It is shown by those species in which only one stage is di- or
polymorphic that not every change in the one stage entails a
corresponding change in the other. Thus, in all seasonally dimorphic
species we find that the caterpillars of butterflies which are often
widely different in the colour and marking of their successive
generations are absolutely identical. On the other hand, many species
can be adduced of which the larvæ are dimorphic whilst the imagines
occur only in one form (compare the first and second essays in this
volume).
There are however facts which directly prove that any one stage can
change independently of the others; I refer to the circumstance that
any one stage may become independently variable--that the property of
greater variability or of greater constancy by no means always occurs
in an equal degree in all the three stages of larva, pupa, and imago,
but that sometimes the caterpillar is very variable and the pupa and
imago quite constant. On the other hand, all three stages may be
equally variable or equally constant, although this seldom occurs.
If variability is to be understood as indicating the period of
re-modelling of a living form, whether in its totality or only in
single characters or groups of characters, from the simple fact of the
heterochronic variability of the ontogenetic stages, it follows that
the latter can be modified individually, and that the re-modelling
of one stage by no means necessarily entails that of the others. It
cannot however be doubted that variability, from whatever cause it may
have arisen, is in all cases competent to produce a new form. From the
continued crossing of variable individuals alone, an equalization of
differences must at length take place, and with this a new, although
not always a widely deviating, constant form must arise.
That the different stages of development of a species may actually be
partly variable and partly constant, and that the variable or constant
character of one stage has no influence on the other stages, is shown
by the following cases, which are, at the same time, well adapted to
throw light on the causes of variability, and are thus calculated to
contribute towards the solution of the main problem with which this
investigation is concerned.
When, in the following pages, I speak of _variability_, I do not
refer to the occurrence of local varieties, or to variations which
occur in the course of time, but I mean a high degree of individual
variability--a considerable fluctuation of characters in the
individuals of one and the same district or of the same brood. I
consider a species to be constant, on the other hand, when the
individuals from a small or large district differ from one another only
to a very slight extent. Constant forms are likewise generally, but not
invariably, such as are poor in local varieties, whilst variable forms
are those which are rich in such variations. Since the terms “variable”
and “constant” are but relative, I will confine myself to the most
extreme cases, those in which the individual peculiarities fluctuate
within very wide or very narrow limits.
As no observations upon the degree of variability shown by a species in
the different stages of its development were available, I was obliged
to fall back upon my own, at least so far as relates to the larval and
pupal stages, whilst for the imaginal stage the wide experience of my
esteemed friend Dr. Staudinger has been of essential service to me.
Let us in the first place confine our attention to the three chief
forms which every Lepidopteron presents, viz. larva, pupa, and imago.
With respect to the constancy or variability of these three forms, we
actually find in nature all the combinations which are theoretically
conceivable.
(1.) There are species which possess a high degree of constancy in
all three stages, such, for example, as _Limenitis Camilla_, _Pieris
Brassicæ_,[171] _Sphinx Ligustri_, and _Euchelia Jacobææ_.
(2.) There are species showing a high degree of variability in all
three stages. This case must be of rare occurrence, as I am only able
to adduce _Araschnia Prorsa-Levana_, a fact which arises from the
circumstance that the pupal stage is, as a rule, but seldom variable.
(3.) There are species which are variable in two stages and constant
in the third. To this class, for example, belongs _Smerinthus Tiliæ_,
of which the larva and imago are very variable, whilst the pupa is
quite constant. The same is the case with _Lasiocampa Pini_, the
well-known fir moth. Many butterflies show this same phenomenon in
other combinations, such, for instance, as _Vanessa Urticæ_ and
_Polychloros_, in which the larva and pupa are very variable, and the
imago very constant. In a less degree the same is also the case with
_Vanessa Atalanta_, whilst in _Pieris Napi_ the pupa and imago are
variable, and the caterpillar remarkably constant, this likewise being
the case with the local form _Bryoniæ_, which, according to my theory,
is to be regarded as the parent form of _Napi_ (See Part I. of the
present volume).
