22 COPE. [VoL. II. 
aborted. Whence, then, has come this different history? It 
has been probably partly due to the substances used as food. 
The softer and often tougher animal tissues permitted the 
shearing motion through their elasticity and extensibility carry- 
ing the friction beyond the opposing transverse edges of the 
crowns, vertically along their sides. The grains and vegeta- 
ble substances, on the other hand, possess no such elastic 
qualities, and are cut or broken by the approach of the edges 
themselves. The pressure would be direct and brief, and not 
a continued shear. Just how this would result in the devel- 
opment of the fold on the posterior side of the crown of the 
superior molar is a question of nutrition not yet explained by 
actual observation; but it is generally observable, however, 
that in dentition, folds of the surface have resulted from ordi- 
nary use, not too severe. In the case of the shearing which 
developed the carnivorous dentition, the animal used force to 
cut the meat in the manner necessary to do it. This shearing 
force is so great as to wear the crown, rather than to encour- 
age growth. The temporary force required by the act of 
crushing vegetable matters (excepting such as approach flesh in 
their characters) is more of the nature of impact, and is of 
very brief duration. The fourth tubercle has been the result, 
and I have described its various complex derivatives elsewhere.! 
Can it be possible that a largely, or exclusively meat diet has 
been the mechanical cause of the development of the trituber- 
cular molar in man? Its great predominance in the Esquimaux 
suggests this explanation. The lower and quadritubercular 
races are largely granivorous and frugivorous, but whether so 
predominantly so as to restrain the modification in question, I do 
not know. It is probable that the tritubercular molar expresses 
a change which is both phylogenetic and physiological.? 
1 In 1874, in the Journal of the Philadelphia Academy of Natural Sciences, I 
homologized the various parts of the mammalian tooth-crown structures and traced 
their phylogeny. The same was done about the same time by Kowalevsky. Not 
being then familiar with the capacity of dense substances, as dentine and enamel, 
to yield their form to continued strains, I did not pursue the question of the origin 
of these forms through use in mastication, although I suspected such origin. This 
was done in 1878 by Prof. J. A. Ryder, in an able paper on “The Mechanical Gene- 
sis of Tooth-Forms,” Proceedings Acad. Phila., p. 45. With minor exceptions, I 
have adopted the views there set forth. 
? After I had nearly completed this investigation, I received during October of this 
year (1886) the admirable monograph of Dr. Wortman on the dentition of the ver- 
