Weismanns theory of Heredity (1891). 51 



distinctive doctrines which go to constitute this 

 theory. 



It may not be immediately apparent that Weis- 

 manns theory of heredity is not per se concerned 

 with either of these two additional postulates of the 

 continuity of germ-plasm as perpetual, and the stability 

 of germ-plasm as absolute ; while both are logically 

 necessary to his further theory of evolution. Gn this 

 account, and also for the sake of clearness in all 

 that is to follow, we had best begin by comparing 

 his theory of heredity with those of his principal 

 predecessors — Darwin and Galton. 



For the purposes of this comparison we may start by 

 again alluding to the fact, that even in the multicellular 

 organisms reproduction is not confined to the sexual 

 methods. Many kinds of invertebrated animals will 

 reproduce entire organisms from the fragments into 

 which a single organism has been chopped : plants 

 of various kinds can be propagated indefinitely by 

 cuttings, grafts, and buds, or even by leaves, as we 

 have already observed in Chapter I. Now, when 

 the whole organism is thus reproduced from a severed 

 portion of somatic-tissue, it reproduces its sexual 

 elements. Whence, then, in such cases are these 

 elements derived ? Obviously they are not derived 

 immediately from the sexual organs — or even from 

 the sexual cells — of their parents : they are derived 

 from the somatic-cells of a single parent, if we choose 

 to retain this term ; and therefore, as Strasburger 

 pointed out soon after Weismann's theory was pub- 

 lished, it seems as if such facts are in themselves 

 destructive of the theory. How, then does Weismann 



E 2 



