146 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



Local or sporadic cases of degenerate loss of parts 

 are seen in variotis parts of the mammalian series, such 

 as toothless Mammalia wherever they occur. Such 

 are cases where the teeth become extremely simple, 

 as in the honey-eating masupialTarsipes, the carnivore 

 Proteles, the pteropod bats, and the aye-aye. Also 

 where teeth are lost from the series, as in the canine 

 genus Dysodus, and in man. The loss of the hallux 

 and pollex without corresponding gain, in various gen- 

 era, may be regarded in the same light. 



In conclusion, the progressive may be compared 

 with the retrogressive evolution of the Vertebrata, as 

 follows : In the earlier periods and with the lower 

 forms, retrogressive evolution predominated. In the 

 higher classes progressive evolution has predominated. 

 When we consider the history of the first class of ver- 

 tebrates, the Tunicata, in this respect, and compare 

 it with that of the last class, the Mammalia, the con- 

 trast is very great. 



//. The Phylogeny of the Horse. 



As an example of special phylogeny I select that 

 of the horse, because it is the most completely repre- 

 sented by specimens in our museums. 



I have already pointed out that the alternate type 

 of carpus and tarsus of the Diplarthra has been derived 

 from the linear of the Taxeopoda by a displacement 

 inwards of the bones of their second rows. In the pos- 

 terior foot this has changed the convex surface of the 

 head of the astragalus into a bifacetted face. Thus 

 was the condylarthrous astragalus modified into that 

 of the Diplarthra. At the beginning of the line of 

 the horses we find the condylarthrous genus of the 

 Wasatch Eocene, Phenacodus Cope, to differ in this 



I 



