If we may consider it to be established that seasonal dimorphism is
nothing else than the splitting up of a species into two climatic
varieties in one and the same locality, the further question at once
arises why all polygoneutic species (those which produce more than
_one_ annual generation) are not seasonally dimorphic.
To answer this, it will be necessary to go more deeply into the
development of seasonal dimorphism. This evidently depends upon a
peculiar kind of periodic, alternating heredity, which we might be
tempted to identify with Darwin’s “inheritance at corresponding
periods of life.” It does not, however, in any way completely agree
with this principle, although it presents a great analogy to it and
must depend ultimately upon the same cause. The Darwinian “inheritance
at corresponding periods of life”--or, as it is termed by Haeckel,
“homochronic heredity”--is characterized by the fact that new
characters always appear in the individuals at the same stage of life
as that in which they appeared in their progenitors. The truth of this
principle has been firmly established, instances being known in which
both the first appearance of a new (especially pathological) character
and its transmission through several generations has been observed.
Seasonally dimorphic butterflies also furnish a further valuable proof
of this principle, since they show that not only variations which
arise suddenly (and which are therefore probably due to purely innate
causes) follow this mode of inheritance, but also that characters
gradually called forth by the influence of external conditions and
accumulating from generation to generation, are only inherited at that
period of life in which these conditions were or are effective. In all
seasonally dimorphic butterflies which I have been able to examine
closely, I found the caterpillars of the summer and winter broods to
be perfectly identical. The influences which, by acting on the pupæ,
split up the imagines into two climatic forms, were thus without effect
on the earlier stages of development. I may specially mention that
the caterpillars, as well as the pupæ and eggs of _A. Levana_, are
perfectly alike both in the summer and winter forms; and the same is
the case in the corresponding stages of _P. Napi_ and _P. Bryoniæ_.
I shall not here attempt to enter more deeply into the nature of the
phenomena of inheritance. It is sufficient to have confirmed the law
that influences which act only on certain stages in the development of
the individual, even when the action is cumulative and not sudden, only
affect those particular stages without having any effect on the earlier
or later stages. This law is obviously of the greatest importance to
the comprehension of metamorphosis. Lubbock[32] has briefly shown in a
very clear manner how the existence of metamorphosis in insects can be
explained by the indirect action of varying conditions on the different
life-stages of a species. Thus the mandibles of a caterpillar are,
by adaptation to another mode of nourishment, exchanged at a later
period of life for a suctorial organ. Such adaptation of the various
development-stages of a species to the different conditions of life
would never give rise to metamorphosis, if the law of homochronic, or
periodic, heredity did not cause the characters gradually acquired at
a given stage to be transferred to the same stage of the following
generation.
The origin of seasonal dimorphism depends upon a very similar law, or
rather form, of inheritance, which differs from that above considered
only in the fact that, instead of the ontogenetic stages, a whole
series of generations is influenced. This form of inheritance may
be formulated somewhat as follows:--When dissimilar conditions
alternatingly influence a series of generations, a cycle is produced
in which the changes are transmitted only to those generations
which are acted upon by corresponding conditions, and not to the
intermediate ones. Characters which have arisen by the action of a
summer climate are inherited by the summer generation only, whilst
they remain latent in the winter generation. It is the same as with
the mandibles of a caterpillar which are latent in the butterfly, and
again make their appearance in the corresponding (larval) stage of the
succeeding generation. This is not mere hypothesis, but the legitimate
inference from the facts. If it be admitted that my conception of
seasonal dimorphism as a double climatic variation is correct, the law
of “cyclical heredity,”[33] as I may term it--in contradistinction
to “homochronic heredity,” which relates only to the ontogenetic
stages--immediately follows. All those cases which come under the
designation of ‘alternation of generation,’ can obviously be referred
to cyclical heredity, as will be explained further on. In the one case
the successive generations deport themselves exactly in the same
manner as do the successive stages of development of the individual in
the other; and we may conclude therefrom (as has long been admitted on
other grounds) that a generation is, in fact, nothing else than a stage
of development in the life of a species. This appears to me to furnish
a beautiful confirmation of the theory of descent.
Now if, returning to questions previously solved, the alternating
action of cold in winter and warmth in summer leads to the production
of a winter and summer form, according to the law of cyclical heredity,
the question still remains: why do we not find seasonal dimorphism in
all polygoneutic butterflies?
