The considerations previously set forth are entirely based on Fritz
Müller’s and Haeckel’s view, that the development of the individual
presents the ancestral history _in nuce_, the ontogeny being a
condensed recapitulation of the phylogeny.
Although this law is generally true--all recent investigations
on development having given it fresh confirmation--it must not
be forgotten that this “recapitulation” is not only considerably
abbreviated, but may also be “falsified,” so that a searching
examination into each particular case is very desirable.
The question thus arises, in the first place, as to whether the
markings of caterpillars, so distinct at the different stages of
growth, are actually to be regarded as residual markings inherited
from the parent-form; or whether their differences do not depend upon
the fact that the caterpillar, in the course of growth, is exposed to
different external conditions of life, to which it has adapted itself
by assuming a different guise.
The former is undoubtedly the case. It can by no means be denied that
the conditions of life in young caterpillars are _sometimes_ different
to those of the adults. It will, in fact, be shown later on, that in
certain cases the assumption of a new guise at an advanced age actually
depends upon adaptation to new conditions of life; but as a rule, the
external conditions remain very similar during the development of the
larva, as follows from the fact that a change of food-plant never takes
place.[126] We should therefore rather expect a complete similarity
of marking throughout the entire larval period, instead of the great
differences which we actually observe.
Different circumstances appear to me to show that the markings of
young larvæ are only exceptionally due to a new adaptation, but that
as a rule they depend upon heredity. In the first place, there is
the fact that closely allied species, exposed to precisely similar
external conditions, as, for instance, _Chærocampa Elpenor_ and
_Porcellus_, possess exactly the same markings when young, these
markings nevertheless appearing at different stages of growth. Thus,
the subdorsal line first appears in _Elpenor_ in the second stage,
whilst in _Porcellus_ it is present during the first stage. If this
line were acquired by the young larva for adapting it at this age to
special conditions of life, it should appear in both species at the
same stage. Since this is not the case, we may conclude that it is
only an inherited character derived from the adult ancestor of the two
species, and now relegated to the young stages, being (so to speak),
pushed further back in one species than in the other.
But the strongest, and, as it appears to me, the most convincing proof
of the purely phyletic significance of the young larval markings, is
to be found in the striking regularity with which these are developed
in a similar manner in all allied species, howsoever different may
be their external conditions of life. In all the species of the
_Chærocampa_ group (the genera _Chærocampa_ and _Deilephila_) the
marking--no matter how different this may be in later stages--arises
from the simple subdorsal line. This occurs even in species which live
on the most diverse plants, and in which the markings can be of no
biological importance as long as the larvæ are so small as to be only
visible through a lens, and where there can be no possible imitation
of leaf-stalks or veins, the leaves and caterpillars being so very
distinct.
Moreover, when in the _Macroglossinæ_ (the genera _Macroglossa_,
_Pterogon_, and _Thyreus_) we see precisely the same simple marking
(the subdorsal) line retained throughout all the stages in two genera,
whilst in the _Smerinthinæ_ this line vanishes at a very early stage,
and in the _Sphinginæ_ is only present in traces, we can give but
one explanation of these facts. We have here a fragmentary series
representing the phyletic development of the Sphinx-markings, which
latter have arisen from one original plan--the simple subdorsal
line--and have then undergone further development in various
directions. As this subsequent development advanced, the older phyletic
stages would always be relegated to younger ontogenetic stages, until
finally they would be but feebly represented even in the youngest stage
(_D. Euphorbiæ_), or else entirely eliminated (most of the species of
the genus _Sphinx_). I believe that no other sufficient explanation of
these facts can be adduced. Granting that the correctness of the above
views can no longer be doubted, we may now take up the certain position
that the ontogeny of larval markings reveals their phylogeny, more or
less completely, according to the number of phyletic stages omitted,
or, in some exceptional cases, falsified. In other words, the ontogeny
of larval markings is a more or less condensed and occasionally
falsified recapitulation of the phylogeny.
Considering this to be established, we have next to deal with the
uniformity of the developmental phenomena, from which we may then
attempt to trace out the inciting causes underlying this development.
The law, or, perhaps better, the line of direction followed by the
development, is essentially the following:--
1. The development commences with a state of simplicity, and advances
gradually to one of complexity.
2. New characters first make their appearance in the last stage of the
ontogeny.
3. Such characters then become gradually carried back to the earlier
ontogenetic stages, thus displacing the older characters, until the
latter disappear completely.
The first of these laws appears almost self-evident. Whenever we
speak of development, we conceive a progression from the simple to
the complex. This result therefore does nothing but confirm the
observation, that we have actually here before us a development in
the true sense of the word, and not simply a succession of different
independent conditions.
