No. 3.] FACTORS IN THE EVOLUTION OF MAMMALIA. 397 



cannot be said to be convincing, though in my opinion it affords 

 some presumption in favor of determinate variation " (p. 5). 



" We have seen that the American school of biologists con- 

 tend that variations, for example in tooth-structure, are deter- 

 minate and not indeterminate. They also contend that these 

 variations are largely due to the inherited effects of use and 

 disuse. They tell us that in a large percentage of cases the 

 new elements of tooth-structure appear in regions of ancestral 

 wear and abrasion. Granting the determinate variations, we 

 may perhaps inquire whether the abrasion may not be due to 

 the presence of incipient points rather than the development of 

 points to increased abrasion. It is admitted that the new points 

 do not always occur where there has been previous abrasion. 

 Granting the determinate variations, therefore, it does not ap- 

 pear to be satisfactorily proved that they are due to the effects 

 of inherited use and disuse. Seeing the nature of tooth-growth 

 and development, one needs very cogent evidence of the pro- 

 duction of new points or cusps at regions of marked ancestral 

 abrasion. The development of certain elements of vertebrate 

 limb-structure and concomitant dwindling of other elements 

 may be adduced as more readily comprehensible effects of 

 inherited use and disuse. But here we have not the same 

 evidence of the determinate nature of the variations, and the 

 theory of selection from among favorable indeterminate varia- 

 tions is not to the same extent, on the showing of the Ameri- 

 can school themselves, excluded. It seems, then, that where 

 the evidence for determinate variations is strong, the theory of 

 use-inheritance is difficult of acceptation, and where use-inheri- 

 tance is more readily comprehensible there is less evidence that 

 the variations are determinate " (pp. 14, 15). 



For my own part, I wish to disclaim the idea of belonging 

 to any "school" whatever. Those American observers who 

 have especially devoted themselves to the morphology of extinct 

 forms have independently reached certain conclusions, in which 

 they agree ; as to other results, they are by no means so con- 

 cordant. It seems to me that the evidence points very strongly 

 in certain directions, but new evidence may at any time alto- 

 gether destroy the force of the old. So far as I can see, the 

 theory of determinate variations and of " use-inheritance " is 

 not antagonistic, but supplementary to natural selection, the 