We might at first suppose that all species are not equally sensitive
to the influence of temperature: indeed, the various amounts
of difference between the winter and summer forms in different
species would certainly show the existence of different degrees of
sensitiveness to the modifying action of temperature. But even this
does not furnish an explanation, since there are butterflies which
produce two perfectly similar[34] generations wherever they occur,
and which, nevertheless, appear in different climates as climatic
varieties. This is the case with _Pararga Ægeria_ (Fig. 23, Plate
II.), the southern variety of which, _Meione_ (Fig. 24, Plate II.),
is connected with it by an intermediate form from the Ligurian coast.
This species possesses, therefore, a decided power of responding to
the influence of temperature, and yet no distinction has taken place
between the summer and the winter form. We can thus only attribute
this different deportment to a different kind of heredity; and we
may therefore plainly state, that changes produced by alternation
of climate are not always inherited _alternatingly_, i.e. by the
corresponding generations, but sometimes _continuously_, appearing
in every generation, and never remaining latent. The causes which
determine why, in a particular case, the one or the other form of
inheritance prevails, can be only innate, i.e. they lie in the organism
itself, and there is as little to be said upon their precise nature as
upon that of any other process of heredity. In a similar manner Darwin
admits a kind of double inheritance with respect to characters produced
by sexual selection; in one form these characters remain limited to the
sex which first acquired them, in the other form they are inherited by
both sexes, without it being apparent why, in any particular case, the
one or the other form of heredity should take place.
The foregoing explanation may obtain in the case of sexual selection,
in which it is not inconceivable that certain characters may not be
so easily produced, or even not produced at all, in one sex, owing
to its differing from the other in physical constitution. In the
class of cases under consideration, however, it is not possible that
the inherited characters can be prevented from being acquired by one
generation owing to its physical constitution, since this constitution
was similar in all the successive generations before the appearance of
dimorphism. The constitution in question first became dissimilar in
the two generations to the extent of producing a change of specific
character, through the action of temperature on the alternating broods
of each year, combined with cyclical heredity. If the law of cyclical
heredity be a general one, it must hold good for all cases, and
characters acquired by the summer generation could never have been also
transmitted to the winter generation from the very first.
I will not deny the possibility that if alternating heredity
should become subsequently entirely suppressed throughout numerous
generations, a period may arrive when the preponderating influence of a
long series of summer generations may ultimately take effect upon the
winter generation. In such a case the summer characters would appear,
instead of remaining latent as formerly. In this manner it may be
imagined that at first but few, and later more numerous individuals,
approximate to the summer form, until finally the dimorphism entirely
disappears, the new form thus gaining ascendency and the species
becoming once more monomorphic. Such a supposition is indeed capable
of being supported by some facts, an observation on _A. Levana_
apparently contradicting the theory having been already interpreted in
this sense. I refer to the fact that whilst some butterflies of the
winter generation emerge in October as _Prorsa_, others hibernate,
and appear the following spring in the _Levana_ form. The winter form
of _Pieris Napi_ also no longer preserves, in the female sex, the
striking coloration of the ancestral form _Bryoniæ_, a fact which may
indicate the influencing of the winter generation by numerous summer
generations. The double form of the spring generation of _Papilio Ajax_
can be similarly explained by the gradual change of alternating into
continuous heredity, as has already been mentioned. All these cases,
however, are perhaps capable of another interpretation; at any rate,
the correctness of this supposition can only be decided by further
facts.
Meanwhile, even if we suppose the above explanation to be correct,
it will not apply to the absence of seasonal dimorphism in cases like
that of _Pararga Ægeria_ and _Meione_, in which only _one_ summer
generation appears, so that a preponderating inheritance of summer
characters cannot be admitted. Another explanation must thus be
sought, and I believe that I have found it in the circumstance that
the butterflies named do not hibernate as pupæ but as caterpillars, so
that the cold of winter does not directly influence those processes of
development by which the perfect insect is formed in the chrysalis.
It is precisely on this point that the origin of those differences of
colour which we designate as the seasonal dimorphism of butterflies
appears to depend. Previous experiments give great probability to this
statement. From these we know that the eggs, caterpillars, and pupæ of
all the seasonally dimorphic species experimented with are perfectly
similar in the summer and winter generations, the imago stage only
showing any difference. We know further from these experiments, that
temperature-influences which affect the caterpillars never entail a
change in the butterflies; and finally, that the artificial production
of the reversion of the summer to the winter form can only be brought
about by operating on the pupæ.
