RELATIONS TO OTHER SCIENCES 563 



evolution of all groups of animals to their prime a tendency towards 

 fixity in the number and regularity (or symmetry) in arrangement of 

 their multiple parts. The assumption of a fixed number of vertebra 

 and fin-rays in the latest and highest families and genera of bony 

 fishes has already been mentioned. An irregular cluster of grinding 

 teeth characterized the Pycnodont fishes of the Lower Lias, while 

 these teeth began to be disposed in definite regular rows in some of the 

 Bathonian forms, and such a symmetrical arrangement henceforth 

 pervaded the highest members of the family. Many of the lower ver- 

 tebrates, both living and extinct, have teeth with multiplied cusps, 

 and in some genera the number of teeth seems to be constant; but in 

 the history of the vertebrates the tooth-cusps never became fixed 

 individual entities, strictly homologous in whole races, until the high- 

 est or mammalian grade had been attained. Moreover, it is only in 

 the same latest phase that the teeth themselves can be treated as defin- 

 ite units, always the same in number (44), except where modified by 

 degeneration or special adaptation. The number of vertebrae in the 

 neck of the lower vertebrates depends on the extent of this part, 

 whereas in the mammal it is almost invariably seven, whatever the 

 total length may be. Equally constant in the artiodactyl ungulate 

 mammalia is the number of nineteen vertebrae between the neck and 

 the sacrum. 



In short, the biologist equipped with an adequate knowledge of 

 paleontology cannot fail to perceive that throughout the evolution of 

 the organic world there has been a periodical succession of impulses, 

 each introducing not only a higher grade of life but also fixing some 

 essential characters that had been variable in the grade immediately 

 below. He must also realize that in the interval between these im- 

 pulses some minor characters in the families similarly acquired fixity 

 in their prime, until old age and extinction approached. The general 

 conclusion is, that if the unknown influence which Cope has termed 

 "bathmic force" were able to act without a succession of checks from 

 the environment and Natural Selection, animals would form much 

 more symmetrical groups than we actually find, and their ultimate 

 grades would display still more instances of numerical fixity in multi- 

 ple parts than can be observed under existing circumstances. 



This result almost tempts a paleontologist to risk the pitfalls of 

 reasoning from analogy, and to compare organic evolution with some 

 purely physical processes. It has already been pointed out more than 

 once that the initial stages of animal races resemble the nascent 

 states of chemical elements in their particular intensity of vigor and 

 unwonted susceptibility to influence; while Cope himself has hinted 

 that the "expression points" in the evolution of races may perhaps 

 be compared with the phenomena of latent heat in the organic world. 

 It now seems reasonable to add that each "phylum," or separate 



