The Evolution Theory, Vol. 2 of 2

by August Weismann

Let us first of all seek to gain clearness as to the composition of the germ-plasm in the case of purely asexual multiplication, and what conclusions may be drawn from this, and then let us compare these with the known observational data, and it will be apparent that in individuals which have arisen by budding the complete germ-plasm of the species must be contained; the number of ids will not only remain the same in the bud as it was in the mother plant, but the number of different ids will not be diminished. The case is analogous to that of pure parthenogenesis, in which the absence of the second maturation-division of the ovum allows the germ-plasm to retain the full complement of ids. Charles Darwin held that purely asexual multiplication was 'closely analogous to long-continued self-fertilization,' yet, as we have seen, according to our theory there must be a not inconsiderable difference between the two processes, depending on the fact that in exclusive self-fertilization the number of different ids is continually decreasing, while in purely asexual reproduction the germ-plasm loses nothing of the diversity of its ids. If, therefore, the germ-plasm in purely asexual reproduction no longer receives fresh ids through amphimixis, it at least loses none of those it formerly possessed. Although we cannot consider it adapted for entering upon new adaptations in many directions, yet we may expect that the species will continue to reproduce unchanged for longer than in the case of exclusive self-fertilization, the more so since all unfavourable variational tendencies which crop up are eliminated as soon as they attain to selection-value, and, as in the case of parthenogenesis, they are eliminated without being mingled with other lines of descent.

Let us take, for instance, the purely asexual reproduction which obtains in Algæ of the genus Laminaria, in regard to which it is stated that it multiplies only through asexual swarm-spores. There are quite a number of species of this large tangle, and if it should be established that in all these the spore-cells really do not conjugate, then the case would prove that the species of a genus can maintain a well-defined existence for a long time after amphimixis has been given up. But this would not be a proof of the possibility of species-formation, for that the ancestral forms of the Laminarians must have possessed amphigony may be assumed, since their nearest relatives exhibit it still. It cannot be proved, but there seems nothing against the assumption that these tangles have existed for a long time under uniform conditions, and have become adapted to these with a high degree of constancy.

The conditions are similar in the marine Algæ of the genus Caulerpa, the nearest relatives of which reproduce sexually, though they themselves, as far as is known, reproduce only by spores.

In the Lichens, which represent, as we have already seen, a life-partnership between Fungi and Algæ, amphimixis appears not to occur at all; the unicellular Alga reproduces by cell-division, the Fungus by producing a great number of swarm-spores, which do not conjugate with one another. As far as the Alga is concerned we might perhaps suppose that the simplicity of its structure makes it possible for it to dispense with a constant recombination of its few characters to bring about the most favourable composition in its idioplasm; in support of this we may note that even the life-long combination with the Fungus has caused no visible variation in the Alga, as we must conclude from the fact that these Algæ can also live independently, and that the same species of Alga may combine with several different Fungi to form different species of lichen, just as the same Fungus may also form part of several species of lichen. We might also imagine that we have here no more than a direct influence of the Alga and Fungus upon one another, and that there is no adaptation to the new conditions of life at all, yet that can hardly be seriously maintained in regard to species which live under such definite and diverse conditions. It now seems to be established—contrary to the older statements—that the lichen-fungus only reproduces asexually, and in face of this it seems to me that nothing remains except to make the assumption that lichens formerly possessed sexual reproduction, but that they have lost it, though whether all have done so is, perhaps, not yet quite certain.

