weismann's theory of heredity. 439 



other member; on the contrary, while both are enormously eomj^lex 

 products, each has luxd a dilVerent ancestral liistory, such that while 

 one iiresents the congenital admixtures of thousands of individuals in 

 one line of descent, the other presents similar admixtures of thousands 

 of other individuals in a different line of descent. Conse(iuently, when 

 in any sexual union two of these enormously complex germinal elements 

 fuse together and constitute a new individual out of their joint endow- 

 ments, it is perfectly certain that that individual can not be exactly 

 like any other individual of the same species or even of the same brood ; 

 the chances must be infinity to one against any single mass of germ- 

 plasm being exactly like any other mass of gcrm-plasiu ; while any 

 amount of latitude as to difference is allowed, up to the point at which 

 the difference becomes too ])ronounced to satisfy the conditions of fer- 

 tilization, in which case, of course, no new individual is born. Ilence, 

 theoretically, we have here a sufficient cause for all individual varia- 

 fions of a congenital kind that can possibly occur within the limits of 

 fertility, and therefore that can ever become actual in living organ- 

 isms. In point of fact, Weismann believes — or at any rate began by 

 believing — that this is the sole and only cause of variations tliat are 

 congenital, and therefore (according to his views) transmissible by hered- 

 ity, ^ow whether or not he is right as regards these latter points, I 

 think there can be no question that sexual propagation is, at all events, 

 one of the main causes of congenital variation ; and seeing of what 

 enormous importance congenital variation must always have been in 

 supi)lying material for the operation of natural selection, we appear to 

 have found a most satisfactory answer to our question, — Why has sex- 

 ual propagation become so universal among all the higher plants and 

 animals? It has become so bec-ause it is thus shown to have been the 

 condition to producing congenital variations, which in turn constitute 

 the condition to the working of natural selection. 



Having got thus far, I should like to make two or three subsidiary 

 remarks. In the first place it ought to be observed that this luminous 

 theory touching the causes of congenital variations was not originally 

 propounded by Professor Weismann, but occurs in the writings of sev- 

 eral previous authors and is expressly alluded to by Darwin. Never- 

 theless, it occupies so prominent a place in Weismann's system of theo- 

 ries and has by him been wrought up so much more elaborately than 

 by any of his i)redecessors that we are entitled to regard it as par ex- 

 cellence the Weismannian theory of variation. In the next place it 

 ought to be observed that Weismann is careful to guard against the 

 seductiv^e fallacy of attributing the origin of sexual projjagation to the 

 agency of natural selection. (U-eat as the benefit of this newer mode 

 of propagation must have been to the ppecies presenting it, the benefit 

 can not have been conferred by natural selecition, seeing that the bene- 

 fit arose from the fact of the new method furnishing material to the 

 operation of natural selection, and therefore insofar as it did this, 