Since many monogoneutic species now hibernate in the caterpillar
stage (e.g. _Satyrus Proserpina_, and _Hermione_, _Epinephele
Eudora_, _Furtina, Ithonus_, _Hyperanthus_, _Ida_, _&c._), we may
admit that during the glacial period such species did not pass the
winter as pupæ. As the climate grew warmer, and in consequence
thereof a second generation became gradually interpolated in many of
these monogoneutic species, there would ensue (though by no means
necessarily) a disturbance of the winter generation, of such a kind
that the pupæ, instead of the caterpillars as formerly, would then
hibernate. It may, indeed, be easily proved _à priori_ that whenever
a disturbance of the winter generation takes place it only does so
retrogressively, that is to say--species which at one time pass the
winter as caterpillars subsequently hibernate in the egg, while those
which formerly hibernate as pupæ afterwards do so as caterpillars.
The interpolation of a summer generation must necessarily delay till
further towards the end of summer, the brood about to hibernate; the
remainder of the summer, which serves for the development of the
eggs and young caterpillars, may possibly under these conditions be
insufficient for pupation, and the species which hibernated in the
pupal state when it was monogoneutic, may perhaps pass the winter in
the larval condition after the introduction of the second brood. A
disturbance of this kind is conceivable; but it is certain that many
species suffer no further alteration in their development than that of
becoming digoneutic from monogoneutic. This follows from the fact that
hibernation takes place in the caterpillar stage in many species of
the sub-family _Satyridæ_ which are now digoneutic, as well as in the
remaining monogoneutic species of the same sub-family. But we cannot
expect seasonal dimorphism to appear in all digoneutic butterflies the
winter generation of which hibernates in the caterpillar form, since
the pupal stage in these species experiences nearly the same influences
of temperature in both generations. We are hence led to the conclusion
that seasonal dimorphism must arise in butterflies whenever the pupæ of
the alternating annual generations are exposed throughout long periods
of time to widely different regularly recurring changes of temperature.
The facts agree with this conclusion, inasmuch as most butterflies
which exhibit seasonal dimorphism hibernate in the pupa stage. Thus,
this is the case with all the _Pierinæ_, with _Papilio Machaon_,
_P. Podalirius_, and _P. Ajax_, as well as with _Araschnia Levana_.
Nevertheless, it cannot be denied that seasonal dimorphism occurs also
in some species which do not hibernate as pupæ but as caterpillars; as,
for instance, in the strongly dimorphic _Plebeius Amyntas_. But such
cases can be explained in a different manner.
Again, the formation of a climatic variety--and as such must we
regard seasonally dimorphic forms--by no means entirely depends
on the magnitude of the difference between the temperature which
acts on the pupæ of the primary and that which acts on those of the
secondary form; it rather depends on the absolute temperature which
the pupæ experience. This follows without doubt from the fact that
many species, such as our common Swallow-tail (_Papilio Machaon_), and
also _P. Podalirius_, in Germany and the rest of temperate Europe,
show no perceptible difference of colour between the first generation,
the pupæ of which hibernate, and the second generation, the pupal
period of which falls in July, whereas the same butterflies in South
Spain and Italy are to a small extent seasonally dimorphic. Those
butterflies which are developed under the influence of a Sicilian
summer heat likewise show climatic variation to a small extent. The
following consideration throws further light on these conditions. The
mean summer and winter temperatures in Germany differ by about 14.9°
R.; this difference being therefore much more pronounced than that
between the German and Sicilian summer, which is only about 3.6° R.
Nevertheless, the winter and summer generations of _P. Podalirius_ are
alike in Germany, whilst the Sicilian summer generation has become a
climatic variety. The cause of this change must therefore lie in the
small difference between the mean summer temperatures of 15.0° R.
(Berlin) and 19.4° R. (Palermo). According to this, a given absolute
temperature appears to give a tendency to variation in a certain
direction, the necessary temperature being different for different
species. The latter statement is supported by the facts that, in the
first place, in different species there are very different degrees of
difference between the summer and winter forms; and secondly, many
digoneutic species are still monomorphic in Germany, first becoming
seasonally dimorphic in Southern Europe. This is the case with _P.
