136 THE AMERICAN NATURALIST [Vol. LVI 



beginnings of opposing cusps in the other jaw; whereby 

 there evolves a continuous reciprocal mechanism not dis- 

 similar to the reciprocal services of the Yale key and the 

 Yale lock. The evolution of the key below proceeds with 

 the evolution of the lock above. The process does not 

 go very far in the Primates, but in the purely herbivo- 

 rous ungulates, like the horse and the elephant, the re- 

 ciprocal grinding mechanism reaches a degree of com- 

 plexity to which the most intricate lock and key devised 

 by man present but a feeble parallel. Every mechanical 

 device in the upper grinding teeth, adapted to the fine 

 comminution of grasses, is reversed in the lower grinding 

 teeth, on the principle of mechanical action and reaction ; 

 nowhere in nature is reciprocal mechanical co-adapta- 

 tion more perfectly evolved than in the upper and lower 

 grinding teeth of mammals. 



Between 1889 (Osbom, 1890.47) and 1891 (Osborn, 

 1891.53) I made what I now believe to be an unsound in- 

 duction from this evidence that this continuous mechani- 

 cal origin tended to support the Lamarckian theory of 

 the inheritance of adaptive reactions. I first termed the 

 orthogenetic process "definite variation"; later I termed 

 it "progressively adaptive variation"; by the year 1908 

 I realized that these new adaptively arising tooth ele- 

 ments were not variations in Darwin's sense at all, and 

 I applied to them the distinctive term rectigradations 

 (Osbom, 1908.314). In the meantime I abandoned the 

 Lamarckian explanation and in 1895 (Osbom, 1895.97) 

 I started out upon a search for the unknoivn factors of 

 evolution, a search in which I am still busily occupied. 



To retum to the difficulty of making sound inductions 

 as to the origin of new characters in hard parts, in 1889 

 I opened a long correspondence with the leading expo- 

 nent of Darwinism in Great Britain, Edward B. Poulton, 

 who admitted the evidence but interpreted the facts in 

 the Darwinian w^ay, namely, as the selection of mechani- 

 cal successes from non-observed mechanical failures. It 

 is a good thing to have a number of skeptical friends 



