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Pages 401-729 and Plates III-VIII are in Volume II.




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  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