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.