Wallace published their theory of evolution by the accumulation of inborn variations, but by Darwin himself his own theory was regarded as merely supplementary to the other, since he believed in the transmissibility of acquired variations. Lately Weismann and others have altogether denied that acquired traits are transmissible, 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 transmission 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 fertilized, divides and redivides many times without conjugation ever occurring among its descendant cells; which cells, though they remain adherent and undergo differentiation 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
