56 ORGANIC EVOLUTION — THE FACTORS 



Wallace published their theory of evolution by the 

 accumulation of inborn variations, but by Darwin him- 

 self his own theory was regarded as merely supplement- 

 ary to the other, since he believed in the transmissibility 

 of acquired variations. Lately Weismann and others 

 have altogether denied that acquired traits are trans- 

 missible, and biologists in general appear to be gradually 

 veering round to this opinion. It is, I think, now 

 admitted on all hands that even if acquired variations 

 are transmitted, they are very much less important as 

 factors in evolution than was formerly supposed. 



On the face of it the theory that acquired variations 

 are transmissible seems very reasonable and probable ; 

 so much so, that before it is understood that such trans- 

 mission must lead to evolution, it is in general very 

 readily accepted by people, who on teleological grounds 

 are opposed to the idea of evolution. For instance, it 

 appears only reasonable to suppose, other things equal, 

 that the offspring of a man who has developed his 

 muscles by exercise will have larger muscles than the 

 offspring of a man who has not so developed his muscles ; 

 or to suppose that the offspring a man has after his 

 lungs are weakened by disease will be weaker in the 

 lungs than the offspring who were born to him before 

 he fell ill. But on closer inspection the theory presents 

 difficulties which I think insuperable. 



We have seen how the germ cell, after being ferti- 

 lized, divides and redivides many times without conju- 

 gation ever occurring among its descendant cells ; which 

 cells, though they remain adherent and undergo differ- 

 entiation in structure and specialization in function, are 

 to be regarded, as the development of the organism 

 proceeds, as homologous to successive generations of the 

 descendants of two conjugated infusorians. No one 

 cell in the body, therefore, is the product of any other 

 co-existing cell or cells. The body cells multiply, as 