(4.) There are species which are constant in two stages, and variable
only in the third. Thus, a few species can be found in which the
larva and pupa are constant and the imago variable. This is the case
with _Saturnia Yamamai_, the imago of which is well known to present
numberless shades of colour, varying from light yellow to greyish
black, whilst the green caterpillar shows only slight individual
differences of marking, and scarcely any differences of colour. The
pupa of this species is quite constant. _Arctia Caja_ and _Hebe_, and
_Chelonia Plantaginis_ belong to this same category.
There are a very large number of species which possess very constant
imagines and pupæ, but extremely variable larvæ. The following are
the cases known to me:--_Macroglossa Stellatarum_, _Fuciformis_
and _Bombyliformis_; _Chærocampa Elpenor_, _Celerio_, and _Nerii_;
_Deilephila Galii_, _Livornica_, Hübn., _Hippophaës_, _Vespertilio_,
and _Zygophylli_; _Sphinx Convolvuli_; _Acherontia Atropos_;
_Smerinthus Ocellatus_ and _Tiliæ_; _Callimorpha Hera_; _Cucullia
Verbasci_ and _Scrophulariæ_.
Cases in which the variability depends entirely upon the pupa, while
the larva and imago are extremely constant, are of great rarity.
_Vanessa Io_ is a case in point, the pupa being light or dark brown, or
bright golden green, whilst in the two other stages scarcely any light
shades of colour or variations in the very complicated marking are to
be met with.
The facts thus justify the above view that the individual stages of
development change independently--that a change occurring in one stage
is without influence on the preceding and succeeding stages. Were this
not the case no one stage could possibly become variable without all
the other stages becoming so. Did there exist a correlation between
larvæ, pupæ, and imagines of such a nature that every change in the
larva entailed a corresponding change in the imago, as soon as a large
number of larval characters became fluctuating (_i.e._ as soon as this
stage became variable), a large number of imaginal characters would
necessarily also become fluctuating (_i.e._ this stage would also
become correspondingly variable).
There is one other interpretation which might perhaps be attempted from
the point of view of the old doctrine of species. It might be said that
it is a special property of certain larval or imaginal markings to be
variable whilst others are constant, and since the larval and imaginal
markings of a species are generally quite distinct, it may easily
happen that a butterfly possessing markings having the property of
constancy may belong to a caterpillar having variable markings.
There is a soul of truth underlying this objection, since it is
true that the various forms of markings which occur in Lepidoptera
apparently reach different degrees of constancy. If we speak of the
constancy or variability of a species, a different meaning is attached
to these expressions according as we are dealing _e.g._ with a species
of _Sphinx_ or a species of _Arctia_. That which in the latter would
be estimated as a high degree of constancy, in the former would be
taken as a considerable amount of variability. It is of interest, in
connection with the question as to the causes of constancy, to note
that the power of any form of marking to attain to a high degree of
constancy is by no means inversely proportional to the complication of
the marking, as would have been expected _à priori_.
Thus, the species of _Sphinx_ and of allied genera possess on their
fore-wings, which are mostly coloured with a mixture of dull grey,
white and black, an exceedingly complicated arrangement of lines
which, in constant species, show a high degree of uniformity: on the
other hand, the checquered fore-wings of our _Arctiidæ_, which are far
more coarsely marked, always show, even in the most constant species,
well-marked individual differences. The different types of marking must
therefore be measured by different standards.
But in granting this, we decidedly refute the statement that constancy
and variability are inherent properties of certain forms of marking.
This reasoning is based on the simple fact that a given type of marking
comprises both species of great constancy and of (relatively) great
variability.
Thus, the fore-wings of _Sphinx Ligustri_ and _S. Convolvuli_ are
extremely constant, whilst the very similarly marked _Anceryx
(Hyloicus) Pinastri_ is exceedingly variable. Similarly _Deilephila
Euphorbiæ_ is known by its great variability of colouring and marking,
whilst _D. Galii_, which resembles this species so closely as to be
sometimes confounded with it, possesses a high degree of constancy,
and further, the Corsican and Sardinian _D. Dahlii_ is very variable.
