Studies in the Theory of Descent, Volume I. Weismann August
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
P. C. Zeller, “Bemerkungen über die auf einer Reise nach Italien und Sicilien gesammelten Schmetterlingsarten.” Isis, 1847, ii. – xii.