224 COPE. [VoLt. III. 
in the wart hog and in the lower jaw of elephants may be asso- 
ciated with the great development of the canine and molar 
teeth in the former, and of the superior incisors in the latter. 
This can be only regarded as a surmise in the present state of 
knowledge. In bats loss of median incisors may be ascribed 
to disuse, as they certainly can have had, for a long time, very 
little functional value. Of the loss of incisors it can be only 
said that it is most easily accounted for by disuse, sinée the 
molars are naturally preferred for mastication of the soft vege- 
table food on which those animals live, owing to their enclosed 
position in the mouth. But why the Edentata should have lost 
inferior incisors, while Artiodactyla retained them, is not clear, 
excepting in the case of genera like Diadomus with approxima- 
ted canines. 
3. DEVELOPMENT OF MOLARS. 
In fishes and reptiles where teeth occasionally present very 
primitive conditions, the theory of the origin of particular types 
of molar teeth is more simple than in the case of Mammalia. 
Under Rule II. direct pressure on a simple tooth crown would, 
if long continued, cause it to expand laterally, or in the direction 
of least resistance, and to grow but little in its vertical axis, ze. 
in the direction of greatest resistance. Constant use will 
account for the increased size of such teeth as compared with 
those in other parts of the jaws. 
In the case of the Mammalia, molar teeth are not traceable 
back to ancestral types of reptilian molars, but to simple conic 
(haplodont) reptilian teeth. The process of the evolution of the 
complex mammalian molars from these, forms the subject of the 
following pages. 
I have already shown that the greater number of the types of 
this series have derived the characters of their molar teeth from 
the stages of the following succession. First a simple cone or 
reptilian crown, alternating with that of the other jaw (haplo- 
dont type). Second, a cone with lateral denticles (the tricono- 
dont type). Third, the denticles to the inner or outer side of 
the crown, forming a three-sided prism, with tritubercular apex, 
which alternates with that of the opposite jaw (tritubercular 
type). Fourth, development of a heel projecting from the pos- 
