268 THE AMERICAN NATURALIST. |. [Vor. XXXVI. 
variations) in course of life, as is so often if not invariably the 
case with new structures in the skeleton. New cusps, folds, 
crests, and styles are invariably congenital. Thus, of all organs 
of the body the teeth most exclusively and purely represent 
the current of stirp, germinal, or constitutional evolution. 
Secondly, the teeth are, nevertheless, among the most pro- 
gressive organs in the body. Whereas the adaptation of the 
skeleton, among the mammals at least, is by a constant loss 
or numerical reduction of parts, the adaptation of the teeth | 
is by a constant addition and modeling of parts (Osborn, '88, 
pp. 1067—1079). 
Thirdly, according to the present paleontological evidence 
many of the different families and orders of mammals diverged 
from each other at a time when they possessed three cusps on 
the upper molar teeth and from three to five cusps on the lower 
molar teeth. This being the case, only the cusps comparable 
in different orders of mammals with these original three upper 
and five lower cusps are derivatives or homogenous. 
Fourthly, it follows that the new cusps of the teeth fur- 
nish an example of Noquopler independent of the individual 
modification. 
Thus, we may say that in the teeth at least omoplasy 
involves a law of latent or potential homology, without profess- 
ing to understand what is its significance. 
We should, a priori, expect that if additional cusps were 
added independently in different families and orders of mam- 
mals in different parts of the world, under highly different 
conditions, the teeth of the higher Mammalia would present 
very great diversity. As a matter of fact, the new cusps in 
different families are absolutely uniform up to a certain limit.! 
In the twenty-three orders of placentals and in the seven mar- 
supial families, many of which are adaptively equivalent to 
orders, the independently developed fourth to eleventh cusps 
of the upper molars, if so many are developed, are uniform 
and may be termed homologous; the eight cusps and folds 
succeeding the original homogenous three arising, if at all, 
1The excess of this limit is. in multituberculism, or es where 
cuspules are indefinitely multiplied. 