The two following laws, on the other hand, lay claim to a greater
importance. They are not now enunciated for the first time, but were
deduced some years ago by Würtemberger[127] from a study of the
ammonites. In this case also the new characters predominate in the
later periods of life, and are then transferred back to the younger
ontogenetic stages in the course of phyletic development. “The change
in the character of the shell in ammonites, first makes itself
conspicuous in the last chamber; but in the succeeding generations
this change continually recedes towards the beginning of the spiral
chambers, until it prevails throughout the greater part of the
convolutions.”
In the same sense must also be conceived the case which Neumayr and
Paul have recently made known respecting certain forms of _Melanopsis_
from the West Sclavonian _Paludina_ bed. In _M. Recurrens_ the last
convolutions of the shell are smooth, this being a new character; the
small upper convolutions, however, are delicately ribbed, as is also
the case with the last convolution of the immediate progenitor. The
embryonic convolutions again are smooth, and the author believes (on
other grounds) that the more remote progenitor possessed a smooth shell.
In this case therefore, and in that of the ammonites, every shell
to a certain extent proclaims the ancestral history of the species;
in one and the same shell we find different phyletic stages brought
into proximity. The markings of caterpillars do not offer similar
facilities; nevertheless I believe that by their means we are led
somewhat further, and are able to enter more deeply into the causes
underlying the processes of transformation, because we can here observe
the living creature, and are thus enabled to study its life-history
with more precision than is possible with a fossil species.
When, in 1873, I received Würtemberger’s memoir, I was not only struck
with the agreement of his chief results with those which I had arrived
at by the study of larval markings, but I was almost as much astonished
at the great difference in the interpretation of the facts. The latter
indicate the gradual backward transference of a new character from the
latest to the earlier ontogenetic stages. Without further confirmation
Würtemberger assumes that it is to a certain extent self-evident
that the force producing this backward transference is the same as
that which, according to his view, first called forth the character
in question in the last stage, viz., natural selection. “Variations
acquired at an advanced age of the organism may, when advantageous,
be inherited by the succeeding generations, in such a manner that they
always appear a little earlier than in the preceding generations.”
It is certainly theoretically conceivable that a newly acquired
character, when also advantageous to the earlier stages, might be
gradually transferred to these stages, since in this case those
individuals in which this character appeared earliest would have the
greatest chance of surviving. In the case of the development of larval
markings, however, there are facts which appear to me to show that such
backward transference of a new character is, in a certain measure,
independent of the principle of utility, and that it must therefore
be referred to another cause--to the innate law of growth which rules
every organism.
When, in the larva of _C. Elpenor_, we perceive that the two eye-spots
which are first formed on the fourth and fifth segments appear
subsequently on the other segments as faint traces of no biological
value whatever, we cannot explain this phenomenon by natural selection.
We should rather say that in segmented animals there is a tendency for
similar characters to be repeated on all the segments; and this simply
amounts to the statement, that an innate law of growth is necessary for
the repetition of such newly acquired characters.
The existence of such a law of growth, acting independently of natural
selection, may therefore be considered as established, and indeed
cannot be disputed (Darwin’s “correlation of growth”). In the present
case it appears to me that an innate law of this kind, determining
the backward transference of new characters, is deducible from the
instances already quoted in another sense, viz., from the fact that in
many cases characters which are decidedly advantageous to the adult
are transferred to the younger stages, where they are at most of but
indifferent value, and can certainly be of no direct advantage. This is
the case with the oblique stripes of _Smerinthus_, which, in the adult
larvæ, resemble the leaf ribs, as will be shown more fully later on,
and, in conjunction with the green coloration, cause these caterpillars
to be very difficult of detection on their food-plants. The insects
are easily overlooked, and can only be distinctly recognized on close
inspection.
Now these oblique stripes appear, in all the _Smerinthus_ caterpillars
known to me, in the second, and sometimes even in the first stage,
_i.e._ in larvæ of from 0.7 to 1 centimeter in length. The stripes
are here much closer together than the ribs of any of the leaves of
either willow, poplar, or lime, and can therefore have no resemblance
to these leaves. The young caterpillars are certainly not rendered more
conspicuous by the oblique stripes, since they can only be recognized
on close inspection. It is for this reason that the stripes have not
been eliminated by natural selection.
The remarkable phenomenon of the backward transference of newly
acquired characters may therefore be formulated as follows:--Changes
which have arisen in the later ontogenetic stages have a tendency to
be transferred back to the younger stages in the course of phyletic
development.
The facts of development already recorded furnish numerous proofs
that this transference occurs gradually, and step by step, taking the
same course as that which led to the first establishment of the new
character in the final ontogenetic stage.
Did this law not obtain, the ontogeny would lose much of the interest
which it now possesses for us. It would then be no longer possible,
from the ontogenetic course of development of an organ or of a
character, to draw a conclusion as to its phylogeny. If, for instance,
the eye-spots of the _Chærocampa_ larvæ, which must have been acquired
at a late age, were transferred back to the younger ontogenetic stages
in the course of phyletic development, as eye-spots already perfected,
and not showing their rudimentary commencement as indentations of the
subdorsal line, the phenomenon would then give us no information as to
the manner of their formation.
