SWIMMING ANIMALS. 65 



are descended from a group of originally terrestrial 

 forms, the special modification having in this case been 

 carried to a considerably greater degree than in Seals, 

 which we know have undergone an analogous develop- 

 ment. Naturalists are, indeed, not altogether in har- 

 mony as to the kind of terrestrial Mammals from 

 which Whales have descended, but the probability is 

 that such ancestral types were more nearly allied to 

 the pig-like Ungulates than to any other type of 

 Mammals with which we are acquainted. The Hippo- 

 potamus shows us how a pig-like animal may become 

 amphibious, and there is no reason why a further 

 development should not go on. It will, however, be 

 understood that the terrestrial ancestors of the Whales 

 have long since disappeared from the face of the earth ; 

 and it should be added that not a trace of the inter- 

 mediate connecting forms has yet revealed itself to 

 reward the anxious search of the palaeontologist. The 

 Cetacea are first known in the upper part of the Eocene 

 division of the Tertiary period, and it thus seems quite 

 clear that they were developed to fill the gap left in 

 the life of the ocean by the disappearance of the 

 Ichthyosaurs and Plesiosaurs at the close of the 

 Secondary period ; the general replacement of a lower 

 by a higher type of organisation being apparently the 

 great life-feature by which the early part of the former 

 period is distinguished from the latter. 