Among the family _Arctiidæ_, _Callimorpha Hera_ and the Alpine _Arctia
Flavia_ are cases of constancy, whilst _A. Caja_, which is so similar
to the last species, is so generally variable that two perfectly
identical specimens can scarcely be found together.
The same can be shown to hold good for the markings of caterpillars.
Thus, the larva of _D. Dahlii_ shows very considerable variability,
whilst that of _D. Galii_ is very constant in marking (disregarding the
ground-colour). So also the larva of _Vanessa Urticæ_ is very variable
and that of _V. Antiopa_ very constant, &c.
The great differences with respect to constancy or variability which
are displayed by the different stages of one and the same species,
must therefore find their explanation elsewhere than in the type of
the marking itself. The explanation must be found in the circumstance
that each stage changes independently of the others, and at different
periods can enter a new phase of variability.
We are here led in anticipation to the main question:--Are changes
produced by internal or external causes? is it the physical nature of
the organism which is compelled to become remoulded spontaneously after
the lapse of a certain period of time? or does such modification only
occur when produced directly or indirectly by the external conditions
of life?
In the cases before us the facts undoubtedly indicate a complete
dependence of the transformations upon external conditions of life.
The independent appearance of variability in the separate stages of the
metamorphosis might, however, be regarded as only apparent. It might
still be attempted to attribute the changes to a purely inherent cause,
_i.e._, to a phyletic vital force, by assuming that the latter acts
periodically in such a manner that at first one and then the following
stage becomes variable, until finally the entire species is transformed.
There is but little to be said in reply to this if we once take refuge
in entirely unknown forces, the operation of which can be arbitrarily
conceived to be either constant or periodic.
But granting that such a transforming power exists and acts
periodically, the variability must always pass over the different
stages in a fixed direction, like a wave over the surface of
water--imago, pupa, and larva, or larva, pupa, and imago, must
_successively_ become variable. Cases like that of _Araschnia
Prorsa_, in which all three stages are variable, may certainly be
thus explained, but those instances in which the larva and imago
are extremely variable, and the pupa quite constant, are entirely
inexplicable from this point of view.
The latter can, however, be very simply explained if we suppose the
changes to be dependent upon external influences. From this standpoint
we not only see how it is possible that an intermediate stage should
remain uninfluenced by the changes which affect the two other stages,
but we can also understand why it should just be the pupal stage that
plays this part so frequently. If we ask why most pupæ are constant
and are relatively but very slightly variable, the answer will be
found in the facts that all pupæ which remain concealed in the earth
or inside plants (_Sesiidæ_), or which are protected by stout cocoons,
show complete constancy, whilst any considerable amount of variability
occurs only in those pupæ which are suspended or openly exposed. This
is closely connected with a fact to which I have called attention on a
former occasion,[172] viz., that dimorphism occurs in certain pupæ,
but only in those which are openly exposed and which are therefore
visible to their foes. I am only acquainted with such cases among the
pupæ of butterflies, and it is likewise only among these that I have
found any considerable amount of variability.
Facts of this kind indicate that Nature does not uselessly sport with
forms, but that at any rate changes of this sort result from external
influences. The greater frequency of variability among larvæ and its
comparative rarity in imagines is also undoubtedly in favour of this
view.
It has already been shown that species with variable larvæ and constant
imagines are extremely common, but that those with constant larvæ
and variable imagines are very rare. This confirms the conclusions,
already drawn above, first, that the variability of the imago cannot
owe its existence to the variability of the larvæ, and secondly, that
the causes which produce variability affect the larval condition more
commonly than that of the imago.
Where can these causes be otherwise sought than in the external
conditions of life, which are so widely different in the two stages,
and which are much more variable for the larva than for the imago?
Let us take the species of one genus, _e.g._ those of _Deilephila_.
The imagines of our European species--as far as we know--all live in
precisely the same manner; they all fly at twilight,[173] showing a
preference for the same flowers and very often frequenting the same
spots, so that in the haunts of one species the others are almost
always to be met with, supposing them to occur in the same locality.
