THEORY OF FERTILISATION. 1 67 



fusion of their nuclei alone. These reproductive cells contain 

 the germinal material or Keimplasma, and this again, in its 

 specific molecular structure, is the bearer of the hereditary 

 tendencies of the organisms from which the reproductive cells 

 originate. Thus in sexual reproduction, two hereditary tend- 

 encies are in a sense intermingled. In this mingling, I see the 

 cause of the hereditary individual characteristics ; and in the 

 production of these characters, the task of sexual reproduction. 

 It has to supply the material for the individual differences from 

 which selection produces new species." 



But this very reasonable contention hardly appears to 

 consist with Weismann's quantitative interpretation of the 

 process of fertilisation. Nor is it evident how the diversities 

 of the male and female plasmas became such as their results 

 indicate them to be, if Weismann be correct in maintaining 

 that no modifications of the body influence the reproductive 

 elements. 



Brooks and Weismann have at any rate maintained a thesis 

 which few will be inclined to oppose, that sexual union is 

 productive of variation. To discuss the relations of this 

 view to other theories of variation is not here relevant, 

 nor can we do more than mention the reasonable sugges- 

 tion of Hatschek, that sexual reproduction is a remedy against 

 the operation of injurious variations. For we can readily 

 imagine, that the excess of some particular line of anabolic or 

 katabolic differentiation may be neutralised through fertilisation. 

 In this way one is led to speculate, whether the constant pairing 

 of diseased individuals may not sometimes be more mercifully 

 condoned by nature than we have been accustomed to think. 



