So DARWIN AND NATURAL SELECTION 



Weismann. This confession of failure, for that 

 is what it amounts to, to account for variations 

 by external agents is very striking and, in my 

 opinion, conclusive. If external agents fail to 

 account for variation, and it is yet clear that 

 variation does occur, then it follows that the change 

 must be caused by some internal factor. Weismann 

 constructed his complicated edifice of " deter- 

 minants," " ids," " biophores " and the rest to 

 avoid assuming the principle of design and to 

 provide a " natural " explanation of the internal 

 factor ; but, though no one can say that these 

 things do not exist, since they are admitted by 

 their author to be invisible, neither can any 

 person say that they do exist, and as science has 

 only to do with what can be demonstrated, it is 

 clear that, until they are demonstrated, they can 

 have nothing to do with science or science with them. 

 An internal, inherent force, an " entelechy " 

 (to use Driesch's Aristotelean term), is therefore 

 being postulated to-day by many men of science 

 besides the distinguished writer whose name has 

 just been quoted, and whose excellent series of 

 Gifford lectures are worthy of the most careful 

 study by all persons interested in the higher 

 problems of biology. Nageli and Korschinsky, as 

 we have seen, not to speak of a number of others, 

 believe that evolution is due to immanent factors 

 in the living cell or the living organism, that these 

 forces work along definite lines and that they cause 

 the variations which we know to occur.* In general 



* In the present writer's work, What is Life? (Sands and Co.) 

 will be found some account of the support for a vitalistic explan- 

 ation of life given by men of science. 



