The Evolution Theory, Vol. 2 of 2

by August Weismann

How far below the Myxomycetes pure plastogamy reaches we do not know; whether it also occurs among non-nucleated organisms (Haeckel's Monera) we cannot tell from experience, since these assumed organisms have not yet been observed with certainty. Perhaps they all lie below the limits of visibility, and then we could never do more than suppose that plastogamic processes occur among them. Logically and purely theoretically we may suppose that amphimixis occurred first between the plasmic bodies of non-nucleated Monera, then between the cell-bodies of true cells, and finally between the nuclei of cells.

Let us hold fast to what we have found to be probable, namely, that the fusion of individually different simple organisms must or may bring about a direct advantage—a stimulation of the metabolism, and at the same time an improvement of the constitution in different directions, and let us go on to the consideration of cell-fusion combined with nuclear fusion, or complete amphimixis. In this something is added which we can recognize as an important advantage, namely, the combination of two hereditary substances, and thus the union of two variation-complexes which, according to our view, is necessary if transformation of species is to take place. In mere plastogamy such a union of two hereditary masses could only take place in Monera, not in nucleated organisms. If then there are really unicellular organisms which exhibit plastogamy without karyogamy (certain Foraminifera), we have a further proof that these processes of plasmic fusion imply direct advantage, which is distinct from the indirect advantage lying in the mingling of two different hereditary contributions, since in these cases of plastogamy there is no demonstrable mingling of hereditary bodies, no karyogamy.

But as soon as karyogamy or nuclear fusion was associated with mere plastogamy, complete amphimixis could never be lost again, because it alone made it possible that there should be harmonious transformation and adaptation in organisms which were becoming ever more complex; the primary effect of the mingling would be more and more transcended, since, without amphimixis, transmutation with harmonious adaptation in all directions would be less and less possible as organisms became more complex in structure. I have already referred to the manifold details in the structure and development of the lowest organisms which make this conclusion appear luminous to us, but we can also infer the necessity for an unceasingly active selection, from a quite different set of facts, namely, from what we know of rudimentary organs in Man.

We may regard Mankind as a species which has its local races and sub-races, but which is fixed in its essential characters, and only fluctuates hither and thither in individual variation in each sub-race, just like any other modern mammal, such as the marmot or the hare. Nevertheless we know that Man, as regards certain fairly numerous parts, is continually and persistently varying in a definite direction. Wiedersheim, in his book On the Structure of Man[23], enumerates a long series of parts and organs of the human body, which are in process of gradual degeneration, and of which it may be predicted that they will disappear from the human structure since they have lost functional significance. Among these dwindling structures are the two last ribs, the eleventh and twelfth, while the thirteenth has already disappeared, and only occurs exceptionally as a small vestige in the adult human being of to-day. The series includes also the seventh cervical rib, the os centrale of the wrist, the wisdom teeth, and the vermiform appendix of the intestine. The last is much larger in many mammals, and represents an important part of the digestive apparatus, but in Man it has dwindled to an unimportant appendage, which is a source of danger when foreign bodies (cherry stones and such like) lodge in it and set up inflammation. The variations in its length warrant us in concluding that it is still in process of degeneration; its average length is about 8½ cm., but it varies from 2 cm. to 23 cm. in length, and in about 25 per cent. of cases a partial or entire closing up of its opening into the intestine may be observed.

[23] Ueber den Bau des Menschen, 2nd ed., Freiburg-i.-Br., 1893. Trans. London, 1896.

Wiedersheim enumerates nearly a hundred parts thus in process of degeneration: this means that nearly a hundred structures in Man are at the present time in process of variation, and this could not be so unless amphimixis were continually mingling the hereditary contributions anew from generation to generation, so that the minus-variations of the parts in question, starting from the germ-plasm in which they arose at one time as chance variations, and confirmed in their direction by means of germinal selection, are gradually being transmitted to all the germ-plasms of the species. We thus see that even in a period of species-life, which we may fairly call a period of constancy, variations of a phyletic kind are continually in process, which could not become general without the co-operation of amphimixis.

Now, we have already seen that personal selection plays no part, or, at least, no important part in such degenerations, because the variations which are here concerned do not usually attain to selection value, but it is just such variations proceeding with infinite slowness that occur in functionally important organs likewise, and in the progressive advance of which personal selection and mutual adaptation probably play a part, so that in this way we can understand why the preservation of amphigony by natural selection must be effected. It is impossible—for obvious reasons—to name particular instances with certainty, as we can do in the case of the rudimentary organs, but even on general considerations we might expect that among the incipient variations of the determinants of the germ-plasm there would be some which were in an ascending direction, and that among these there would be some which, advanced by germinal selection, would go on ascending until they attained selection value. Wiedersheim reckons, for instance, the gradually increasing differentiation of the cortical zone of the human brain among the parts which are still in process of ascending variation, and he is probably right in doing so.

But if variations, so slow as to be unnoticeable, are still of abundant occurrence in Man, we have no reason to doubt that similar processes are going on in other animals; among the higher Vertebrates at least there is hardly a species which does not exhibit regressive variations even now, and in many cases progressive variations also are occurring, although we cannot give definite proofs of this.

The appearance of fixity which most species have is, therefore, illusory; in reality they exhibit a slow flux, gradually setting aside the superfluities they received from their ancestors, perfecting the important parts to more precise adaptation and greater functional capacity, and at the same time endeavouring to maintain all the parts in constant harmony. We can understand that as long as this process of gradual perfecting goes on, amphimixis will not readily be given up. Those that retain it must always, in the long run, have the preference. Moreover, as we have seen, it cannot be given up, when it has existed through æons, because of the power of persistence which the germ-plasm has gradually acquired in the course of such a long hereditary succession. It could only be given up if an advantage decisive as to survival were associated with its abandonment, such as can be actually recognized in most cases of parthenogenesis, among animals at least.

In my opinion this indirect effect of amphimixis, that is, the increasing of the possibilities of adaptation by new combinations of individual variational tendencies, is the main one, while the direct nutritive effect of the two germ-cells upon one another is quite subsidiary. In this opinion I find myself in opposition to the views of many if not most naturalists, who assume that amphimixis has a direct, sometimes, indeed, only a direct effect, and believe that they can prove it by facts.