PRESIDENTIAL ADDRESS. 2G3 



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 appear 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 production 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 favourable indeterminate variations is 

 not to the same extent, on the showing of the American school 

 themselves, excluded. It seems, then, that where the evi- 

 dence 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. To carry conviction 

 it must be shown (1) that the variations are determinate, and 

 (2) that the determination is due to the inherited effects of 

 use and disuse. And of this there is at present no adequate 

 proof. 



To account for the diminution of organs or structures no 

 longer of use, apart from any inherited effects of disuse, Mr. 

 Romanes has invoked the cessation of selection : and Mr. 



