102 An Examination of Weismannism. 



a single Stylonichia is potentially capable of yield- 

 ing a billion descendants within a week, we should 

 need some extraordinarily good evidence to make 

 us believe that as regards this organism natural 

 selection is inoperative. But the point at present is 

 that, quite apart from all general and a priori con- 

 siderations of this kind, Weismann's doctrine that 

 unicellular organisms cannot be influenced by natural 

 selection must be abandoned. For this doctrine 

 followed deductively from the premiss that in the 

 multicellular organisms congenital variations can only 

 be due to admixtures of germ-plasms in acts of 

 sexual fertilization ; so that, in the absence of such 

 admixtures, there could be no material for natural 

 selection to work upon. But now we have found that 

 this premiss must be given up ; and, therefore, the 

 deduction with respect to the unicellular organisms 

 falls to the ground. Although it is true that the 

 unicellular organisms propagate by fission, and 

 although we grant, for the sake of argument, that 

 they never propagate by way of sexual unions — even 

 so this can no longer be taken to argue that none of 

 their innumerable species owe their origin to natural 

 selection. And, although it is probably true that the 

 sexual methods of propagation constitute one source 

 of hereditary individual variation among the multi- 

 cellular organisms, there is no vestige of any indepen- 

 dent reason for supposing that this is the only source 

 of such variation ; while the sundry facts which have 

 now been given amount to nothing short of a demon- 

 stration to the contrary 1 . 



1 In this connexion it ought to be observed that Darwin believed 

 the causes of variation to be internal as well as external — or arising 



