It has been shown that the phenomenon of seasonal dimorphism has the
same proximate cause as climatic variation, viz. change of climate,
and that it must be regarded as identical in nature with climatic
variation, being distinguished from ordinary, or, as I have designated
it, simple (monomorphic) climatic variation by the fact that, besides
the new form produced by change of climate, the old form continues to
exist in genetic connexion with it, so that old and new forms alternate
with each other according to the season.
Two further questions now present themselves for investigation, viz.
(1) by what means does change of climate induce a change in the marking
and colouring of a butterfly? and (2) to what extent does the climatic
action determine the nature of the change?
With regard to the former question, it must, in the first place, be
decided whether the true effect of climatic change lies in the action
of a high or low temperature on the organism, or whether it may not
perhaps be produced by the accelerated development caused by a high
temperature, and the retarded development caused by a low temperature.
Other factors belonging to the category of external conditions of life
which are included in the term “climate” may be disregarded, as they
are of no importance in these cases. The question under consideration
is difficult to decide, since, on the one hand, warmth and a short
pupal period, and, on the other hand, cold and a long pupal period,
are generally inseparably connected with each other; and without great
caution one may easily be led into fallacies, by attributing to the
influence of causes now acting that which is but the consequence of
long inheritance.
When, in the case of _Araschnia Levana_, even in very cold summers,
_Prorsa_, but never the _Levana_ form, emerges, it would still be
erroneous to conclude that it is only the shorter period of development
of the winter generation, and not the summer warmth, which occasioned
the formation of the _Prorsa_ type. This new form of the species
did not come suddenly into existence, but (as appears sufficiently
from the foregoing experiments) originated in the course of many
generations, during which summer warmth and a short development period
were generally associated together. From the fact that the winter
generation always produces _Levana_, even when the pupæ have not been
exposed to cold but kept in a room, it would be equally erroneous to
infer that the cold of winter had no influence in determining the type.
In this case also the determining causes must have been in operation
during innumerable generations. After the winter form of the species
has become established throughout such a long period, it remains
constant, even when the external influence which produced it (cold) is
occasionally withdrawn.
Experiments cannot further assist us here, since we cannot observe
throughout long periods of time; but there are certain observations,
which to me appear decisive. When, both in Germany and Italy, we see
_Polyommatus Phlæas_ appearing in two generations, of which both the
German ones are alike, whilst in Italy the summer brood is black,
we cannot ascribe this fact to the influence of a shorter period of
development, because this period is the same both in Germany and Italy
(two annual generations), so that it can only be attributed to the
higher temperature of summer.
Many similar cases might be adduced, but the one given suffices for
proof. I am therefore of opinion that it is not the duration of the
period of development which is the cause of change in the formation
of climatic varieties of butterflies, but only the temperature to
which the species is exposed during its pupal existence. In what
manner, then, are we to conceive that warmth acts on the marking
and colouring of a butterfly? This is a question which could only
be completely answered by gaining an insight into the mysterious
chemico-physiological processes by which the butterfly is formed in the
chrysalis; and indeed only by such a complete insight into the most
minute details, which are far beyond our scrutiny, could we arrive
at, or even approximate to, an explanation of the development of any
living organism. Nevertheless an important step can be taken towards
the solution of this problem, by establishing that the change does not
depend essentially upon the action of warmth, but upon the organism
itself, as appears from the nature of the change in one and the same
species.
If we compare the Italian summer form of _Polyommatus Phlæas_ with its
winter form, we shall find that the difference between them consists
only in the brilliant coppery red colour of the latter being largely
suffused in the summer form with black scales. When entomologists speak
of a “black dusting” of the upper side of the wings, this statement
must not of course be understood literally; the number of scales is the
same in both forms, but in the summer variety they are mostly black,
a comparatively small number being red. We might thus be inclined
to infer that, owing to the high temperature, the chemistry of the
material undergoing transformation in _Phlæas_ is changed in such a
manner that less red and more black pigment is produced. But the case
is not so simple, as will appear evident when we consider the fact
that the summer forms have not originated suddenly, but only in the
course of numerous generations; and when we further compare the two
seasonal forms in other species. Thus in _Pieris Napi_ the winter is
distinguished from the summer form, among other characters, by the
strong black dusting of the base of the wings. But we cannot conclude
from this that in the present case more black pigment is produced in
the winter than in the summer form, for in the latter, although the
base of the wings is white, their tips and the black spots on the
fore-wings are larger and of a deeper black than in the winter form.
The quantity of black pigment produced does not distinguish between the
two forms, but the mode of its distribution upon the wings.
Even in the case of species the summer form of which really possesses
far more black than the winter form, as, for instance, _Araschnia
Levana_, one type cannot be derived from the other simply by the
expansion of the black spots present, since on the same place where
in _Levana_ a black band crosses the wings, _Prorsa_, which otherwise
possesses much more black, has a white line. (See Figs. 1-9, Plate
I.) The intermediate forms which have been artificially produced
by the action of cold on the summer generation present a graduated
series, according as reversion is more or less complete; a black spot
first appearing in the middle of the white band of _Prorsa_, and then
becoming enlarged until, finally, in the perfect _Levana_ it unites
with another black triangle proceeding from the front of the band,
and thus becomes fused into a black bar. The white band of _Prorsa_
and the black band of _Levana_ by no means correspond in position; in
_Prorsa_ quite a new pattern appears, which does not originate by a
simple colour replacement of the _Levana_ marking. In the present case,
therefore, there is no doubt that the new form is not produced simply
because a certain pigment (black) is formed in larger quantities, but
because its mode of distribution is at the same time different, white
appearing in some instances where black formerly existed, whilst in
other cases the black remains. Whoever compares _Prorsa_ with _Levana_
will not fail to be struck with the remarkable change of marking
produced by the direct action of external conditions.
