Palccontology to Biology. 317 



lifii wliosc influence is independent of temporary equilibrium. 

 Equally inevitable and irreversible are the essential cliaiif^es 

 which may be observed during tlio evolution of each family 

 of organisms. As the late Professor Bcechcr pointed out *, 

 all animals with skeletons tend to produce a superfluity of 

 dead matter, wliich accumulates in the form of spines as 

 soon as the race to wliicli they belong lias passed its prime 

 and begins to be on the downgrade ; all vertebrates tend to 

 Jose their teeth when they reach the culmination of tlicir life- 

 history ; nearly all groups of fishes end their career with 

 eel-shaped representatives ; and when a structural character 

 has been definitely lost in the course of evolution it never 

 reappears, but, if actually wanted again, is reproduced in a 

 secondary makeshift. Finally, and perhaps most important 

 of all, there is in tlie course of 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 

 vertebrse and fin-rays in the latest and highest families and 

 genera of bony fishes has already been mentioned. An 

 irregular cluster of grinding-tccth 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 vertebrates, 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 highest or mammalian 

 grade had been attained. Moreover, it is only in the same 

 latest phase that the teeth themselves can be treated as 

 definite units, always the same in number (44), except Avhere 

 modified by degeneration or special adaptation. The number 

 of vertebra? 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 vertebra} between the neck and the sacrum. 



In short, the biologist equipped with an adequate know- 

 ledge of Paleontology cannot fail to perceive that throughout 

 the evolution of the organic Avorld there has been a periodical 

 succession of impulses, each introducing not only a hio^her 

 grade of life, but also fixing some essential characters that 



* C. E. Beeclier, " The Origin and Significance of Spines," Amer. 

 Journ. Science, [4] vol. vi. (1898), July to October. 



Ann. & Mag. N. Hist. Ser. 7. Vol. xviii. 23 



