Summary. 107 



theory of heredity is, as regards all essential points, 

 indistinguishable from that of Galton. 



The truly scientific attitude of mind with regard to 

 the problem of heredity is to say, as Galton says. 

 " that we might almost reserve our belief that the 

 structural [i.e., somatic] cells can react on the 

 sexual elements at all, and we may be confident that 

 at most they do so in a very faint degree ; in other 

 words, that acquired modifications are barely, if at 

 all, inherited, in the correct sense of that word." But 

 for Weismanns further theory of evolution, it is 

 necessary to postulate the two additional doctrines 

 in question ; and it makes a literally immeasurable 

 difference to the theory of evolution whether or not 

 we entertain these two additional postulates. For no 

 matter how faintly or how fitfully the substance of 

 heredity may be modified by somatic tissues, by 

 external conditions of life, or even by so-called 

 spontaneous changes on the part of this substance 

 itself, numberless causes of congenital variation are 

 thus admitted, while even the Lamarckian principles 

 are hypothetically allowed some degree of play. And 

 although this is a lower degree than Darwin supposed, 

 their influence in determining the course of organic 

 evolution may still have been enormous ; seeing that 

 their action in any degree must always have been 

 directive on the one hand, and ctimidative on the 

 other. 



Having thus pointed out the great distinction 

 between the theories of stirp and of germ-plasm, 

 it became needful to note that Weismann himself 

 is not consistent in observing it. On the con- 

 trary, in some passages he apparently expresses 



