It is thus clear that amphimixis is an essential factor in the fixing of forms, but that it certainly does not of itself determine these, and that it is not always the average of the variations that is the most frequent, but that the form of the curve of frequency is determined by other factors also, namely, by germinal and personal selection and by the directive control which these exert on variations.
The equalizing effect of amphigony may perhaps be expressed thus: In the case of every new adaptation there is at first a large area of variation, but this gradually decreases owing to a continual restriction on the part of natural selection, until ultimately—when the highest degree of constancy of the character or species has been attained—it only extends very little beyond the 'adaptation-playground' or the 'area of exemption.'
One of the effects of amphimixis is thus to bring about an increasing restriction of the area of variation, or, as we usually say, a constancy of the facies of a given form, a condensation into a species. How far this result is necessary or useful, and therefore how far it may be regarded as accounting for the persistence of amphimixis, we shall discuss in the chapter on the formation of species. My own view is that even the fact that new adaptations are rendered possible through amphimixis and amphigony, the mode of reproduction associated with it, affords in itself a sufficient reason why amphimixis should have been retained when once it had been introduced.
LECTURE XXIX
THE GENERAL SIGNIFICANCE OF AMPHIMIXIS (continued)
Association of amphimixis with reproduction—Origin of amphimixis—Its lowest forms—Amphimixis in Coccidia—Chromosomes in unicellular organisms—Coccidium proprium—'Amœba-nests' as a preliminary stage to amphimixis—Plastogamy of the Myxomycetes—Result: a strengthening of the power of adaptation—Strengthening of the power of assimilation—Use of complete amphimixis—Proof of its constant efficacy to be found in the rudimentary organs of Man—Allogamy—Means taken to prevent the mingling of nearly related forms—Amphimixis is not a 'formative' stimulus—Attraction of the germ-cells—Effects of inbreeding compared with those of parthenogenesis—Nathusius's case of injurious inbreeding—Hindrances to fertilization in the crossing of species—Probable reason for the injurious effect of inbreeding.
We have endeavoured to understand why amphimixis should have been established among the processes of life, and we have now to turn to the question when and how, that is, in what form, it was first introduced. But first I should like to refer for a little to the association of amphimixis with reproduction, which we find in all multicellular organisms, and among the higher types so unexceptionally that, until not very long ago, amphimixis and reproduction were looked on as one and the same thing, and all multiplication was believed to be associated with 'fertilization.' We have seen that this is not the case, that on the other hand the two processes are quite distinct, and may be called contrasts rather than equivalents, for reproduction always means an increase in the number of individuals, while amphimixis implies—originally at least—their diminution by a half.
Accordingly we found that, in unicellular organisms, amphimixis is not associated with reproduction, but interpolated between the divisions, and not even in such a manner that amphimixis precedes every multiplication by division, but so that the conjugation of two animals occurs only from time to time, after numerous divisions, sometimes hundreds, have occurred. It is obvious that this must be so, since, if amphimixis occurred regularly between every two divisions, no increase in the number of individuals would be brought about, at least if the fusion of the two conjugating individuals were complete.