They conceal themselves by day in similar places, and are attacked by
similar foes.
It is quite different with the caterpillars. These, even in the case of
the most closely allied species, live under different conditions, as
appears from the fact that they feed on different plants. The latter
can, however, produce changes both directly and indirectly. The larvæ
may acquire adaptive colours and markings, and these would vary in
accordance with the colour and structure of the food-plant; or they may
become brightly coloured as a sign of distastefulness in cases where
they are inedible. Then again the colour of the soil on which the larvæ
live would act upon their colours making these adaptive. Certain habits
of the caterpillars may also be dependent upon the nature of their
food-plants. Thus, _e.g._ _Deilephila Hippophaës_ feeds only at night,
and conceals itself by day under moss and among the leaves at the base
of the food-plant; but _D. Euphorbiæ_ could not acquire such a habit,
because _Euphorbia Cyparissias_ generally grows on arid soil which is
poor in vegetation, and which therefore affords no concealment, and
furthermore, because a caterpillar, as long as it continues to feed,
cannot, and as a matter of fact does not, ever wander far from its
food-plant. A habit of concealment by burying in the earth also, such
for example as occurs in _Acherontia Atropos_, could not be acquired by
_D. Euphorbiæ_, because its food-plant generally grows on hard, dry,
and stony ground.
In addition to these considerations, the foes would be different
according as the caterpillar lived on plants which formed dense
thickets covering large extents of the shore (_Hippophae_) or grew
isolated on dry hillocks and declivities where the herbage was scanty
or altogether absent; or again, according as the insect, in conjunction
with such local differences, fed by day or had acquired the habit
of feeding only by night. It must in fact be admitted that new and
improved adaptations, or, in more general terms, that inducements to
change, when depending on the environment, must be more frequently
dissimilar for larvæ than for the imagines. We must accordingly expect
to find actual change, or that condition of variability which may be
regarded as initiative to change, occurring more commonly in larvæ than
in perfect insects.
Since facts are in complete accordance with the results of these _à
priori_ considerations we may also venture to conclude that the basis
of the considerations is likewise correct, viz., the supposition that
the changes of colour and marking in caterpillars, pupæ, and imagines
result from external influences only.
This must not be taken as signifying that the single stages of the
larval development are also only able to change through the action of
external influences. The larval stages are correlated with each other,
as has already been shown (see the previous essay): new characters
arise in the adult caterpillar at the last stage and are then gradually
transferred back to the younger stages quite independently of external
influences, this recession being entirely brought about by the laws of
correlation. Natural selection here only exerts a secondary action,
since it can accelerate or retard this transference, according as the
new characters are advantageous or disadvantageous to the younger
stages.
Now as considerable individual differences appear in the first
acquisition of a new character with respect to the rapidity and
completeness with which the individuals acquire such a character, the
same must obtain for the transference of an improvement acquired in
the last stage to the next younger stage. The new character would be
acquired by different individuals in different degrees and at different
rates--it would have, to a certain extent, to struggle with the older
characters of the stage; in brief, the younger stage would become
variable.
Variability of this kind might well be designated as _secondary_,
in contradistinction to _primary_ variability; the latter (primary)
depends upon an unequal reaction of the individual organisms to
external influences, the former (secondary) results from the unequal
strength and rate of the action of the innate laws of growth governing
the organism. In both cases alike exceeding variability may occur, but
the causes producing this variability are dissimilar.
The different stages of larval development would thus frequently
display independent variability in a manner similar to the pupal or
imaginal stages, since they can show individual variability while
the other stages of development remain constant. This appearance
of independent variability in the different stages of the larval
development, however, is in truth deceptive--we have here in fact
a kind of wave of variability, which passes downwards through the
developmental stages, becoming gradually weaker, and finally dying out
completely.
In accordance with this, we very frequently find that only the last or
two last stages are variable, while the younger stages are constant.
