The Evolution Theory, Vol. 2 of 2

by August Weismann

A proof of this may be found in the results of experiments made upon the little red-gold fire-butterfly (Polyommatus phlæas), to which I have briefly referred in a former lecture. This little diurnal butterfly of the family Lycænidæ has a wide distribution and occurs in two climatic varieties. In the far north and also in the whole of Germany the upper surface is red-gold with a narrow black outer margin, but in the south of Europe the red-gold has been almost crowded out by the black. I reared caterpillars in Germany from eggs of P. phlæas found at Naples and exposed them directly after they had entered on the pupa-stage to a relatively low temperature (10° C.). Butterflies emerged which were not quite so black as those of Naples, but considerably darker than the German form. Conversely, German pupæ were exposed to greater warmth (38° C.), and these gave rise to butterflies which were rather less fiery gold and considerably blacker than the ordinary German form. If I had to repeat these experiments I should use a much lower temperature in the case of the cold experiments, because we now know from the experiments of Standfuss, E. Fischer, and Bachmetjeff, that most of the pupæ of diurnal butterflies can stand a temperature below zero for a considerable time; probably the results would be even more marked then.

But even from the results of my former experiments we are justified in concluding that the blackening of the upper surface of the wing is really the direct result of the increased temperature during pupahood, and that the pure red-gold results from the lowered temperature. Similar experiments made by Merrifield with English Phlæas pupæ agree exactly with mine. But we may conclude further from these experiments that both warmth and cold only give rise to slight variations in the individual pupæ, and that the pure red-gold of the northern form and the black of the southern are the result of a long process of inheritance and accumulation, in which the germ-plasm has been caused to vary in as far as the relevant determinants are concerned, so that these yield the respective northern and southern forms even in less extreme temperatures.

As it is to be assumed that these determinants are present not only in the primordium of the wing in the pupa, but also in the germ-cells, both must be affected by the varying temperature, and, in accordance with the continuity of the germ-plasm, each variation of these determinants, however slight, would be continued in the next generation. It is thus intelligible that somatic variations like the blackening of the wings through warmth appear to be directly inherited and accumulate in the course of generations; in reality, however, it is not the somatic change itself which is transmitted, but the corresponding variation evoked by the same external influence in the relevant determinants of the germ-plasm within the germ-cells, in other words, in the determinants of the following generation.

This interpretation of these experiments, which I offered some years ago, has been confirmed in several ways in regard to various other diurnal Lepidoptera. By employing a temperature as low as 8° C. in the case of fresh pupæ of various species of Vanessa Standfuss and Merrifield, and especially E. Fischer, succeeded in getting great deviations in the marking and colour of the full-grown insects,—so-called aberrations, such as had previously been found only very rarely and singly under natural conditions. The deviations from the normal must undoubtedly be ascribed to the effect of cold, but it does not follow that they are new forms which have suddenly sprung into existence, as many have assumed without further experiment. Dixey, on the other hand, has attempted to establish, by a comparison of the different species of Vanessa, the phyletic development of their markings, and has found that these aberrations due to cold are more or less complete reversions to earlier phyletic stages. As regards the common small painted lady (Vanessa cardui), the small tortoise-shell butterfly (Vanessa urticæ), the 'Admiral' (Vanessa atalanta), the peacock (Vanessa io), and the large tortoise-shell (Vanessa polychioros), I can agree with this interpretation, and I do so the more readily because some years ago I suggested that the alternation of differently coloured generations of seasonally dimorphic Lepidoptera might be considered as a reversion. But this by no means excludes the possibility that other than atavistic aberrations may be produced by cold or heat. There is nothing against this theoretically. Yet we must not, without due consideration, compare these abruptly occurring variations to the sport-varieties of plants which we have already discussed; there is an important difference between the two sets of cases. In the Lepidoptera a single interference, lasting only for a short time, modifies the wing-marking, but in the plant varieties the visible appearance of the variation is preceded by a long period of preparatory change within the germ-plasm. This period required for the external influences to take effect was already recognized by Darwin, and it has recently been named by De Vries the 'premutation period.'