It is well known to all who have studied the developmental history of
any group of animals, that no organ, or no character, however complex,
appears suddenly in the ontogeny; whereas, on the other hand, it
appears certain that new, or more advanced, but simpler characters,
predominate in the last stage of development. We are thus led to the
following modification of the foregoing conclusion:--Newly acquired
characters undergo, as a whole, backward transference, by which means
they are to a certain extent displaced from the final ontogenetic stage
by characters which appear later.
This must be a purely mechanical process, depending on that innate law
of growth, the action of which we may observe without being able to
explain fully. Under certain conditions the operation of this law may
be prevented by natural selection. Thus, for instance, if the young
caterpillars of _Anceryx Pinastri_ have not acquired the characteristic
marking of the adults, it is probably because they are better protected
by their resemblance to the green pine-needles than they would be if
they possessed the pattern of the larger caterpillars in their last
stage.
The backward transference of newly acquired characters may also
possibly be accelerated when these characters are advantageous to the
younger stages; but this transference takes place quite independently
of any advantage if the characters are of indifferent value, being then
entirely brought about by innate laws of growth.
That new characters actually predominate in the last stage of the
ontogeny, may also be demonstrated from the markings of caterpillars.
It is, of course, not hereby implied, that throughout the whole animal
kingdom new characters can only appear in the last ontogenetic stage.
Haeckel is quite correct in maintaining that the power of adaptation of
an organism is not restricted to any particular period. Under certain
circumstances transformations may occur at any period of development;
and it is precisely insects undergoing metamorphosis that prove this
point, since their larvæ differ so widely from their imagines that the
earlier stages may be completely disguised. It is here only signified
that, with respect to the development of caterpillars, new characters
first appear in the adult. The complexity of the markings, which so
frequently increases with the age of the caterpillar, can scarcely
bear any other interpretation than that the new characters were always
acquired in the last stage of the ontogeny. In certain cases we are
able, although with some uncertainty, to catch Nature in the act of
adding a new character.
I am disposed to regard the blood-red or rust-red spots which occur
in the last stage of the three species of _Smerinthus_ larvæ in the
neighbourhood of the oblique stripes as a case in point. It has
already been shown that these red spots must be regarded as the first
rudiments of the linear coloured edges which reach complete development
in the genus _Sphinx_. In some specimens of _Smerinthus Tiliæ_ the
spots coalesce so as to form an irregular coloured edge to the oblique
stripes. In _S. Populi_ they occur in many individuals, but remain
always in the spot stage; whilst _S. Ocellatus_ is but seldom, and _S.
Quercus_ appears never to be spotted.
The spots both of _S. Tiliæ_ and _Populi_ certainly do not show
themselves exclusively in the fifth (last) stage, but also in the
fourth, and sometimes in _Populi_ even as early as the third stage,
from which we might be disposed to conclude that the new character did
not first appear in the last stage. But the majority of the spotted
individuals first acquire their spots in the fifth stage, and only a
minority in the fourth; so that their occasional earlier appearance
must be ascribed to the backward transference of a character acquired
in the fifth stage. Moreover, the fourth and fifth stages of the
caterpillars are closely analogous both in size, mode of life, and
marking, and are therefore analogous with reference to the environment,
so that it is to be expected that new characters, when depending on
adaptation, would be rapidly transferred from the fifth stage to the
fourth.[128] We should thus have a case of the acceleration by natural
selection, of processes determined by innate causes. Why changes should
predominate in the last stage, is a question closely connected with
that of the causes of larval markings in general, and may therefore
be investigated later.
But if we here assume in anticipation that all new markings depend
on adaptation to the conditions of life, and arise through natural
selection, it will not be difficult to draw the conclusion that such
new characters must prevail in the last stage. There are two conditions
favouring this view; the size of the insect, and the longer duration
of the last stage. As long as the caterpillar is so small as to be
entirely covered by a leaf, it only requires a good adaptation in
colour in order to be completely hidden; independently of which, it is
also possible that many of its foes do not consider it worth attacking
at this stage. The last stage, moreover, is of considerably longer
duration than any of the four preceding ones; in _Deilephila Euphorbiæ_
this stage lasts for ten days, whilst the remaining stages have a
duration of four days; in _Sphinx Ligustri_ the last stage also extends
over ten days, and the others over six days.
In its last stage, therefore, a caterpillar is for a longer period
exposed to the danger of being discovered by its foes; and since, at
the same time, its enemies become more numerous, and its increased size
makes it more easy of detection, it is readily conceivable that a
change in the conditions of life, such, for instance, as removal to a
new food-plant, would bring about the adaptation of the _adult larva_
as its chief result.
I shall next proceed to show how far the assumption here made--that all
markings depend on natural selection--is correct.