476 Causes and Course of Organic Evolution 



were relatively, small. Swamp-loving animals, like many of 

 the batrachians, reptiles, and early mammals, stood greater 

 chance of being more quickly buried from disintegrating change. 



Every known fact points to the conclusion that the prede- 

 cessors of mammals were essentially land animals. Though 

 the remains must represent a mere fraction of those that at 

 one time inhabited the earth, we already know of many and 

 diverse types of fossil batrachians, reptiles, and mammals, 

 which, while serAdng at times to link together isolated groups 

 now li\dng, in the majority of cases rather suggest that great 

 cohorts of families have been swept out of existence. The 

 greatest destruction too must undoubtedly have come to land 

 species. This therefore complicates and makes difficult the 

 task of bridging over the gaps now existing amongst living 

 species. 



But we may be helped toward a right estimate of phylo- 

 genetic lines of descent if we pass in review the leading char- 

 acters of the three great classes, Batrachia,* Reptilia, and 

 Mammalia, that alone now concern us. These can be set 

 out in correlated series as below: 



In Batrachia (higher Urodela).* 



(1) The skin is smooth, scaleless, richly glandular, 

 with abundant sensory nerve endings and sensory hairs; 

 pigment cells are also frequent. 



(2) The limbs are tetrapodous and four- to five-dactyl. 

 When digital reduction occurs, digit I always disappears 

 first, and later digit V (159: 1). 



(3) Larval gills and gill clefts are formed, but these 

 are replaced functionally by lungs in the adult. 



(4) There is a lateral line connected with abundant 

 sense organs. 



(5) The skull articulates with the first vertebra by two 

 exoccipital condyles. 



(6) An auditory columellar apparatus fits into the fen- 

 estra ovalis. 



* For reasons that will appear later we give the characters here of the higher 

 Urodela, and neglect the simpler groups. 



