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. It might be objected that, through such a continual weeding-out of rebellious determinants, the germ-plasm would become so constant in its constitution that it would ultimately be secure from all such aberrations of it on the part of its determinants, and therefore would in time become quite incapable of diverging from its proper path at all, and would thus no longer require this continual correction through amphimixis.