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.