Studies in the Theory of Descent, Volume I. Weismann August
conclusion can be founded on them; still, this conclusion may be correct, in so far as everything depends on whether, from the beginning, the formative processes in the pupa tended to this or that direction, the final result of which is the Prorsa or Levana form. If once there is a tendency to one or the other direction, then temperature might exert an accelerating or a retarding influence, but the tendency cannot be further changed.
It is also possible – indeed, probable – that a period may be fixed in which warmth or cold might be able to divert the original direction of development most easily; and this is the next problem to be attacked, the answer to which, now that the main points have been determined, should not be very difficult. I have often contemplated taking the experiments in hand myself, but have abandoned them, because my materials did not appear to me sufficiently extensive, and in all such experiments nothing is to be more avoided than a frittering away of experimental materials by a too complicated form of problem.
There may indeed be a period most favourable for the action of temperature during the first days of the pupal stage; it appears from Experiment No. 12 that individuals tend in different degrees to respond to such influences, and that the disposition to abandon the ordinary course of development is different in different individuals. In no other way can it be explained that, in all the experiments made with the first and second generations of Prorsa, only a portion of the pupæ were compelled by cold to take the direction of development of Levana, and that even from the former only a few individuals completely reverted, the majority remaining intermediate.
If it be asked why in the corresponding experiments with Pieris Napi complete reversion always occurred without exception, it may be supposed that in this species the summer form has not been so long in existence, and that it would thus be more easily abandoned; or, that the difference between the two generations has not become so distinct, which further signifies that here again the summer form is of later origin. It might also be finally answered, that the tendency to reversion in different species may vary just as much as in different individuals of the same species. But, in any case, the fact is established that all individuals are impelled by cold to complete reversion, and that in these experiments it does not depend so particularly upon the moment of development when cold is applied, but that differences of individual constitution are much more the cause why cold brings some pupæ to complete, and others to partial, reversion, while yet others are quite uninfluenced. In reference to this, the American Papilio Ajax is particularly interesting.
This butterfly, which is somewhat similar to the European P. Podalirius, appears, wherever it occurs, in three varieties, designated as var. Telamonides, var. Walshii, and var. Marcellus. The distinguished American entomologist, W. H. Edwards, has proved by breeding experiments, that all three forms belong to the same cycle of development, and in such a manner that the first two appear only in spring, and always come only from hibernating pupæ, while the last form, var. Marcellus, appears only in summer, and then in three successive generations. A seasonal dimorphism thus appears which is combined with ordinary dimorphism, winter and summer forms alternating with each other; but the first appears itself in two forms or varieties, vars. Telamonides and Walshii. If for the present we disregard this complication, and consider these two winter forms as one, we should thus have four generations, of which the first possesses the winter form, and the three succeeding ones have, on the other hand, the summer form, var. Marcellus.
The peculiarity of this species consists in the fact that in all three summer generations only a portion of the pupæ emerge after a short period (fourteen days), whilst another and much smaller portion remains in the pupal state during the whole summer and succeeding winter, first emerging in the following spring, and then always in the winter form. Thus, Edwards states that out of fifty chrysalides of the second generation, which had pupated at the end of June, forty-five Marcellus butterflies appeared after fourteen days, whilst five pupæ emerged in April of the following year, and then as Telamonides.
The explanation of these facts is easily afforded by the foregoing theory. According to this, both the winter forms must be regarded as primary, and the Marcellus form as secondary. But this last is not yet so firmly established as Prorsa, in which reversion of the summer generations to the Levana form only occurs through special external influences; whilst in the case of Ajax some individuals are to be found in every generation, the tendency of which to revert is still so strong that even the greatest summer heat is unable to cause them to diverge from their original inherited direction of development, or to accelerate their emergence and compel them to assume the Marcellus form. It is here beyond a doubt that it is not different external influences, but internal causes only, which maintain the old hereditary tendency, for all the larvæ and pupæ of many different broods were simultaneously exposed to the same external influences. But, at the same time, it is evident that these facts are not opposed to the present theory; on the contrary, they confirm it, inasmuch as they are readily explained on the basis of the theory, but can scarcely otherwise be understood.
If it be asked what significance attaches to the duplication of the winter form, it may be answered that the species was already dimorphic at the time when it appeared in only one annual generation. Still, this explanation may be objected to, since a dimorphism of this kind is not at present known, though indeed some species exhibit a sexual dimorphism,20 in which one sex (as, for instance, the case of the female Papilio Turnus) appears in two forms of colouring, but not a dimorphism, as is here the case, displayed by both sexes.21 Another suggestion, therefore, may perhaps be offered.
In A. Levana we saw that reversion occurred in very different degrees with different individuals, seldom attaining to the true Levana form, and generally only reaching the intermediate form known as Porima. Now it would, at all events, be astonishing if with P. Ajax the reversion were always complete, as it is precisely in this case that the tendency to individual reversion is so variable. I might, for this reason, suppose that one of the two winter forms, viz. the var. Walshii, is nothing else than an incomplete reversion-form, corresponding to Porima in the case of A. Levana. Then Telamonides only would be the original form of the butterfly, and this would agree with the fact that this variety appears later in the spring than Walshii. Experiments ought to be able to decide this.22 The pupæ of the first three generations placed upon ice should give, for the greater part, the form Telamonides, for the lesser portion Walshii, and for only a few, or perhaps no individuals, the form Marcellus. This prediction is based on the view that the tendency to revert is on the whole great; that even with the first summer generation, which was the longest exposed to the summer climate, a portion of the pupæ, without artificial means, always emerged as Telamonides, and another portion as Marcellus. The latter will perhaps now become Walshii by the application of cold.
One would expect that the second and third generations would revert more easily, and in a larger percentage, than the first, because this latter first acquired the new Marcellus form; but the present experiments furnish no safe conclusion on this point. Thus, of the first summer generation only seven out of sixty-seven pupæ hibernated, and these gave Telamonides; while of the second generation forty out of seventy-six, and of the third generation twenty-nine out of forty-two pupæ hibernated. But to establish safer conclusions, a still larger number of experiments is necessary. According to the experience thus far gained, one might perhaps still be inclined to imagine that, with seasonal dimorphism, external influences operating on the individual might directly compel it to assume one or the other form. I long held this view myself, but it is, nevertheless, untenable. That cold does not produce the one kind of marking, and warmth the other, follows from the before-mentioned facts, viz. that in Papilio Ajax every generation produces both forms; and, further, in the case of A. Levana I have frequently reared the fourth (hibernating) generation entirely in a warm room, and yet I have always obtained the winter form. Still, one might be inclined not to make the temperature directly responsible, but rather the retardation or acceleration of development produced through the action of temperature. I confess that I for a long time believed that in this action I had found the true cause of seasonal
[For other remarkable cases of sexual dimorphism (not
[Eng. ed. Dimorphism of this kind has since been made known: the North American
[Eng. ed. Edwards has since proved experimentally that by the application of ice a large proportion of the pupæ do indeed give rise to the var.