256 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



in Elotlierium, HalWierium, and Tricliechus they are few ; in 

 Trachytherium and Dinotlierium more numerous ; in Mastodon, 



Stegodon, and Elephas, most 

 numerous. The tubercles are 

 united into serrated cross- 

 crests in Halitherium, the ex- 

 tinct sea-cow ; in Tracliytlie- 

 rium, another fossil ally of the 

 Manati, the tubercles are not 

 united. The succession from 

 Mastodon to Elephas may be 

 represented by the accompany- 

 ing figures : Fig. 26 (from 

 Cuvier) is a molar of M. an- 



FiG. 26. — Mastodon angusHdens. 



gustidens, where, beside the principal tubercles, numerous lesser 

 ones appear. Fig. 27 represents Mastodon oMoticus, in which 



Fig. 27. — Mastodon oMoticus. 



Fig. 28. — Dinotherium giganteum. 



the opposite tubercles are nearly united into transverse crests. 

 In Dinotlierium (Fig. 28) and Elephas (Fig. 29) the union is 

 complete. The relation of these 

 genera has been described as 

 one of *' inexact parallelism" ; a 

 condition supposed by the writer 

 to de23end on modification in 

 descent under the law of acceler- 

 ation. The language used is :* 

 ^* The young tooth of Elephas, 

 moreover, is represented by a 



» . n T , TT 1 Fig, 29. — Elephas indicns. 



series of independent parallel ^ 



laminee at first, which, when they unite, form a series of crests 



* "Origin of Genera," p. 17. 



