no FOOT-NOTES TO EVOLUTION. 



tion before us, then, is : How can extrinsic causes 

 modify the structure of the germinal protoplasm ? 



Since by his own admissions, as Romanes has shown, 

 the most characteristic features of Weismann's system, 

 both as to inheritance and evolution, have been virtually 

 abandoned, it seems to some that his theories have been 

 of no real value, and that, like an ignis fatuus, they have 

 only served to lead biologists astray far from the path 

 of science into the dangerous quagmires of speculation. 

 I do not share any such opinion. Apart from his splen- 

 did observations and the great stimulus to investigation 

 which Weismann's theories have furnished, there remain 

 many elements of permanent value in his work. 



Osborn* thinks that Weismann's most "permanent 

 service to biology is his demand for direct evidence of 

 the Lamarckian principle." It seems to me that his 

 greatest service consists in the emphasis which he has 

 laid upon the intrinsic factors of development and evo- 

 lution as opposed to the extrinsic factors, a thing which 

 he. has indeed overemphasized but which has sadly 

 needed a strong defender in these later years. Largely 

 as an outcome of his work we now recognise the possi- 

 bilities and the limitations of the selection theory as 

 never before, and we also recognise that many of the 

 evidences which were adduced in support of the La- 

 marckian factors are not conclusive, while the method 

 of securing conclusive evidence is clearly marked out. 

 Whatever we may think of his theories, this certainly is 

 no slight service. 



5. It is by no means an easy task to determine 

 whether the influence of extrinsic forces has really 

 reached the germinal protoplasm and modified its struc- 

 ture ; much more difficult is it to determine how that 



* Osborn. The Unknown Factors of Evolution. Biological 

 Lectures, 1894. 