Thus in _Macroglossa Stellatarum_, the larvæ are constant in the first,
second, and third stages, but become variable in the fourth, and in
the fifth stage first show that high degree of variability which has
already been described in detail (See. Pl. III., Figs. 3-12). The
larvæ, of _Vanessa Cardui_ also, according to my notes, are extremely
constant in the first four stages in spite of their complicated
marking, but become variable in the fifth stage, although to no very
great extent.
In _Smerinthus Tiliæ_, _Ocellatus_ and _Populi_ also, the greatest
larval variability is shown only in the last stage, the preceding
stages being very constant. These cases by no means depend upon the
marking of the young stages being simpler and therefore being less
capable of varying. The reverse case also occurs. In a somewhat similar
manner as the young of the tapir and wild hog are striped, while the
adult animals are plainly coloured, the young caterpillars of _Saturnia
Yamamai_ possess longitudinal black lines on a yellow ground, while as
early as in the second stage a simple green colour appears in the place
of this complicated but perfectly constant marking. If the young stages
are so frequently constant, this rather depends upon the fact that the
transference of a new character to these stages not only takes place
gradually, but also with continually diminishing energy, in a manner
somewhat similar to physical motion, which continually diminishes in
speed by the action of resistance till it is completely arrested. This
constancy of the younger stages may further be due to the circumstance
that the characters would only be transferred when they had become
fixed in the last stage, and were consequently no longer variable. The
transferred characters may thus have acquired a greater regularity,
_i.e._ a less degree of variability, than they possessed at their first
origination. Extensive investigations in this special direction must
be made if the precise laws, in accordance with which the backward
transference of new characters takes place, are to be discovered. By
such researches only should we arrive with certainty at the causes
which determine the lesser variability of the young larval stages.
It may also occur that the early stages are variable, whilst the
later stages are constant, although this case appears to happen less
frequently. Thus, the caterpillars of _Gastropacha Quercifolia_ vary
considerably in the second stage but are constant at a later period,
and the same is the case with _Spilosoma Urticæ_, which in the second
stage may be almost considered to be dimorphic, but which subsequently
becomes constant.
Cases in which the first stage is variable appear to be of the least
frequent occurrence. I know of only one such instance, viz., _Anceryx
Pinastri_, of which the newly hatched larvæ (Pl. VI., Fig. 53) show
considerable differences in the brownish-black crescentic spots. The
second (Fig. 54), third, and fourth stages are then tolerably constant,
while the fifth stage again is very variable.
An instance of this kind can be easily explained by two waves of
variation, the first of which now affects only the first stage,
while the second has just commenced to affect the fifth stage. Such
a supposition is not opposed to any theoretical considerations, but
rather has much probability in its favour, since we know that species
are from time to time subject to be remodelled; and further, that the
coalescence of several stages of phyletic development in the ontogeny
of one and the same species (see p. 226, development of the genus
_Deilephila_) shows that during the backward transference of one
character, new characters may appear in the last stage of the ontogeny,
and indeed very frequently at a time when the next youngest character
has not been transferred back so far as to the first stage.
That this secondary variability is to a certain extent brought about by
the conflict between the old and new characters, the latter striving to
suppress the former, is shown by the caterpillar of _Saturnia Carpini_
which I have observed for many years from this point of view, and than
which I do not know a more beautiful illustration.
When these larvæ leave the egg they are black, but in the adult state
are almost bright green--this at least being the case in a local form
which, from the district in the vicinity of Genoa where it is found,
I will designate as the var. _Ligurica_. Now whilst these two extreme
stages of development are relatively constant, the intermediate stages
show a variability which becomes greater the nearer the last stage
is approached, this variation in the marking depending simply on the
struggle between the green colour and the more anciently inherited
black. In this manner there arises, especially in the fourth stage
of the German local form, an incredible mixture of the most diverse
markings, all of which can, however, be very easily explained from the
foregoing point of view.
