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

reared the rarer golden-brown variety of the caterpillar separately, but precisely the same forms of butterfly were developed as from black caterpillars bred at the same time under similar external conditions. The same experiment was performed, with a similar result, in the last century by Rösel, the celebrated miniature painter and observer of nature, and author of the well-known “Insect Diversions” – a work in use up to the present day.

      The question next arises, as to whether the causes originating the phenomena are not the same as those to which we ascribe the change of winter and summer covering in so many mammalia and birds – whether the change of colour and marking does not depend, in this as in the other cases, upon the indirect action of external conditions of life, i.e., on adaptation through natural selection. We are certainly correct in ascribing white coloration to adaptation7– as with the ptarmigan, which is white in winter and of a grey-brown in summer, both colours of the species being evidently of important use.

      It might be imagined that analogous phenomena occur in butterflies, with the difference that the change of colour, instead of taking place in the same brood, alternates in different broods.8 The nature of the difference which occurs in seasonal dimorphism, however, decidedly excludes this view; and moreover, the environment of butterflies presents such similar features, whether they emerge in spring or in summer, that all notions that we may be dealing with adaptational colours must be entirely abandoned.

      I have elsewhere9 endeavoured to show that butterflies in general are not coloured protectively during flight, for the double reason that the colour of the background to which they are exposed continually changes, and because, even with the best adaptation to the background, the fluttering motion of the wings would betray them to the eyes of their enemies.10 I attempted also to prove at the same time that the diurnal Lepidoptera of our temperate zone have few enemies which pursue them when on the wing, but that they are subject to many attacks during their period of repose.

      In support of this last statement I may here adduce an instance. In the summer of 1869 I placed about seventy specimens of Araschnia Prorsa in a spacious case, plentifully supplied with flowers. Although the insects found themselves quite at home, and settled about the flowers in very fine weather (one pair copulated, and the female laid eggs), yet I found some dead and mangled every morning. This decimation continued – many disappearing entirely without my being able to find their remains – until after the ninth day, when they had all, with one exception, been slain by their nocturnal foes – probably spiders and Opilionidæ.

      Diurnal Lepidoptera in a position of rest are especially exposed to hostile attacks. In this position, as is well known, their wings are closed upright, and it is evident that the adaptational colours on the under side are displayed, as is most clearly shown by many of our native species.11

      Now, the differences in the most pronounced cases of seasonal dimorphism – for example, in Araschnia Levana– are much less manifest on the under than on the upper side of the wing. The explanation by adaptation is therefore untenable; but I will not here pause to confute this view more completely, as I believe I shall be able to show the true cause of the phenomenon.

      If seasonal dimorphism does not arise from the indirect influence of varying seasons of the year, it may result from the direct influence of the varying external conditions of life, which are, without doubt, different in the winter from those of the summer brood.

      There are two prominent factors from which such an influence may be expected – temperature and duration of development, i.e., duration of the chrysalis period. The duration of the larval period need not engage our attention, as it is only very little shorter in the winter brood – at least, it was so with the species employed in the experiments.

      Starting from these two points of view, I carried on experiments for a number of years, in order to find out whether the dual form of the species in question could be traced back to the direct action of the influences mentioned.

      The first experiments were made with Araschnia Levana. From the eggs of the winter generation, which had emerged as butterflies in April, I bred caterpillars, and immediately after pupation placed them in a refrigerator, the temperature of the air of which was 8°-10° R. It appeared, however, that the development could not thus be retarded to any desired period by such a small diminution of temperature, for, when the box was taken out of the refrigerator after thirty-four days, all the butterflies, about forty in number, had emerged, many being dead, and others still living. The experiment was so far successful that, instead of the Prorsa form which might have been expected under ordinary circumstances, most of the butterflies emerged as the so-called Porima (Figs. 3, 4, 7, 8, and 9, Plate I.); that is to say, in a form intermediate between Prorsa and Levana sometimes found in nature, and possessing more or less the marking of the former, but mixed with much of the yellow of Levana.

      It should be here mentioned, that similar experiments were made in 1864 by George Dorfmeister, but unfortunately I did not get this information12 until my own were nearly completed. In these well-conceived, but rather too complicated experiments, the author arrives at the conclusion “that temperature certainly affects the colouring, and through it the marking, of the future butterfly, and chiefly so during pupation.” By lowering the temperature of the air during a portion of the pupal period, the author was enabled to produce single specimens of Porima, but most of the butterflies retained the Prorsa form. Dorfmeister employed a temperature a little higher than I did in my first experiments, viz. 10°-11° R., and did not leave the pupæ long exposed, but after 5½-8 days removed them to a higher temperature. It was therefore evident that he produced transition forms in a few instances only, and that he never succeeded in bringing about a complete transformation of the summer into the winter form.

      In my subsequent experiments I always exposed the pupæ to a temperature of 0°-1° R.; they were placed directly in the refrigerator, and taken out at the end of four weeks. I started with the idea that it was perhaps not so much the reduced temperature as the retardation of development which led to the transformation. But the first experiment had shown that the butterflies emerged between 8° and 10° R., and consequently that the development could not be retarded at this temperature.

      A very different result was obtained from the experiment made at a lower temperature.13 Of twenty butterflies, fifteen had become transformed into Porima, and of these three appeared very similar to the winter form (Levana), differing only in the absence of the narrow blue marginal line, which is seldom absent in the true Levana. Five butterflies 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

7

[In 1860 Andrew Murray directed attention to the disguising colours of species which, like the Alpine hare, stoat, and ptarmigan, undergo seasonal variation of colour. See a paper “On the Disguises of Nature, being an inquiry into the laws which regulate external form and colour in plants and animals.” Edinb. New Phil. Journ., Jan. 1860. In 1873 I attempted to show that these and other cases of “variable protective colouring” could be fairly attributed to natural selection. See Proc. Zoo. Soc., Feb. 4th, 1873, pp. 153–162. R.M.]

8

[A phenomenon somewhat analogous to seasonal change of protecting colour does occur in some Lepidoptera, only the change, instead of occurring in the same individual, is displayed by the successive individuals of the same brood. See Dr. Wallace on Bombyx Cynthia, Trans. Ent. Soc. Vol. v. p. 485. R.M.]

9

“Über den Einfluss der Isolirung auf die Artbildung.” Leipzig, 1872, pp. 55–62.

10

[Mr. A. R. Wallace maintains that the obscurely coloured females of those butterflies which possess brightly coloured males have been rendered inconspicuous by natural selection, owing to the greater need of protection by the former sex. See “Contributions to the Theory of Natural Selection,” London, 1870, pp. 112–114. It is now generally admitted that the underside of butterflies has undergone protectional adaptation; and many cases of local variation in the colour of the underside of the wings, in accordance with the nature of the soil, &c., are known. See, for instance, Mr. D. G. Rutherford on the colour-varieties of Aterica Meleagris (Proc. Ent. Soc. 1878, p. xlii.), and Mr. J. Jenner Weir on a similar phenomenon in Hipparchia Semele (loc. cit. p. xlix.) R.M.]

11

[The fact that moths which, like the Geometræ, rest by day with the wings spread out, are protectively marked on the upper side, fully corroborates this statement. R.M.]

13

See Exp. 9, Appendix I.