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