462 COMPARATIVE ZOOLOGY 



the individual, has a certain shorter or longer life period, 

 after which it perishes, never to reappear.* 



The genealogical history, or line of descent, of several 

 animals has been very completely established since 

 fossils came to be studied in the light of the theory 

 of evolution. The ancestry of the horse has been 

 traced through forms which follow one another in 

 linear series from remote geological periods. The 

 earliest form was about as large as a sheep and had 

 five toes and short molar teeth, ' with a comparatively 

 simple arrangement of the ridges on their crowns. 

 This was succeeded by four-toed, three-toed, and eventu- 

 ally the single-toed horse of modern times. The dimi- 

 nution in the number of the digits was accompanied by 

 gradually increasing stature and growing complexity of 

 the crown patterns of the teeth. An even more com- 

 plete series of remains from the bed of an ancient 

 lake establishes the genealogy of the fresh-water snail, 

 Planorbis. " In passing from the lowest to the highest 

 strata the species change greatly and many times, the 

 extreme forms being so different that, were it not for 

 the intermediate forms, they would be called not only 

 different species, but different genera. And yet the 

 gradations are so insensible that the whole series is 

 nothing' less than a demonstration, in this case at least, 

 of origin of species by derivation with modifications. "f 

 Other series show the evolution of the horn-bearing 

 ruminants from hornless ancestors. Casts of the brain 

 cavities of the early mammals show that their brains 

 were much smaller than those of living species and had 

 fewer, if any, convolutions. 



Of the " missing links " none is more instructive than 



* Quoted from Williams's " Geological Biology," pp. 82-83. 

 t Quoted from Le Conte's "Evolution and its Relations to Religious 

 Thought," pp. 254-255. 



