1070 Evolution of Mammalian Molars. 
(2.) The majority of Mesozoic mammals showed trituberculy in 
some of its stages. Present evidence goes to show that the remain- 
ing, or aberrant types, if such existed, did not persist. The majority 
of the persisting forms of later periods were derived from the forms, 
with simple tritubercular molars, of earlier periods. It follows that 
_trituberculy was an important factor in survival. 
(3.) The definite homologies of the primary and to some degree 
of the secondary cusps in the upper and lower molars can be estab- 
lished. 
(4.) The mode of succession of tooth forms favors the kinetogen- 
esis theory advanced by Ryder and Coope. 
There are three general observations to be made:— 
First—In attempting to complete the history of each of the 
cusps, we naturally find that the paleontological record is not suffi- 
ciently perfect to admit of our following a certain type alonga single 
phylum back to the primitive type. We must at the outset pro- 
-ceed upon the principle of similar effects, similar causes. For 
example, since the history of the development of the intermediate 
tubercles in the superior molars of the Lemuroidea (Pseudolemu- 
roidea, Schlosser) is perfectly clear during the Wasatch and Bridger 
epochs—it is safe to infer that the intermediate tubercles of the 
Ungulate molars, which are fully developed in the underlying 
Puerco, had the same history. Second.—There are in each period 
Aberrant types which embrace either incomplete or degenerate tri- 
tubercular stages, i.e., a high specialization in which the past rec- 
ord is obliterated, or, finally, stages in non: tritubercular lines of 
development. Third.—In the parallel evolution of trituberculy 
in different phyla we find that the progression is by no means uni- 
form. In every geological period in which the fauna is well known 
_we observe progressive genera which outstrip the others in reaching 
a certain stage of molar development, contrasted with persistent typ® 
which represent arrested lower stages of development, while between 
them are the central types which represent the degree of evolution 
attained by the majority of genera. The latter may be said to 
constitute the stage which is characteristic of the period. 4 
_ The Stages of trituberculy may now be defined as seen in different — 
types in their order of succession :— 
