Page:The Scientific Monthly vol. 3.djvu/511

 ORIGIN AND EVOLUTION OF LIFE 505

Bearctes on heredity each of these character complexes is now believed to have a corresponding physico-chemical determiner or group of deter- miners in the germ chromatin, the chromatin existing not as a minia- ture but as an individual potential and causal.

The principle of individuality, namely, of separate development and eziBtence, which we have seen to be the prime characteristic of the first chemical assemblage into an organism (p. 179), also governs each of these character complexes. In acme vertebrates we observe an infinit}' of similar character complexes evolving in an exactly Binular

��Fio, 1. Bbbll, Fio. 2. Tooth.

manner, as in the beautiful markings of the shell and the exquisite enamel pattern of the teeth of the glyptodon, in which every portion of the shell evolves similarly, and every one of the teeth evolve simi- larly, from which we might conclude that there is an absence of indi- viduality in form-characters and that some homomorphic (similarly formative) impulse is present in all characters of similar ancestry; but this rash conclusion is offset by the existence of other character complexes of similar ancestry which each evolve differently, or are in a high degree heteromorphic (diversely formative), as, for example, in the grinding teeth of mammals.

This individuality and separability inherent in character-fonn is equally observed in character-velocity, and is at the basis of the shifting of characters and of all the proportionate and quantitative changes which make up four-fifths of vertebrate evolution. For example, two character-forms side by side may evolve with equal velocity and main- tain a perfect symmetry or one may be accelerated into very rapid

�� �