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.