The Evolution Theory, Vol. 2 of 2

by August Weismann

This is true of most of the Epidendra of South America, and of Coryanthus triloba of New Zealand, two hundred blossoms of which only yielded five seed-capsules, and also of our Ophrys muscifera and O. aranifera, the latter of which yielded only a single seed-capsule from 3,000 flowers gathered in Liguria. We might expect that the species in question must have become very rare, but this is not always the case, since each of these capsules contains an enormous number of seeds, sometimes many thousands. As soon as the visits of insects cease altogether, the species must die out in the particular locality concerned, unless it can revert to self-pollination and self-fertility. There is a whole series of species in which the stigma of the flower is sensitive to its own pollen, and in many of these an adaptation to self-fertilization has actually been effected, for the pollinia detach themselves from their anthers at maturity and fall upon the stigma. I have already mentioned Ophrys apifera, which, according to Charles Darwin, is no longer visited by insects, although its flowers still possess the structure required for insect-fertilization. This species has saved itself from extinction by the normal occurrence of self-fertilization.

This seems to me noteworthy in two respects. In the first place, it shows that pure self-fertilization need not necessarily result in a weakening of the species, and secondly, it affords a clear instance of a species being transformed in one minute character only, all the other characters remaining unaltered. In this case it was only the pollinia that required to vary a little in their mode of attachment and maturation, in order to effect the transformation of the flower for self-fertilization, and in point of fact that is all that has varied. The case is not relevant to our investigation at this moment, but cases of the kind can so rarely be clearly demonstrated that I cannot lose the opportunity of calling attention to it. The germ-plasm of this Ophrys must have varied at an earlier stage, for otherwise the detachment of the pollinia would not have become normal and hereditary, but it can only have varied to the extent that the structure of this one small part of the flower was affected by the variation; something must have varied in the germ-plasm that had no influence upon the other parts of the flower, that is, solely the determinants of the pollinia.

Let us return after this digression to our previous train of thought; we have to inquire how we can interpret the fact of continued self-fertilization without any visible injurious results to the species. If cross-fertilization be a material advantage as regards the continuance of the species, how can it be transformed into its opposite without evil effects? And there are no visible evil effects in Ophrys apifera. It is indeed not so abundant as Ophrys muscifera, or other allied species, but it certainly does not follow from that that it is on the way to extinction; certainly no decrease either of vigour of growth or of fertility can be observed.

If we inquire from the standpoint of our theory, how the composition of the germ-plasm must have altered through continual inbreeding, we have already found the answer—that through the reduction of the number of ids at the maturation of every germ-cell the diversity of the germ-plasm would gradually be lessened, that the number of different ids would thereby be lessened possibly even to the identity of the whole of the ids.

The consequences of such extreme uniformity of the germ-plasm would not, according to our theory, necessarily be that the species would be incapable of continued existence, but it would be that the species would become incapable of adaptations in many directions. Adaptations in one direction, such, for instance, as the variation in the mode of attachment and detachment of the pollinia of an Orchid, would still be possible. Thus a species which has long been perfectly adapted will be able to make the transition to inbreeding without injury to its chances of continued existence, if it be compelled by circumstances to do so. Species, on the other hand, which are still undergoing considerable transformations in many directions must be exposed by these to the danger of degeneration, just as happens in the artificial experiments with domesticated animals, whose secret weaknesses are greatly exaggerated by inbreeding.

We might be inclined to regard the effects of inbreeding as similar to those of parthenogenesis; they are certainly analogous, for both modes of reproduction must lead to a certain degree of uniformity in the germ-plasm. But there seems to me to be a difference and one which is not without importance.

In parthenogenesis no amphimixis occurs, but neither does any reduction of the number of the ids to one-half; all the ids present at the beginning of parthenogenesis are retained; they are only no longer mingled with strange ids. In inbreeding both amphimixis and reduction take place, but the former soon ceases to convey any really strange ids to the germ-plasm, but only the same as those which it already contains, so that a rapidly increasing monotony of the germ-plasm must result. To this must be added the possibility that among the few ids which now—many times repeated—form the germ-plasm, some must occur which exhibit unfavourable variational tendencies in one or many determinants, and then the same thing will occur which usually occurs in experimental inbreeding of domesticated animals, namely, degeneration of the progeny. In parthenogenesis the case is otherwise; unfavourable variational tendencies, as soon as they attain selection-value, are, so to speak, eliminated root and branch, because the individuals which exhibit them, and their whole lineage, are exterminated, without their having any effect upon the other collateral lines of descent. A purely parthenogenetic species will, therefore, not degenerate as long as individuals of normal constitution are present, for these reproduce with perfect purity. But if in later generations unfavourable variational tendencies crop up in the germ-plasm through germinal selection, the process of personal selection will be reinforced on these or on their descendants, and it is conceivable, and even probable, that in perfectly adapted species parthenogenesis may last for a very long time without doing any injury to the constitution of the species.

The same is true of purely asexual reproduction, to the investigation of which we shall now turn.

Let us leave out of account the simplest animals (Monera) without amphimixis, which we have already discussed. In simple animals reproduction by budding or by fission is frequent, or it occurs in alternation with sexual reproduction; in higher animals, Arthropods, Mollusca, and Vertebrates, asexual reproduction is wholly absent. In plants it plays an enormously greater part, and what is called 'vegetative reproduction,' which is purely asexual without any amphimixis, is to be found in all groups of plants, especially in the form of budding and spore-formation, besides which there is multiplication by runners, rhizomes, tubers, bulbs, and bulbils. In most cases there is, in addition to the purely asexual reproduction, so-called sexual reproduction associated with amphimixis, and often the sexual and asexual generations alternate with each other, so that 'alternation of generations' occurs, as is common in lower animals, especially polyps, medusæ, and worms.

But it sometimes happens among plants that the sexual reproduction is absent, and that a species reproduces by the asexual mode only, and this is the case which we must now consider more closely.