The simpler and, as I am inclined to believe, the older form of the
transformation is presented to us in the local variety _Ligurica_. In
the last stage, when 7.5 centimeters long, this form is of a beautiful
bright green colour without any trace of black marking[174] (Pl. VIII.,
Fig. 77). The colour of the six orange warts which are situated on
each segment is also similar in all specimens, so that this stage is
perfectly constant.
Our German _S. Carpini_ shows different characters in the fifth stage.
It is true that individual specimens occur which are entirely green
without any black, but these are rare; the majority possess a more or
less broad black ring encircling the middle of each segment (Pl. VIII.,
Figs. 78 and 79). Those specimens in which the black ring has become
broken up into large or small spots surrounding the base of the warts
constitute intermediate forms (Fig. 80). The last stage of the German
local form, unlike that of the Genoese local form, is therefore very
variable.
The two forms, moreover, do not simply differ in being more or less
advanced in phyletic development, but also in several other points. As
it is of great theoretical interest to show that a species can develop
local differences only in the stage of larva, I will here subjoin the
plain facts.
The differences consist in that the Genoese local form goes through
five moults whilst the German local form, like most caterpillars,
has only four moults. Further, in the Genoese form the light green,
which is also possessed by the German form in the fourth stage, when
it once appears, is retained to the end of the larval development,
whilst in the fifth stage of the German form this colour is replaced
by a dull greyish-green (compare Figs. 77 and 78). There is further
a very considerable difference in the earlier stages which shows
that the phyletic transforming process has taken a quite independent
course in the two forms. Since the struggle between the green and
black--retaining this idea--appears to be quite finished in the last
stage of the Genoese form, we should expect that the new colour, green,
would now also have encroached further upon the younger stages than
in the German form. Nevertheless, this is not the case, but quite
the reverse happens, the black maintaining its ground longer in the
Italian than in the German form.
In the Genoese form the two first stages are completely black, and
in the third stage an orange-yellow lateral stripe first appears. In
the German form this stripe appears in the second stage, and there
is not subsequently added, at least on the middle segments, a yellow
border surrounding some of the warts of the median series. In the
third stage, however, the yellow (which is but the precursor of the
later green colour) becomes further extended, so that the caterpillars
often appear of an orange colour, some or all of the warts and certain
spots and stripes only being black (Figs. 66 and 68). The warts are
also often yellow while the ground remains in most part black--in
brief, the bright colour is in full struggle with the black, and an
endless series of variations is the result of this conflict, whilst in
the corresponding stage of the Genoese form almost complete constancy
prevails.
This constancy remains also in the following (fourth) stage, the
caterpillar still being deep black, only the yellow (sulphur-coloured)
lateral stripe, which has now become brighter, indicating the impending
change (Fig. 67). This takes place in the fifth stage, in which the
ground-colour suddenly becomes bright green, the black remaining at
most only in traces on the anterior edges of the segments.
This is the same marking as is shown by the fourth stage of the German
form, only in this case individuals quite destitute of black do not
occur. In many specimens indeed black forms the ground-colour, the
green only appearing in certain spots (Figs. 71 to 75); in others the
green predominates, and these two extremes are connected by innumerable
intermediate forms, so that this stage must be regarded as the most
variable of all.
The sixth stage of the Genoese and the fifth of the German form have
already been compared together. The results may be thus tabulated:--
_A. German form._ _B. Genoese form._
STAGE I. 9 days. 9 days.
Black; constant. Black; constant.
STAGE II. 8 days. 11 days.
Black, with orange-yellow Black; constant.
lateral stripe; variable.
STAGE III. 5 days (in some 12 days.
cases as much as 16 days).
Black, with yellow; very Black, with orange-yellow
variable. lateral stripes; constant.
STAGE IV. 16 days (in some 6 days.
cases only 5 days).
Bright green and black, Black, with bright yellowish
mixed; very variable. lateral stripe; constant.
STAGE V. 6 days (frequently 6 days.
longer).
Dark green, with or without Bright green, small traces
black bands; variable. of black; variable.
STAGE VI. Pupation. 18 days.
Bright green, without any
black; constant.
STAGE VII. Pupation.
