Studies in the Theory of Descent, Volume I. Weismann August

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СКАЧАТЬ were uninfluenced by the cold, and remained unchanged, emerging as the ordinary summer form (Prorsa). It thus appeared from this experiment, that a large proportion of the butterflies inclined to the Levana form by exposure to a temperature of 0°-1° R. for four weeks, while in a few specimens the transformation into this form was nearly perfect.

      Should it not be possible to perfect the transformation, so that each individual should take the Levana form? If the assumption of the Prorsa or Levana form depends only on the direct influence of temperature, or on the duration of the period of development, it should be possible to compel the pupæ to take one or the other form at pleasure, by the application of the necessary external conditions. This has never been accomplished with Araschnia Prorsa. As in the experiment already described, and in all subsequent ones, single specimens appeared as the unchanged summer form, others showed an appearance of transition, and but very few had changed so completely as to be possibly taken for the pure Levana. In some species of the sub-family Pierinæ, however, at least in the case of the summer brood, there was, on the contrary, a complete transformation.

      Most of the species of our ‘Whites’ (Pierinæ) exhibit the phenomenon of seasonal dimorphism, the winter and summer forms being remarkably distinct. In Pieris Napi (with which species I chiefly experimented) the winter form (Figs. 10 and 11, Plate I.) has a sprinkling of deep black scales at the base of the wings on the upper side, while the tips are more grey, and have in all cases much less black than in the summer form; on the underside the difference lies mainly in the frequent breadth, and dark greenish-black dusting, of the veins of the hind wings in the winter form, while in the summer form these greenish-black veins are but faintly present.

      I placed numerous specimens of the summer brood, immediately after their transformation into chrysalides, in the refrigerator (0°-1° R.), where I left them for three months, transferring them to a hothouse on September 11th, and there (from September 26th to October 3rd) sixty butterflies emerged, the whole of which, without exception – and most of them in an unusually strong degree – bore the characters of the winter form. I, at least, have never observed in the natural state such a strong yellow on the underside of the hind wings, and such a deep blackish-green veining, as prevailed in these specimens (see, for instance, Figs. 10 and 11). The temperature of the hothouse (12°-24° R.) did not, however, cause the emergence of the whole of the pupæ; a portion hibernated, and produced in the following spring butterflies of the winter form only. I thus succeeded, with this species of Pieris, in completely changing every individual of the summer generation into the winter form.

      It might be expected that the same result could be more readily obtained with A. Levana, and fresh experiments were undertaken, in order that the pupæ might remain in the refrigerator fully two months from the period of their transformation (9–10th July). But the result obtained was the same as before – fifty-seven butterflies emerged in the hothouse14 from September 19th to October 4th, nearly all of these approaching very near to the winter form, without a single specimen presenting the appearance of a perfect Levana, while three were of the pure summer form (Prorsa).

      Thus with Levana it was not possible, by refrigeration and retardation of development, to change the summer completely into the winter form in all specimens. It may, of course, be objected that the period of refrigeration had been too short, and that, instead of leaving the pupæ in the refrigerator for two months, they should have remained there six months, that is, about as long as the winter brood remains under natural conditions in the chrysalis state. The force of this last objection must be recognized, notwithstanding the improbability that the desired effect would be produced by a longer period of cold, since the doubling of this period from four to eight weeks did not produce15 any decided increase in the strength of the transformation. I should not have omitted to repeat the experiment in this modified form, but unfortunately, in spite of all trouble, I was unable to collect during the summer of 1873 a sufficient number of caterpillars. But the omission thus caused is of quite minor importance from a theoretical point of view.

      For let us assume that the omitted experiment had been performed – that pupæ of the summer brood were retarded in their development by cold until the following spring, and that every specimen then emerged in the perfect winter form, Levana. Such a result, taken in connexion with the corresponding experiment upon Pieris Napi, would warrant the conclusion that the direct action of a certain amount of cold (or of retardation of development) is able to compel all pupæ, from whichever generation derived, to assume the winter form of the species. From this the converse would necessarily follow, viz. that a certain amount of warmth would lead to the production of the summer form, Prorsa, it being immaterial from which brood the pupæ thus exposed to warmth might be derived. But the latter conclusion was proved experimentally to be incorrect, and thus the former falls with it, whether the imagined experiment with Prorsa had succeeded or not.

      I have repeatedly attempted by the application of warmth to change the winter into the summer form, but always with the same negative result. It is not possible to compel the winter brood to assume the form of the summer generation.

       A. Levana may produce not only two but three broods in the year, and may, therefore, be said to be polygoneutic.16 One winter brood alternates with two summer broods, the first of which appears in July, and the second in August. The latter furnishes a fourth generation of pupæ, which, after hibernation, emerge in April, as the first brood of butterflies in the form Levana.

      I frequently placed pupæ of this fourth brood in the hothouse immediately after their transformation, and in some cases even during the caterpillar stage, the temperature never falling, even at night, below 12° R., and often rising during the day to 24° R. The result was always the same: all, or nearly all, the pupæ hibernated, and emerged the following year in the winter form as perfectly pure Levana, without any trace of transition to the Prorsa form. On one occasion only was there a Porima among them, a case for which an explanation will, I believe, be found later on. It often happened, on the other hand, that some few of the butterflies emerged in the autumn, about fourteen days after pupation; and these were always Prorsa (the summer form), excepting once a Porima.

      From these experiments it appeared that similar causes (heat) affect different generations of A. Levana in different manners. With both summer broods a high temperature always caused the appearance of Prorsa, this form arising but seldom from the third brood (and then only in a few individuals), while the greater number retained the Levana form unchanged. We may assign as the reason for this behaviour, that the third brood has no further tendency to be accelerated in its development by the action of heat, but that by a longer duration of the pupal stage the Levana form must result. On one occasion the chrysalis stage was considerably shortened in this brood by the continued action of a high temperature, many specimens thus having their period of development reduced from six to three months. The supposed explanation above given is, however, in reality no explanation at all, but simply a restatement of the facts. The question still remains, why the third brood in particular has no tendency to be accelerated in its development by the action of heat, as is the case with both the previous broods?

      The first answer that can be given to this question is, that the cause of the different action produced by a similar agency can only lie in the constitution, i.e., in the physical nature of the broods in question, and not in the external influences by which they are acted upon. Now, what is the difference in the physical nature of these respective broods? It is quite evident, as shown by the experiments already described, that cold and warmth cannot be the immediate causes of a pupa emerging in the Prorsa or Levana form, since the last brood always gives rise to the Levana form, whether acted on by cold or warmth. The first and second broods only can be made to partly assume, more СКАЧАТЬ