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.