286 EVOLUTION OF TO-DAY. 



although the collateral evidence for this view is not 

 wanting, there is some difficulty in getting direct 

 evidence, since it is even denied that the effects of 

 use and disuse are inherited at all. In order that 

 this explanation should hold, it is therefore necessary 

 to show that such features are transmitted from one 

 generation to another. It is therefore necessary to 

 study carefully the other series of data which we 

 have to work with viz., heredity. In doing this 

 Weismann has found an explanation of heredity, 

 which causes him to believe that the effects of use 

 and disuse are not transmitted. He is, conse- 

 quently, obliged to fall back upon the principle of 

 natural selection acting upon the variations of the 

 ovum. This view is open to even greater difficulty 

 than that of Darwin. Finally, Brooks has advanced 

 a theory of heredity which, if true, is of assistance to- 

 all of the explanations considered. By an hypothesis 

 he explains heredity and variation. He shows that 

 we may understand how simultaneous variations can 

 appear in such organs as need change. The theory 

 thus offers to natural selection the simultaneous 

 variations which are necessary. It aids the theory 

 of Mivart in showing how extraordinary births may 

 be explained, and it is of great assistance to the 

 Neo-Lamarckian view, by showing how the effects of 

 use and disuse may be transmitted from father to 

 children. But, at the same time, this theory of 

 Brooks, though valuable, is only an hypothesis, and 

 many objections arise to prevent it from being ac- 

 cepted in its unmodified form. 



Perhaps all of the factors enumerated above have 