The numerous intermediate forms which can be produced artificially
appear to me to furnish a further proof of the gradual character
of the transformation. Ancestral intermediate forms can only occur
where they have once had a former existence in the phyletic series.
Reversion may only take place completely in some particular characters,
whilst in others the new form remains constant--this is in fact the
ordinary form of reversion, and in this manner a mixture of characters
might appear which never existed as a phyletic stage; but particular
characters could certainly never appear unless they were normal to the
species at some stage of phyletic development. Were this possible it
would directly contradict the idea of reversion, according to which
new characters never make their appearance, but only such as have
already existed. If, therefore, the ancestral forms of _A. Levana_
(which we designate as _Porima_) present a great number of transitional
varieties, this leads to the conclusion that the species must have gone
through a long series of stages of phyletic development before the
summer generation had completely changed into _Prorsa_. The view of the
slow cumulative action of climatic influences already submitted, is
thus confirmed.
If warmth is thus without doubt the agency which has gradually changed
the colour and marking of many of our butterflies, it sufficiently
appears from what has just been said concerning the nature of the
change that the chief part in the transmutation is not to be attributed
to the agency in question, but to the organism which is affected by it.
Induced by warmth, there begins a change in the ultimate processes of
the matter undergoing transformation, which increases from generation
to generation, and which not only consists in the appearance of the
colouring matter in one place instead of another, but also in the
replacement of yellow, in one place by white and in another by black,
or in the transformation of black into white on some portions of the
wings, whilst in others black remains. When we consider with what
extreme fidelity the most insignificant details of marking are, in
constant species of butterflies, transmitted from generation to
generation, a total change of the kind under consideration cannot but
appear surprising, and we should not explain it by the nature of the
agency (warmth), but only by the nature of the species affected. The
latter cannot react upon the warmth in the same manner that a solution
of an iron salt reacts upon potassium ferrocyanide or upon sulphuretted
hydrogen; the colouring matter of the butterfly’s wing which was
previously black does not become blue or yellow, nor does that which
was white become changed into black, but a new marking is developed
from the existing one--or, as I may express it in more general terms,
the species takes another course of development; the complicated
chemico-physical processes in the matter composing the pupa become
gradually modified in such a manner that, as the final result, a new
marking and colouring of the butterfly is produced.
Further facts can be adduced in support of the view that in these
processes it is the constitution of the species, and not the external
agency (warmth), which plays the chief part. The latter, as Darwin
has strikingly expressed it, rather performs the function of the
spark which ignites a combustible substance, whilst the character of
the combustion depends upon the nature of the explosive material.
Were this not the case, increased warmth would always change a given
colour[29] in the same manner in all butterflies, and would therefore
always give rise to the production of the same colour. But this does
not occur; _Polyommatus Phlæas_, for example, becoming black in the
south, whilst the red-brown _Vanessa Urticæ_ becomes black in high
northern latitudes, and many other cases well known to entomologists
might be adduced.[30] It indeed appears that species of similar
physical constitution, i.e., nearly allied species, under similar
climatic influences, change in an analogous manner. A beautiful example
of this is furnished by our _Pierinæ_. Most of the species display
seasonal dimorphism; as, for instance, _Pieris Brassicæ_, _Rapæ_,
_Napi_, _Krueperi_, and _Daplidice_, _Euchloe Belia_ and _Belemia_,
and _Leucophasia Sinapis_, in all of which the difference between the
winter and the summer forms is of a precisely similar nature. The
former are characterized by a strong black dusting of the base of the
wings, and by a blackish or green sprinkling of scales on the underside
of the hind wings, while the latter have intensely black tips to the
wings, and frequently also spots on the fore-wings.
Nothing can prove more strikingly, however, that in such cases
everything depends upon the physical constitution, than the fact that
in the same species the males become changed in a different manner to
the females. The parent-form of _Pieris Napi_ (var. _Bryoniæ_) offers
an example. In all the _Pierinæ_ secondary sexual differences are
found, the males being differently marked to the females; the species
are thus sexually dimorphic. Now the male of the Alpine and Polar var.
_Bryoniæ_, which I conceive to be the ancestral form, is scarcely to
be distinguished, as has already been mentioned, from the male of our
German winter form (_P. Napi_, var. _Vernalis_), whilst the female
differs considerably.[31] The gradual climatic change which transformed
the parent-form _Bryoniæ_ into _Napi_ has therefore exerted a much
greater effect on the female than on the male. The external action on
the two sexes was exactly the same, but the response of the organism
was different, and the cause of the difference can only be sought for
in the fine differences of physical constitution which distinguish the
male from the female. If we are unable to define these differences
precisely, we may nevertheless safely conclude from such observations
that they exist.
I have given special prominence to this subject because, in my idea,
Darwin ascribes too much power to sexual selection when he attributes
the formation of secondary sexual characters to the sole action of
this agency. The case of _Bryoniæ_ teaches us that such characters may
arise from purely innate causes; and until experiments have decided
how far the influence of sexual selection extends, we are justified in
believing that the sexual dimorphism of butterflies is due in great
part to the differences of physical constitution between the sexes.
It is quite different with such sexual characters as the stridulating
organs of male Orthoptera which are of undoubted importance to that
sex. These can certainly be attributed with great probability to sexual
selection.
It may perhaps not be superfluous to adduce one more similar case, in
which, however, the male and not the female is the most affected by
climate. In our latitudes, as also in the extreme north, _Polyommatus
Phlæas_, already so often mentioned, is perfectly similar in both
sexes in colour and marking; and the same holds good for the winter
generation of the south. The summer generation of the latter, however,
exhibits a slight sexual dimorphism, the red of the fore wings of the
female being less completely covered with black than in the male.