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 :f
"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 defined one from the
other."
Each
of the elephant's molars displays a number of transverse ridges of
dentine. These are bounded by enamel and are united by cement. The
number of these transverse plates varies markedly in different
specimens and different species and varieties, ranging all the way from
' four to twenty-seven. % The marked difference apparent in the
ridges on the molars of the African and Indian elephants,
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 für
Mineralogie, Geologie, und Paläontologie, No. 8, April 15, Stuttgart,
1915.
fLoc. cit., p. 248.
% Arthur
Hopewell-Smith, "An Introduction to Dental Anatomy and Physiology,
Descriptive and Applied," Philadelphia and New York, 1913, pp. 832,
333; see p. 91, Fig. 61, for coronal aspects of molars from the African
and Indian elephant.
