The Evolution Theory, Vol. 2 of 2

by August Weismann

Why, then, is there such an intimate, and in the case of the higher types, such an indissoluble, association between reproduction and amphimixis that 'fertilization' appears to be a sine qua non of reproduction, and not very long ago seemed to us to be the 'quickening of the ovum,' the 'burning spark' which causes the powder-barrel to explode?

The reason of this is not difficult to discover; it lies in the structure of multicellular animals, and in their differentiation according to the principle of division of labour, for since only particular cells are capable of reproduction, that is, of giving rise to the whole, it is in these necessarily that the process of amphimixis has to occur if its significance lies in its effects on the succeeding generations. It is true that in the lowest multicellular organisms, such as the species of Volvox, there are, in addition to the sex-cells, other reproductive cells quite similar to the ova, whose development into a new colony takes place without amphimixis, but the higher we ascend in the animal and plant series the rarer are these 'asexual' germ-cells or 'spores,' and in the highest animal types they are entirely absent and reproduction occurs only by means of the 'sex-cells.'

I am inclined to look for the cause of this striking phenomenon mainly in the fact that, if amphimixis had to be retained, this was effected with increasingly great difficulty the more highly and complexly differentiated the organisms became, and that more complicated adaptations were therefore necessary in order that the union of the two germ-cells might be rendered possible at all. There is first of all the separation into two kinds of sex-cells, whose far-reaching differentiations and precise adaptations to the most minute conditions we have already discussed; then follow the innumerable adaptations to bring about the meeting of the sex-cells, the arrangements for copulation, and, finally, the instincts which draw the two sexes together, the means of attraction which are employed, whether decorative colours or attractive shapes, stimulating odours or musical notes, in short, all the diverse and intricate arrangements, which seem to be more subtly elaborated the higher the organism stands upon the ladder of life. When we call to mind that sexual differentiations finally go so far that they dominate the whole organism, alike in its external appearance and in its internal nature, its feelings, inclinations, instincts, its will and ability, as well as its structure down to the finest nerve-elements, we can understand that a mode of reproduction which demands such a composite disposition of details, involving a moulding of the whole organism, so to speak, from birth till death, must of necessity remain the only one, and that there was no room for the persistence of any essentially different mode of reproduction with quite different adaptations. Or, to speak metaphorically, the power of adaptation which is innate in the organism so exhausted itself in the establishment of this marvellous amphimixis adjustment that the possibility of any other was totally excluded.

It is true that it is only among the Vertebrates that we find 'the reproductive apparatus' so highly developed, but even among Molluscs and Arthropods 'sexual' reproduction, that is, reproduction associated with amphimixis, is the prevailing mode. In these, indeed, parthenogenesis does occasionally occur, that is to say, sexually differentiated female germ-cells are, by means of some slight variations in the maturation of the egg, rendered capable of developing without previous amphimixis, but this happens only in quite special cases as an adaptation to quite special circumstances, and can only be regarded as a temporary cessation of the association between reproduction and amphimixis. In some cases it is a moiety of the ova adapted for amphimixis which develop parthenogenetically, as it is the same sexually differentiated animals, true females, which produce both sorts, and this is often true to some extent when the differentiation in the direction of parthenogenesis has advanced further, and the ova have been separated into those requiring fertilization and those which are parthenogenetic (e.g. the winter and the summer eggs of the Daphnidæ). Parthenogenesis is not asexual but unisexual reproduction, a mode of multiplication which shows us that even in highly differentiated animals the apparently indissoluble association between reproduction and amphimixis can be dissolved if circumstances require it.

But if amphimixis had to be retained in the higher animal forms—and we have seen reasons why this must be—it could only be effected by means of unicellular germs, for amphimixis is in essence a fusion of nuclei, and this is the reason why 'vegetative' reproduction, so-called, becomes less and less prominent in animals at least, and above the level of the Arthropods disappears almost entirely.

Let us now return to the question we asked at the beginning—When and in what form was amphimixis first introduced into the world of organisms? The best way to answer this is by observation. We must turn to the lowest forms which now exhibit it, and see whether it occurs in them in a simpler form, so that we may draw conclusions as to its origin and its primitive significance, for it would be possible, a priori, that this was something different from what it is now in the relatively higher organisms, and that a change of function has gradually come about.

Assuredly the whole intricate complex of adaptations which is now exhibited on the conjugation of the two sex-cells in animals and plants, the differentiation of two kinds of 'sexually' antagonistic cells, with all their special adaptations, the reduction of the chromosomes, the institution of the karyo-kinetic apparatus, together with the centrospheres and so on, cannot possibly have arisen all at once by fortuitous variation, but can only have arisen gradually, step by step, and as the result of 'innumerable external and internal influences.' But why should not these arrangements, nowadays so complex, have had a simple beginning? Why might not this beginning have been the simple union of the protoplasmic bodies of two non-nucleated Monera; followed, after the origin of nuclear substances, by the union of these, and, finally, after the differentiation of a nucleus with a definite number of chromosomes, with a dividing apparatus, with a membrane, and so on, by complete amphimixis as we now know it? And how many transition stages may not be added to fill up the gaps between these three main stages?

But how much we can actually prove in regard to these conceivable preliminary stages of amphimixis is another matter. If we take a survey of the observations that have been made up till now, we are confronted at first by the undoubtedly striking fact that very little is known about it as yet, for in fact the whole process is gone through even in quite lowly forms of life in a manner very similar to that in the higher forms. Amphimixis has been shown to be widespread even among unicellular organisms, yet not in an essentially simpler mode than among multicellulars. We have seen that even in ciliated Infusorians reducing division obtains, and that of the four nuclei which arise from twofold division of the original nucleus three break up again, and only the fourth, by a further division, separates into a male and a female pronucleus, 'which then complete the amphimixis with the corresponding pro-nuclei of another animal' (compare [Fig. 85], 4-7, vol. i. p. 321). This, and the existence of a dividing apparatus and of chromosomes, make the process appear very little less complicated than the fertilization of higher animals. The case is similar even in much lower unicellular organisms, such as Noctiluca ([Fig. 83], vol. i. p. 317). In this form and in Rhizopods it is true that reducing divisions have not yet been made out, but their occurrence in the lower Algæ (Basidiobolus), and above all in those simple unicellular organisms which give rise to malaria, and their allies, which live as 'Coccidia' in the blood-cells and intestinal cells of animals, leads us to expect that they may prove to be of general occurrence among unicellulars.