obviously depends the statement of that extremely
accurate observer Rösel, that the caterpillar of _Euphorbiæ_ is but
very slightly variable (“Insektenbelustigungen,” Bd. iii. p. 36). I
formerly held the same opinion, till I convinced myself that this
species is very constant in some localities, but very variable in
others. It appears that local influences make the caterpillar variable.

[91] The green is considerably too light in Fig. 45.

[92] “Die Pflanzen und Raupen Deutschlands.” Berlin, 1860, p. 83.

[93] Fig. 62, Pl. VII., is copied from Boisduval.

[94] The fading of the red anteriorly has not been represented in the
figure.

[95] [The caterpillar of _Deilephila Euphorbiarum_, figured by
Burmeister (Lép. Rép. Arg., Pl. XVI, Fig. 1) belongs to this stage.
R.M.]

[96] [In concluding this account of the _Chærocampinæ_ I may call
attention to the following species, which have since been figured by
Burmeister:--_Pachylia Ficus_, Linn. (_loc. cit._ Pl. XIV., Figs. 1
and 2); during the three first stages the larva is uniformly green,
with a yellow subdorsal line, and below this ten oblique yellow
stripes slanting away from the head; after the third moult the colour
completely changes, the whole area of the body being divided into two
distinct portions by the subdorsal line, above which the colour is
red, and underneath of a pale green; the oblique stripes have almost
disappeared; no occelli nor annuli are present. _Pachylia Syces_, Hübn.
(_loc. cit._ Fig. 3); very similar to the last species in its young
stages (figured also by Mérian, Surin. pl. 33). _Philampelus Vitis_,
Linn. (_loc. cit._ Figs. 4 and 5); two stages represented; between
first and second moults green, with oblique paler stripes slanting in
same direction as in _Pachylia_, and each one containing a red streak
surrounding the spiracle. When adult, the ground-colour is yellow above
and green beneath, the whole surface being mottled with deep black and
red transverse markings; the oblique stripes whitish, bordered with
black at their lower extremities (figured also by Mérian, pls. 9 and
39). _Philampelus Anchemolus_, Cram. (_loc. cit._ Pl. XV., Fig. 1;
Mérian, pl. 47); green when young, with seven oblique red stripes; when
adult, uniformly brown, with seven pale yellow lateral markings, the
first four of which are spots, and the remainder broad oblique stripes
slanting forwards. _Philampelus Labruscæ_, (see note 82, p. 195). R.M.]

[97] [_Mimas Tiliæ_ of Butler’s revision. The author states that this
genus is “easily distinguished from _Laothoë_ by the form of the wings,
the outer margin of secondaries deeply excavated below the apex, and
the secondaries narrow and not denticulated.” Here again we have a
clashing of the results arrived at by a study of the ontogeny of the
larvæ, on the one hand, and the founding of genera on the characters of
the imagines only, on the other. Of the three species discussed by Dr.
Weismann, Mr. Butler, following other authors, refers _Tiliæ_ to the
genus _Mimas_, _Populi_ to _Laothoë_, and _Ocellatus_ to _Smerinthus_.
It is to be hoped that when our knowledge of the developmental history
of larvæ is more complete in all groups, a reconciliation between the
results of the biological investigator and the pure systematist will be
brought about, so that a genus may not, as at present, have such very
different values when regarded from these two points of view. R.M.]

[98] The caterpillar is thus figured by Rösel.

