The 'reducing divisions' of the germ-plasm and the mingling of the moieties of parental germ-plasm in fertilization must be of the utmost importance in this connexion, for they secure the constant presence of an abundance of very varied combinations of primary constituents.
I should, moreover, suppose that perfect harmony of the primary constituents of the germ, such as would render the gradual adaptation of parts during the development of the organism unnecessary, is never in any case attained to. This seems to me as little possible as that any organ should ever become absolutely perfect. No adaptation is more than relatively perfect: this fact is involved in the principle of selection, which I believe to be unable to carry improvement further than the point at which the species becomes capable of maintaining its existence. And in like manner the harmony of the primary constituents which are contained in the germ-plasm can never become more complete than is necessary just to produce, with the help of intra-selection, an individual sufficiently capable of acting its part.
A complete harmony of the primary constituents can therefore never exist in the germ-plasm of sexually produced individuals; for this germ-plasm is always composed of two individually distinct halves. If, at any rate, those are right who agree with Darwin, Galton, de Vries, and myself in believing in a pre-formative arrangement of the germ-substance—that is, in a germ-substance composed of primary constituents (Analgen)—it follows that in every act of fertilization
