XL] EVOLUTION IN BIOLOGY. 309 



nature and extent of the evidence bearing on these 

 questions may consult the works of Kiitimeyer, 

 Gaudry, Kowalewsky, Marsh, and the writer of the 

 present article. It must suffice, in this place, to say 

 that the successive forms of the Equine type have 

 been fully worked out ; while those of nearly all the 

 other existing types of Ungulate mammals and of 

 the Carnivora have been almost as closely followed 

 through the Tertiary deposits; the gradations be- 

 tween birds and reptiles have been traced; and the 

 modifications undergone by the Crocodilia, from the 

 Triassic epoch to the present day, have been demon- 

 strated. On the evidence of palaeontology, the evo- 

 lution of many existing forms of animal life from 

 their predecessors is no longer an hypothesis, but an 

 historical fact; it is only the nature of the physio- 

 logical factors to which that evolution is due which is 

 still open to discussion. 