From this comparison we perceive that the process of transformation
has at least become preliminarily concluded in the Genoese form. Why
the backward transference of the newly-acquired character to the young
stages has not yet occurred, or, at least, why it is not in progress,
does not appear; neither can it be stated whether this will take place
later, although we may venture to suppose that such will be the case.
At first sight but a relatively short time appears necessary for the
single stage V., which is still in a state of fluctuation (variable),
to become constant by continued crossing, like all the other stages.
That the transformation is still in full progress in the German form,
is shown by the fact that in this case all the stages are variable with
the exception of the first--the second stage being only variable to a
small extent, the third to a much greater extent, and the fourth to the
highest degree conceivable, whilst the fifth and last stage is again
less variable--so that the greatest struggle between the old and new
characters takes place in the fourth stage.
Among the innumerable variations presented by this last stage a
complete series of transitional forms can be arranged so as to show the
gradual conquest of the black by the green, and thus indicating, step
by step, the course which the latter colour has taken.
In the blackest specimens there is nothing green but the lateral
(infra-spiracular) line which was yellow in the preceding stage, and a
crescent-shaped streak at the base of the middle warts together with
a still smaller crescent at the base of the upper warts (Figs. 71 and
81). These spots become extended in lighter specimens and approximate
so as to leave only narrow black bridges, a third spot being added at
the posterior edge of the warts (Figs. 72 and 82). The three spots
then extend on all sides, still leaving for a long period narrow
black lines at the boundaries where their growth has caused them to
abut. In this manner there frequently arises on the green ground a
true hieroglyphic-like marking (Figs. 85 and 86). Finally the black
disappears from the anterior edge and diminishes on the middle line of
the back where it still partly remains as a T-shaped figure (Figs. 73
and 74), although generally replaced elsewhere by the green with the
exception of small residues.
One point remained for a long time inexplicable to me, viz., the change
of the light green into dark grey-green which appeared in the last
stage in connection with a total change of the black marking.
Supposing that new characters are actually acquired only in the last
stage, and that from this they are transferred to the younger stages,
we should expect to find completely developed in the last stage the
same colouring and markings as are possessed more or less incompletely
in the fourth stage. Now since the developmental tendency to the
removal of black and to the predominance of green--if we may thus
venture to express it--is obvious in the fourth stage, we may expect to
find in the fifth stage a bright green ground-colour, either without
any mixture of black or with such black spots and streaks as were
retained in the fourth stage as residues of the original ground-colour.
But instead of this the fifth stage shows a dark green colour, and a
more or less developed black marking which cannot in any way be derived
from that of the fourth stage.
The Genoese local form observed last year first gave me an explanation
to the extent that in this form the last stage is actually only the
potential penultimate stage, or, more correctly expressed, that
the same characters which at present distinguish the last stage of
this form, are already more or less completely transferred to the
penultimate stage.
The apparently paradoxical behaviour of the German form can be
explained by supposing that before the pure bright green had become
completely transferred to the penultimate stage a further change
appeared in the last stage, the green ground-colour becoming darker,
and black transverse bands being formed. The marking of the last
stage would then be regarded as the reverse of that of the preceding
stage; the absence of black would be the older, simple black spots at
the base of the warts the next in succession, and a connected black
transverse band the most advanced state of the development.
Whether this explanation is correct, and if so, what causes have
produced the second change, may perhaps be learnt at some future
time by a comparison with the ontogeny of other _Saturniidæ_; in the
meantime this explanation receives support from another side by the
behaviour of the Genoese local form. If the last stage of the German
form has actually commenced to be again re-modelled, then this variety
is further advanced in phyletic development than the Genoese form; and
this corresponds entirely with the theory that in the former the light
colour (the orange considered as preliminary to the transformation into
green) has already been carried down into the second stage, whilst in
the Genoese variety even in the fourth stage only the first rudiments
of the colour-transformation show themselves.