The same assumption must be made in regard to the Basidiomycetes among the Fungi, and for most of the Ascomycetes, for in these groups of Fungi sexual reproduction has only been demonstrated 'with certainty in a few genera.' That in these cases also there has been a degeneration of amphigony, until it has completely disappeared, seems probable from the two other groups of Fungi, the Zygomycetes and Oomycetes, since in these 'a reduction of sexuality amounting in some cases to complete disappearance' can be demonstrated even in existing forms. But whether it may be assumed that the Fungi which are now asexual are no longer capable of new adaptations, and whether their parasitic habit may be regarded as making up in some way for the lack of the remingling of the germ-plasm, as the botanist Möbius supposes, I am not able to decide. It is obvious that data in regard to amphimixis among the Fungi are still incomplete, and recent investigations lead us to suspect that sexual mingling may not be absent, but only disguised. Dangeard, Harold Wager, and others have observed that a fusion of nuclei precedes the formation of spores, and this may be regarded as amphimixis, although the conjugating nuclei belong to cells of the same plant and sometimes even to the same cell. But although we are here dealing with a set of facts which cannot yet be satisfactorily formulated in terms of our theory, it is nevertheless not contradictory to it that amphimixis should be wholly absent in the higher Fungi. But the fact would be contradictory to the unadulterated rejuvenescence-theory, for if amphimixis were really a condition of the continuance of life, no species—as we have already said—could continue to exist without it for countless generations.

Fig. 38 (repeated). A fragment of a Lichen (Ephebe kerneri), magnified 450 times. a, the green alga-cells. P, the fungoid filaments. After Kerner.

The same argument holds true for the higher plants, which have become purely asexual under the influence of cultivation. I refer to many of the well-marked varieties of our cultivated plants which multiply exclusively, or almost exclusively, by means of tubers and slips, as is the case with the potato, the manioc, the sugar-cane, the arrowroot-plant (Maranta arundinacea), and others. All these facts can easily be reconciled with our interpretation of the meaning of amphimixis, although the attempt to range them as evidence against our theory has more than once been made. We have thus arrived at the conclusion that while many-sided adaptations, that is, variations which transform the plant in accordance with the indirect influences of new conditions of life, cannot be brought about without a persistent mingling of germ-plasms, simple modifications may readily appear although amphimixis is altogether absent. If a wild plant be permanently transferred to a well-manured culture-bed, it is probable that certain changes will occur in it, either gradually or at once. But these are not adaptations; they are, so to speak, direct reactions of the organism which do not even require selection to make them increase, but depend upon the influencing of certain determinants of the germ-plasm, and which, like all germinal variations, will follow their course steadily until a halt is called either by germinal or by personal selection. When a given plant is exposed to these new and artificial conditions, the changes in question make their appearance sooner or later, and follow their course, and go on increasing as long as that is compatible with the harmony of the structure and functioning of the plant, this depending, as in all individual development, on the struggle between the parts, that is to say, on histonal selection. Only in this respect is the utility or injuriousness of the change of importance, for personal selection, the struggle between individuals, does not affect plants which are under cultivation.

That such modifications may increase and may persist through many generations, even with asexual multiplication, depends upon the fact that the budding cells contain germ-plasm, as well as the germ-cells, and if particular determinants of the germ-plasm in general are caused to vary by these new influences, the variation may be transmitted from bud to bud, from shoot to shoot, and so go on increasing as long as the new conditions persist, as well as in amphigonic (bisexual) reproduction, where they are transmitted from germ-cell to germ-cell. It is not inconceivable that an individual adaptation, that is to say a useful adjustment, might be effected in the course of asexual reproduction, although it is improbable that direct influences would give rise to just those changes which would be useful under the new conditions. But there are a number of cases which have been interpreted in this way. In several of the cultivated plants named, the reproductive organs have themselves degenerated, either only the male, or only the female, or both at the same time; and some observers, accepting the hypothesis of an inheritance of functional modifications, have regarded this as the direct result of disuse during the long period of asexual reproduction.

Leaving out of account this erroneous presupposition, we may ask how asexual reproduction, such as that of the potato by tubers instead of by seed, which has gone on exclusively for several centuries, could exercise any influence upon the flowers and seed-forming of this species? In point of fact it has exercised none in most potatoes, for the flowers and seeds are just as fertile now as they were when the potato was first discovered.