It may be objected that, apart from amphimixis, variation can be brought about in many parts of an organism, as in purely asexual reproduction. A plant, for instance, may vary when it is transferred to a strange soil or climate; and even in that case the variations seem to be harmonious, at least the harmony of the parts is so far maintained that the plant continues to flourish, at any rate under cultivation. A plant species may be incited by abundant nourishment to gigantic growth, and caused to vary in many of its parts, and the abundant food may even directly affect the germ-plasm so that all or some of these variations may become hereditary; and yet this is far from being a case of adaptation, it is merely a case of simultaneous variations, and it is questionable whether they will make the continued existence of the plant under the new conditions possible or not. It might easily happen, for instance, that the plant, though it became larger and bore more abundant blossoms, would be sterile, and therefore unfitted for continued existence in a natural state. Variations are not necessarily adaptations; the latter can never come about solely through direct influence upon the germ-plasm. What direct influence upon the germ-plasm could, for instance, make the hind-legs of a mammal long and strong and the fore-legs short and weak? Obviously neither an increase nor decrease in the food-supply, nor a higher or lower temperature—in short, no direct influence, because all these affect the germ-plasm as a whole, and therefore cannot possibly influence two homologous groups of determinants in opposite directions.
This, it seems to me, is only possible when amphimixis brings about in one individual a favourable coincidence of the chance germinal variations of the determinants of the fore- and hind-limbs; and just as it is with the two variations in this simple hypothetical case, so it will be in the actual processes of adaptation where there are involved numerous—we know not how numerous—variations essential to a 'harmonious adaptation.'
It need not be objected that the very number of variations necessary to a 'harmonious adaptation' makes its occurrence impracticable; for it is the complete harmony of the parts that makes the adaptation, and without this the individual was only imperfectly adapted, and therefore incapable of survival. It is certainly not mathematically demonstrable that this is the case, but as the whole process of transformation which makes an old adaptation into a new one begins with minimal fluctuations of the determinants, which must first be brought by germinal selection to the level of selection-value, and must then be subject to personal selection, so the whole process goes on so gradually and by such small steps that the harmony of the parts is maintained by functional adaptation during the individual life in a great number of individuals. But these are just the individuals which survive in the struggle for existence, and at the same time possess at every stage of the process the best combination of favourably varying determinants. As these favourable variations are, in consequence of germinal selection, not mere isolated variations of fluctuating importance, but variations in a definite direction, the whole process of variation must persist in every single part in the direction imposed upon it by personal selection. But since at every reducing division the ids of the germ-cells are brought down to half their number, a possibility is offered for gradually removing the unfavourable ids from the germ-plasm of the species, since the descendants resulting from the most unfavourable id-combinations always perish, and so from generation to generation the germ-plasm gets rid of its unfavourably varying ids, and the most propitious combinations afforded by amphimixis are preserved, till ultimately there remain only those combinations which are varying appropriately, or at least only those in which the appropriately varying determinants are in the majority, and so have controlling influence.
Logically this deduction is undoubtedly indisputable, from the standpoint of the germ-plasm theory; but whether it may be regarded as a sufficient reason for the introduction of amphimixis, and for its extremely tenacious persistence throughout the course of the long and intricate phylogeny, cannot be maintained without special investigation.
Against my position the objection has often been urged that an arrangement cannot arise or be maintained through natural selection unless it is of direct use to the individual in which it occurs. Sexual reproduction cannot therefore have been established simply because it advances, or even because it makes possible the adaptations of species, for these adaptations only came about occasionally, perhaps once in a thousand generations or even less frequently; thus the intervening generations could derive no advantage of any kind from the arrangement in question, and therefore, according to the law of the degeneration of unused characters, it must have long since been lost. I mentioned this objection before, but was obliged to postpone a detailed consideration of it until we had discussed germinal selection.
We admit, of course, that characters are only preserved intact as long as they are of advantage sufficient to turn the scale in favour of their possessors, and that they begin to fall from their height of perfection when that is no longer the case; we admit also that new adaptations are not continually necessary, but are so only at intervals of long series of generations, and yet the objection cited seems to me baseless.
Leaving out of account, for the moment, the first introduction of amphimixis, let us deal with it as an existing occurrence, for the tenacious persistence of which we wish to find reasons.
Is it really the case that amphimixis is only of importance in connexion with the new adaptation of a species, and that it has nothing to do with the persistence of the species in the state of adaptation already attained? According to the conception of the processes within the germ-plasm which we have already stated, it is impossible that this should be the case, for continual slight fluctuations are occurring in the determinants in consequence of the fluctuations of the nutritive stream, and these slight variations, plus or minus, do not in many cases equalize one another or counteract one another by turning again in a contrary direction; they go on increasing in the direction in which they have begun. It is only when personal selection opposes them that they come to a standstill, and this can only happen when they attain to selection-value, that is to say, when they reach a level at which they become disadvantageous in the struggle of persons. But as germinal variations of this kind are continually occurring, personal selection must keep continual watch over them, and eradicate them as soon as they have attained selection-value.
Therefore, when a species is most perfectly adapted to its conditions, it would of necessity begin to degenerate if personal selection were not continually guarding it, and setting aside everything that is in excess or deficient as soon as it begins to be prejudicial. But the adaptation of a species does not depend upon one character persisting at its normal level, but on the persistence of very many, and many of these vary simultaneously upwards or downwards, and reach the limit of selection-value at one time or another. If there were no amphimixis, then either all individuals with any excessive variant would be at once eliminated, or the species would go on deteriorating until this excessive variant was so numerously and strongly represented in all its individuals that it would perish through degeneration. But even in the first of these cases the species would drift towards the fate of extinction, because excessive variations do occur even in every asexual generation, and would appear in an increasingly large number of determinants if there were no possibility of rejecting them and eliminating them from the lineage of the species.
This is made possible through the periodic intervention of amphimixis; it is actually effected thereby; and in this way alone the species is kept at its high-water mark of adaptation. It is not necessary to assume that every single determinant which is varying in an unfavourable direction is at once eliminated as soon as it becomes prejudicial, that is, reaches negative selection-value, or—to make use of an expression introduced by Ammon—as soon as it oversteps the boundaries of the 'playground of variations,' the limits within which variations are neither favourable nor unfavourable. But in the course of generations they are unfailingly eliminated, especially when a large number of unfavourably varying determinants are coincident in the germ-plasm. Then the individuals which arise from a germ-plasm thus composed must perish in the struggle for existence, and thus the id-combinations with excessive determinants are eliminated from the germinal constitution of the species. As this is repeated as often as excesses of the ids occur, the species is kept pure.