Machaon_ and _P. Podalirius_, as already mentioned, and likewise with
_Polyommatus Phlæas_. Zeller in 1846-47, during his journey in Italy,
recognized as seasonally dimorphic in a small degree a large number of
diurnal Lepidoptera which are not so in our climate.[35]
In a similar manner the appearance of seasonal dimorphism in species
which, like _Plebeius Amyntas_, do not hibernate as pupæ, but as
caterpillars, can be simply explained by supposing that the winter
generation was the primary form, and that the increase in the summer
temperature since the glacial period was sufficient to cause this
particular species to become changed by the gradual interpolation
of a second generation. The dimorphism of _P. Amyntas_ can,
nevertheless, be explained in another manner. Thus, there may have
been a disturbance of the period of development in the manner already
indicated, the species which formerly hibernated in the pupal stage
becoming subsequently disturbed in its course of development by the
interpolation of a summer generation, and hibernating in consequence
in the caterpillar state. Under these circumstances we must regard the
present winter form (var. _Polysperchon_) as having been established
under the influence of a winter climate, this form, since the supposed
disturbance in its development, having had no reason to become changed,
the spring temperature under which its pupation now takes place not
being sufficiently high. The interpolated second generation on the
other hand, the pupal period of which falls in the height of summer,
may easily have become formed into a summer variety.
This latter explanation agrees precisely with the former, both starting
with the assumption that in the present case, as in that of _A. Levana_
and the _Pierinæ_, the winter form is the primary one, so that the
dimorphism proceeds from the said winter form and does not originate
the winter but the summer form, as will be explained. Whether the
winter form has been produced by the action of the winter or spring
temperature is immaterial in judging single cases, inasmuch as we are
not in a position to state what temperature is necessary to cause any
particular species to become transformed.
The reverse case is also theoretically conceivable, viz., that in
certain species the summer form was the primary one, and by spreading
northwards a climate was reached which still permitted the production
of two generations, the pupal stage of one generation being exposed
to the cold of winter, and thus giving rise to the production of a
secondary winter form. In such a case hibernation in the pupal state
would certainly give rise to seasonal dimorphism. Whether these
conditions actually occur, appears to me extremely doubtful; but it may
at least be confidently asserted that the first case is of far more
frequent occurrence. The beautiful researches of Ernst Hoffmann[36]
furnish strong evidence for believing that the great majority of the
European butterflies have immigrated, not from the south, but from
Siberia. Of 281 species, 173 have, according to Hoffmann, come from
Siberia, 39 from southern Asia, and only 8 from Africa, whilst during
the greatest cold of the glacial period, but very few or possibly no
species existed north of the Alps. Most of the butterflies now found
in Europe have thus, since their immigration, experienced a gradually
increasing warmth. Since seasonal dimorphism has been developed in
some of these species, the summer form must in all cases have been the
secondary one, as the experiments upon the reversion of _Pieris Napi_
and _Araschnia Levana_ have also shown.
All the seasonally dimorphic butterflies known to me are found in
Hoffmann’s list of Siberian immigrants, with the exception of two
species, viz., _Euchloe Belemia_, which is cited as an African
immigrant, and _Pieris Krueperi_, which may have come through Asia
Minor, since at the present time it has not advanced farther west
than Greece. No considerable change of climate can be experienced by
migrating from east to west, so that the seasonal dimorphism of _Pieris
Krueperi_ can only depend on a cause similar to that which affected the
Siberian immigrants, that is, the gradual increase of temperature in
the northern hemisphere since the glacial period. In this species also,
the winter form must be the primary one. In the case of _E. Belemia_,
on the other hand, the migration northwards from Africa certainly
indicates removal to a cooler climate, which may have originated a
secondary winter form, even if nothing more certain can be stated. We
know nothing of the period of migration into southern Europe; and even
migration without climatic change is conceivable, if it kept pace with
the gradual increase of warmth in the northern hemisphere since the
glacial epoch. Experiments only would in this case be decisive. If the
summer generation, var. _Glauce_, were the primary form, it would not
be possible by the action of cold on the pupæ of this brood to produce
the winter variety _Belemia_, whilst, on the other hand, the pupæ of
the winter generation by the influence of warmth would be made to
revert more or less completely to the form _Glauce_. It is by no means
to be understood that the species would actually comport itself in this
manner. On the contrary, I am of opinion that in this case also, the
winter form is primary. The northward migration (from Africa to south
Spain) would be quite insufficient, and the winter form is now found in
Africa as well as in Spain.