We may explain these remarkable aberrations theoretically in the following way: The determinants of the wing-scales in the wing-primordium of the young pupa are influenced by the cold in different ways, some kinds of determinants being strengthened by it, others markedly weakened, even crippled so to speak, and in this way one colour-area spreads itself out more than is normal on the surface of the wing, and another less, while a third is suppressed altogether. That this disturbance of the equilibrium between the determinants leads usually to the development of a phyletically older marking pattern leads us to the conclusion that in the germ-plasm of the modern species of Vanessa a certain number of determinants of the ancestors must be contained in addition to the modern ones. We might even inquire whether these were not better able to endure cold than their modern descendants, since their original possessors, the old species of the Ice age, were accustomed to greater cold, but this idea is contradicted by the experiments of E. Fischer, which go to show that the same aberrations are evoked by abnormally high temperature. That the old ancestral determinants are present in different numbers in the germ-plasm of the modern species, I am inclined to infer from the fact that among a large number of experiments made by me in the course of several years the aberrations have always occurred in very different numbers in the different broods, although the greatest care was taken to have the conditions as nearly alike as possible; absolutely alike, of course, they never can be.

But it would lead me too far if I were to enter on a detailed discussion of these cases, which have not yet been fully worked up; only one thing more need be mentioned, that is, that the aberrations induced by cold are to a certain extent transmissible. Standfuss first succeeded in making some aberrant specimens of Vanessa urticæ reproduce, and from their eggs he procured butterflies which showed a much slighter deviation from the normal, which however was still so decided that it could not be regarded as due to chance. I myself succeeded in doing the same, but the deviation in this case was much slighter. But that these observed cases are rightly referred to the cold to which their parents had been subjected is proved by other observations recently published by E. Fischer. These refer to one of the Bombycidæ (Arctia caja), which flies by day, and accordingly has a gay and very definite marking and coloration. A large number of pupæ were exposed to cold at 8° C., and some of these resulted in striking and very dark aberrant forms (Fig. 129, A). A pair of these yielded fertilized eggs; in the progeny, which were reared at a normal temperature, there were among the much more numerous normal forms a few (17) which exhibited the aberration of the parents, though to a considerably less degree (Fig. 129, B).

This shows that the cold had affected not only the wing-primordia of the parental pupæ, but the germ-plasm as well, and at the same time that this latter variation was less marked than that of the determinants of the wing-rudiments. This gives rise to an appearance of the transmission of acquired characters.

In the case of many of these cold-aberrations in Lepidoptera the cold gives rise to variations, but does so not by creating anything new, but by giving the predominance to primary constituents which have long been present, but are usually suppressed, and so it is also among the plants. I have in mind, for instance, the interesting experiments of Vöchting on the influence of light in the production of flowers in phanerogams. These showed that the common balsam (Impatiens noli me tangere) produces its familiar open flowers in a strong light, but in weak light only bears small, closed, so-called 'cleistogamous' flowers. But it would be utterly erroneous to suppose that the strong or weak light is the real cause, the causa materialis, of these two forms of flowers: the degree of illumination is merely the stimulus which provokes one or other of the primary constituents to development, both kinds being present in the constitution of the plant. As has long been known, the balsam normally possesses two kinds of flowers, and the slumbering primary constituents of these are so arranged that the open flowers develop where there is a prospect of insect visits and cross-fertilization, that is, in sunny weather or in a strong light, while closed and inconspicuous flowers adapted for self-fertilization develop in weak light, that is, in shady places and in concealed parts of the plant, where insect visits are not to be expected.

Fig. 129. A, an aberration of Arctia caja, produced by low temperature. B, the most divergent member of its progeny. After E. Fischer.