[99] [In 1879 Mr. E. Boscher found about thirty full-grown caterpillars
of this species in the neighbourhood of Twickenham, ten to twelve
of which were feeding on _Salix viminalis_, and the remainder, from
a locality not far distant, on _Salix triandra_. The whole of the
specimens taken on the plant first named, had the red-brown spots above
and below the oblique stripes more or less completely developed, as
I myself had an opportunity of observing. In these spotted specimens
the ground-colour was bright yellowish-green, and in the others this
colour was dull whitish-green above, passing into bluish-green below.
Should these observations receive wider confirmation, it would be
fair to conclude that this species is now in two states of phyletic
development, the more advanced stage being represented by the brighter
spotted variety. (See also Proc. Ent. Soc. 1879, p. xliv.). Mr. Peter
Cameron has recently suggested (Trans. Ent. Soc. 1880, p. 69) that the
reddish-brown spots on the _Smerinthus_ caterpillars may serve for
purposes of disguise, as they closely resemble, both in colour and
form, certain galls (_Phytoptus_) of the food-plants of these species.
If this view be admitted, these spots must be considered as a new
character, now being developed by natural selection. The variation in
the ground-colour of the two forms of _S. Ocellatus_ may possibly be
phytophagic, but this can only be decisively settled by a series of
carefully conducted experiments. R.M.]

[100] “Insekten-Belustigungen,” Suppl. Pl. 38, Fig. 40.

[101] “Catalogue of Lepidop.” British Museum. [Butler divides the
subfamily _Smerinthinæ_ into 17 genera, containing 79 species, viz.
_Metamimas_, 2; _Mimas_, 4; _Polyptychus_, 7; _Lophostethus_, 1;
_Sphingonæpiopsis_, 1; _Langia_, 2; _Triptogon_, 23; _Laothoë_, 2;
_Cressonia_, 3; _Paonias_, 2; _Calasymbolus_, 5; _Smerinthus_, 5;
_Pseudosmerinthus_, 2; _Daphnusa_, 4; _Leucophlebia_, 5; _Basiana_, 10;
_Cæquosa_, 1. R.M.]

[102] “Cabinet Orient. Entom.,” p. 13, Pl. VI., Fig. 2. [Butler places
this species doubtfully among the _Sphinginæ_. R.M.]

[103] “Catalogue of the Lepidop. Insects of the E.I. Co.,” by Horsfield
and Moore. Pl. VIII., Fig. 6.

[104] [The larvæ of four other species of this subfamily have since
been made known through Mr. Butler’s figures. _Smerinthus Tatarinovii_,
Ménetriés (_loc. cit._ Pl. XC., Fig. 16), from Japan, is “pale
sea-green, tuberculated with white, with seven lateral, oblique,
crimson-edged white stripes.” There is no trace of the subdorsal line
shown in the figure, so that this species thus appears to be in the
third phyletic stage of development. _Smerinthus Planus_, Walker, from
China (_loc. cit._ Pl XCII., Fig. 11), is “pale green, with white or
yellow lateral stripes.” A trace of the subdorsal line remains on
the front segments, thus showing that the species is in the second
phyletic stage of development. _Triptogon Roseipennis_, Butler, from
Hakodadi (_loc. cit._ Pl. XCI., Fig. 6), is represented as yellow,
with seven oblique white stripes, with large irregular triangular red
spots extending from the anterior edge of the stripes, nearly across
each segment. It is probably in the third phyletic stage. The Indian
_Polyptychus Dentatus_, Cramer (_loc. cit._ Pl. XCI., Fig. 10), is
“bluish-green at the sides, with oblique purple stripes, with a broad,
dorsal, longitudinal, golden-green band, bordered by subtriangular
purple spots, one above each stripe.” The dorsal band is bordered by
coloured stripes, which may be the subdorsal lines; but the position
in which it is figured, and its very different mode of coloration,
make it very difficult to compare satisfactorily with the foregoing
species. The genus _Ambulyx_ is closely allied to the _Smerinthinæ_,
and the two following species may be here mentioned: _A. Gannascus_,
Stoll, figured by Burmeister (_loc. cit._ Pl. XIII., Fig. 5), is green,
with a yellow subdorsal line, and seven oblique white lateral stripes,
edged with red. _A. Liturata_, Butl. (_loc. cit._ Pl. XCI., Fig. 2), is
yellowish-green above, passing into bluish-green below. The subdorsal
is present on the three front segments