VI EVOLUTION IN BIOLOGY 225 



Both Mr. Darwin and Mr. Wallace lay great 

 stress on the close relation which obtains between 

 the existing fauna of any region and that of the 

 immediately antecedent geological epoch in the 

 same region ; and rightly, for it is in truth in- 

 conceivable that there should be no genetic 

 connection between the two. It is possible to put 

 into words the proposition that all the animals and 

 plants of each geological epoch were annihilated 

 and that a new set of very similar forms was 

 created for the next epoch ; but it may be doubted 

 if any one who ever tried to form a distinct mental 

 image of this process of spontaneous generation on 

 the grandest scale, ever really succeeded in real- 

 ising it. 



Within the last twenty years, the attention of 

 the best palaeontologists has been withdrawn from 

 the hodman'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 presented 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, 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 



