EVOLUTION OF ELEPHANTS 329 



comparative homogeneity of the skull type of the first- 

 named species and especially by the lack of variation in 

 the molars, this homogeneity precluding the idea that two 

 differentiated types, such as E. meridionalis and E. antiquus, 

 could be evolved without one or more intermediate stages 

 of development. This confirms the view that the antiquus 

 type and the Elephas primigenius are derivable from E. 

 meridionalis as a common ancestor.* Of the period in 

 which the progressive differentiation of Elephas meridionalis, 

 or rather at least of some form of this species, into the two 

 species E. antiquus and E. primigenius took place, Soergel 

 writes:! 



"However far back in the Upper Pliocene we may place 

 the differentiation of the two main diluvial species, there 

 can be no doubt of the fact that the divergence of both 

 lines of descent first appears strongly marked at the end 

 of the Pliocene age, and that it is only with the beginning 

 of the Glacial Period that these two types, long closely 

 associated through all their variations, become sharply de- 

 fined one from the other." 



Each of the elephant's molars displays a number of trans- 

 verse ridges of dentine. These are bounded by enamel 

 and are united by cement. The number of these trans- 

 verse plates varies markedly in different specimens and 

 different species and varieties, ranging all the way from 

 four to twenty-seven. I The marked difference apparent 

 in the ridges on the molars of the African and Indian ele- 

 phants, respectively, has been explained as due to the fact 

 that the food of the former is usually of a softer kind and 



*W. Soergel, "Die Stammesgeschichte der Elephanten"; Centralblatt fiir Mineralogie, 

 Geologie, und Palaontologie, No. 8, April 15, Stuttgart, 1915. 



tLoc. cat., p. 248. 



lArthur Hopewell-Smith, "An Introduction to Dental Anatomy and Physiology, 

 Descriptive and Applied," Philadelphia and New York, 1913, pp. 332, 333; see p. 91, Fig. 

 61, for coronal aspects of molars from the African and Indian elephant. 



