466 H. F. Osborn — Mammalia in North A 



merica. 



prosperous and attaining a great size, it was really passing into 

 a great corral of inadaptation to the grasses which were intro- 

 duced in the middle Miocene. So with other families and 

 lesser lines, extinction came in at the end of a term of develop- 

 ment and high specialization. With other families no causes 

 for extinction can be assigned, as in the lopping off of the 

 smaller Miocene perissodactyls. The point is that a certain 

 trend of development is taken leading to an adaptive or in- 

 adaptive final issue — but extinction or survival of the fittest 

 seems to exert little influence en route. 



The changes en route lead us to believe either in predestina- 

 tion — a kind of internal perfecting tendency, or in kineto- 

 genesis. For the trend of evolution is not the happy resultant 

 of many trials, but is heralded in structures of the same form 

 all the world over and in age after age, by similar minute 

 changes advancing irresistibly from inutility to utility. It is 

 an absolutely definite and lawful progression. The infinite 

 number of contemporary developing degenerating and station- 

 ary characters preclude the possibility of fortuity. There is 

 some law introducing and regulating each of these variations, 

 as in the variations of individual growth. 



The limits of variation seem to lie partly in what I have 

 called the " potential of evolution." As the oosperm or fer- 

 tilized ovum is the potential adult, so the Eocene molar is the 

 potential Miocene molar. We have seen that the variations of 

 the horse and rhinoceros molars, apparently so diverse, are 

 really uniform,— is not this evidence that the stem peris- 

 sodactyl had these variations in potential, waiting to be called 

 forth by certain stimuli ? This capacity of similar develop- 

 ment under similar stimuli is part of the law of mammalian 

 evolution, but this does not decide the crucial point whether 

 the stimulus is spontaneous in the germ or inherited from the 

 parent. I incline to the latter opinion. 



Columbia College, August 3, 1893. 



