If, as I have attempted to show, seasonal dimorphism originates
through the slow operation of a changed summer climate, then is this
phenomenon nothing else than the splitting up of a species into two
climatic varieties in the same district, and we may expect to find
various connexions between ordinary simple climatic variation and
seasonal dimorphism. Cases indeed occur in which seasonal dimorphism
and climatic variation pass into each other, and are interwoven in
such a manner that the insight into the origin and nature of seasonal
dimorphism gained experimentally finds confirmation. Before I go more
closely into this subject, however, it is necessary to come to an
understanding as to the conception “climatic variation,” for this term
is often very arbitrarily applied to quite dissimilar phenomena.
According to my view there should be a sharp distinction made between
climatic and local varieties. The former should comprehend only such
cases as originate through the direct action of climatic influences;
while under the general designation of “local forms,” should be
comprised all variations which have their origin in other causes--such,
for example, as in the indirect action of the external conditions of
life, or in circumstances which do not owe their present existence to
climate and external conditions, but rather to those geological changes
which produce isolation. Thus, for instance, ancient species elsewhere
long extinct might be preserved in certain parts of the earth by the
protecting influence of isolation, whilst others which immigrated in
a state of variability might become transformed into local varieties
in such regions through the action of ‘amixia,’[24] i.e. by not being
allowed to cross with their companion forms existing in the other
portions of their habitat. In single cases it may be difficult, or for
the present impossible, to decide whether we have before us a climatic
form, or a local form arising from other causes; but for this very
reason we should be cautious in defining climatic variation.
The statement that climatic forms, in the true sense of the word, do
exist is well known to me, and has been made unhesitatingly by all
zoologists; indeed, a number of authentically observed facts might
be produced, which prove that quite constant changes in a species may
be brought about by the direct action of changed climatic conditions.
With butterflies it is in many cases possible to separate pure climatic
varieties from other local forms, inasmuch as we are dealing with
only unimportant changes and not with those of biological value, so
that natural selection may at the outset be excluded as the cause of
the changes in question. Then again the sharply defined geographical
distribution climatically governed, often furnishes evidence of
transition forms in districts lying between two climatic extremes.
In the following attempt to make clear the relationship between simple
climatic variation and seasonal dimorphism, I shall concern myself only
with such undoubted climatic varieties. A case of this kind, in which
the winter form of a seasonally dimorphic butterfly occurs in other
habitats as the only form, i.e., as a climatic variety, has already
been adduced in a former paragraph. I allude to the case of _Pieris
Napi_, the winter form of which seasonally dimorphic species occurs
in the temperate plains of Europe, whilst in Lapland and the Alps it
is commonly found as a monomorphic climatic variety which is a higher
development of the winter type, viz., the var. _Bryoniæ_.
Very analogous is the case of _Euchloe Belia_, a butterfly likewise
belonging to the _Pierinæ_, which extends from the Mediterranean
countries to the middle of France, and everywhere manifests a very
sharply pronounced seasonal dimorphism. Its summer form was, until
quite recently, described as a distinct species, _E. Ausonia_.
Staudinger was the first to prove by breeding that the supposed two
species were genetically related.[25] This species, in addition to
being found in the countries named, occurs also at a little spot
in the Alps in the neighbourhood of the Simplon Pass. Owing to the
short summer of the Alpine climate the species has in this locality
but one annual brood, which bears the characters of the winter form,
modified in all cases by the coarser thickly scattered hairs of the
body (peculiar to many Alpine butterflies,) and some other slight
differences. The var. _Simplonia_ is thus in the Alps a simple climatic
variety, whilst in the plains of Spain and the South of France it
appears as the winter form of a seasonally dimorphic species.
This _Euchloe_ var. _Simplonia_ obviously corresponds to the var.
_Bryoniæ_ of _Pieris Napi_, and it is highly probable that this form of
_E. Belia_ must likewise be regarded as the parent-form of the species
surviving from the glacial epoch, although it cannot be asserted, as
can be done in the case of _Bryoniæ_, that the type has undergone no
change since that epoch, for _Bryoniæ_ from Lapland is identical with
the Alpine form,[26] whilst _E. Simplonia_ does not appear to occur in
Polar countries.
Very interesting also is the case of _Polyommatus Phlæas_, Linn.,
one of our commonest _Lycænidæ_, which has a very wide distribution,
extending from Lapland to Spain and Sicily.[27] If we compare specimens
of this beautiful copper-coloured butterfly from Lapland with those
from Germany, no constant difference can be detected; the insect has,
however, but one annual generation in Lapland, whilst in Germany it is
double-brooded; but the winter and summer generations resemble each
other completely, and specimens which had been caught in spring on the
Ligurian coast were likewise similarly coloured to those from Sardinia.
(Fig. 21, Plate II.). According to these facts we might believe this
species to be extraordinarily indifferent to climatic influence; but
the South European summer generation differs to a not inconsiderable
extent from the winter generation just mentioned, the brilliant coppery
lustre being nearly covered with a thick sprinkling of black scales.
(Plate II., Fig. 22.) The species has thus become seasonally dimorphic
under the influence of the warm southern climate, although this is
not the case in Germany where it also has two generations in the
year.[28] No one who is acquainted only with the Sardinian summer form,
and not with the winter form of that place, would hesitate to regard
the former as a climatic variety of our _P. Phlæas_; or, conversely,
the north German form as a climatic variety of the southern summer
form--according as he accepts the one or the other as the primary form
of the species.
Still more complex are the conditions in another species of _Lycænidæ_,
_Plebeius Agestis_ (= _Alexis_ Scop.), which presents a double seasonal
dimorphism. This butterfly appears in three forms; in Germany A and B
alternate with each other as winter and summer forms, whilst in Italy
B and C succeed each other as winter and summer forms. The form B
thus occurs in both climates, appearing as the summer form in Germany
and as the winter form in Italy. The German winter variety A, is
entirely absent in Italy (as I know from numerous specimens which I
have caught), whilst the Italian summer form, on the other hand, (var.
_Allous_, Gerh.), does not occur in Germany. The distinctions between
the three forms are sufficiently striking. The form A (Fig. 18, Plate
II.) is blackish-brown on the upper side, and has in the most strongly
marked specimens only a trace of narrow red spots round the borders;
whilst the form B (Fig. 19, Plate II.) is ornamented with vivid red
border spots; and C (Fig. 20, Plate II.) is distinguished from B by the
strong yellowish-brown of the under side. If we had before us only the
German winter and the Italian summer forms, we should, without doubt,
regard them as climatic varieties; but they are connected by the form
B, interpolated in the course of the development of both, and the two
extremes thus maintain the character of mere seasonal forms.