The Genoese form is to a certain extent intermediate between the
German form of _Saturnia Carpini_ and the nearly related _S. Spini_,
a species inhabiting East Germany. In this latter the larvæ, even in
the adult state, are completely black with yellow warts. This form of
caterpillar must therefore be regarded as phyletically the oldest, and
this very well agrees with the character of the moth, which differs
essentially from _S. Carpini_ only in not being sexually dimorphic. In
_Carpini_ the male possesses a far more brilliant colouring than the
female, the latter agreeing so completely with the female of _Spini_
that it can hardly be distinguished therefrom, especially in the case
of the somewhat larger South European specimens of the last species.
Now as the more simple colouring of the female must in any case be
regarded as the original form, we must consider _Spini_, both sexes
of which possess this colouring, to be phyletically the older form,
and _Carpini_, the male of which has become differently coloured, must
be considered as the younger type. This completely accords with the
characters of the larvæ.
I must here mention that I have also asked myself the question whether
the variations of the different larval stages are connected together as
cause and effect--whether the lightest specimens of the fifth stage may
perhaps not also have been the lightest individuals of the third and
fourth stages.
Such relationship is only apparent between the third and fourth stages;
the darkest larvæ of the third stage become the darker varieties of the
fourth stage, although it is true that the lighter forms of the third
sometimes also become dark varieties in the fourth stage. Between the
fourth and fifth stages there is scarcely any connection of this kind
to be recognized. Thus, the darkest varieties of the fourth stage
sometimes become the lightest forms of the fifth stage, whilst in other
cases from the lightest individuals of the fourth stage there arise
all the possible modifications of the fifth stage. Further details may
be omitted: the negative result cannot cause any surprise, as it is a
necessary consequence of the continued crossing that must take place.
We thus see that the three chief stages of development (larva, pupa,
and imago) actually change in colour independently of each other,
the single stages of the larval development being however in greater
dependence upon one another, and being connected indeed in such a
manner that a new character cannot be added to the last stage without
being transferred in the course of time to the preceding stage, and at
a later period from this again even to the youngest stage, supposing
it not to be previously delayed in the course of its transference by
unknown opposing forces. On this last point, however, the facts at
present available do not admit of any certain decision.
But why do the individual larval stages behave in this respect so
very differently to the chief stages of the whole development? why
are the former so exactly correlated whilst the latter are not? If
new characters have a general tendency to become transferred to the
younger ontogenetic stages, why are not new imaginal characters first
transferred to the pupa, and finally to the larva?
The answer to these questions is not far to find. The wider two stages
of a species differ in structure, the less does correlation become
possible; the nearer the two stages are morphologically related, the
more powerful does the action of correlation become. It is readily
conceivable that the more widely two succeeding stages deviate
in structure and mode of life, the less possible does it become
for characters to be transferred from one to the other. How is it
possible, for example, that a new character in the proboscis or on the
wings of a butterfly can be transferred to the caterpillar? If such
correlation existed it could only manifest itself by some other part
of the caterpillar changing in correspondence with the change of the
proboscis or wings of the butterfly. That this is not the case has, in
my opinion, been conclusively shown by all the foregoing considerations
respecting the independent variability of the chief stages of the
metamorphosis.
There are, moreover, an endless number of facts which prove the
independence of the individual stages of development--I refer to
the multitudinous phenomena presented by metamorphosis itself.
The existence of that form of development which we designate as
metamorphosis is alone sufficient to prove incontestably that the
single stages are able to change independently of one another to a most
remarkable extent.
If we now ask the question: how has the so-called “complete”
metamorphosis of insects arisen? the answer can only be: through the
gradual adaptation of the different stages of development to conditions
of life which have continually deviated more and more widely from each
other.[175]
But if individual stages of the post-embryonic development can finally
attain to such complete diversity of structure as that of the larva and
imago through gradual adaptations to continually diverging conditions
of life, this shows that the characters acquired by the single stages
are always only transferred to the same stages of the following
generation, whilst the other stages remain uninfluenced thereby. This
depends upon that form of heredity designated by Darwin “inheritance at
corresponding periods of life,” and by Haeckel “homochronic heredity.”