54 HEREDITY 



formulating laws of variation and explaining the 

 causes of variation, but he gives us no further 

 help. 



Let us now return to Weismann himself, and see 

 whether he has any further light to throw on our 

 problem besides that which we have already rejected 

 — the statement that true variations are due to 

 amphimixis. Weismann's theory of the continuity 

 of the germ-plasm was first advanced in 1883. Ten 

 years later, in his work, " The Germ-Plasm," Weis- 

 mann explained true variations as due to the direct 

 action of external influences upon the hypothetical 

 " biophors " and " determinants " of the germ-plasm. 

 The relation between the germ-plasm and their 

 environment is nutritive, as we shall see in discussing 

 the possibilities of modification of the germ-plasm 

 by the body of the individual.^ Weismann supposes 

 the constituent structures of the germ -plasm to 

 undergo incessant changes of composition during 

 their residence in the body of the individual, and 

 these changes are the essential causes of variation. 

 The immediate cause of these changes is to be found 

 in the " inequalities of nutrition " to which the deter- 

 minants are subject. Thus though it is amphimixis^ 

 that Weismann regards as causing the constant 

 occurrence of variations as seen in the individual, 

 he inclines to the view that amphimixis is not the 

 primary cause of these variations, "but that the 



1 Chapter X. 



2 Amphimixis is the term applied not only to the union of the 

 male and female nuclei in bi- parental reproduction, but also to the 

 conjugation or union of two entire individuals, often observed in 

 unicellular organisms. 



