<rsi> RKCTIGRADATIONS 



235 



teeth in different families of iii;mnn;ils. The still more significant 

 fact that this definite and determinate evolution was proceeding 

 independently in a great many different families of mammals did 

 not at the time impress itself so strongly upon my mind. 



If molar teeth are found independently evolving in exactly similar 

 ways in such remote parts of the world as Switxerland, Wyoming, 

 and Patagonia, it is obvious that the process is not go\enied by 

 chance hut represents the operation of some similar or uniform law 

 deduced from the four following considerations : 



Firstly, the teeth differ from all the other tissues and organs of 

 the body in being preformed, beneath the gum. 1 Unlike all other 

 organs they are not modilied, improved, or rendered more adaptive 

 by use ; on the contrary, after the first stage of wear, the longer 

 they are used the more useless and less adaptive they become. 



Fii.. -2 10. Superior molar of JI/e'/v/c/(//>/"'---. showing styles ps, ms, ruts, and conules pi, ml, 

 homoplastic with those of the wholly unrelated primate molar, Fig. u, C. 



Thus, new structures in the teeth do not first appear as modifications 

 (as distinguished from congenital 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 con- 

 genital. 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 progressive 

 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 

 modelling of parts (Osborn, '88, pp. 1067-1079). 



Thin////, 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 



1 The importance of this fact was first pointed out to me l>y Prof. 10. B. Poulton 

 of Oxford. 



