316 EVOLUTION IN BIOLOGY. 



ever tried to form a distinct mental image of this process 

 of spontaneous generation on the grandest scale, ever 

 really succeeded in realising it. 



"Within the last twenty years, the attention of the 

 best palaeontologists has been withdrawn from the hod- 

 man's work of making " new species " of fossils, to the 

 scientific task of completing our knowledge of individual 

 species, and tracing out the succession of the forms pre- 

 sented by any given type in time. 



Those who desire to inform themselves of the nature 

 and extent of the evidence bearing on these questions 

 may consult the works of Riitimeyer, Gaudry, Kowalew- 

 sky, 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 Un- 

 gulate mammals and of the Carnivora have been nearly 

 as closely followed through the Tertiary deposits ; the 

 gradations between 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 evolu- 

 tion 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 physiological factors 

 to which that evolution is due which is still open to 

 discussion. 



