INHERITANCE OF ACQUIRED CHARACTERS 105 



results of man's external heritage are often such," says 

 Professor J. A. Thomson, " as might have come about if 

 acquired characters were heritable/' On the other hand, 

 we shall see presently that progress is by no means im- 

 possible if the idea of use-inheritance be excluded. 



III. CONFUSION OF THE ISSUES. 



Seeing that all the instances brought forward as proofs 

 of the inheritance of acquired characters are capable of 

 another interpretation, we are driven to the conclusion that 

 the positive contention has, to say the least, not been 

 made out. On the contrary, the evidence rather tends into 

 the opposite direction. For, as we cannot disprove a 

 negative, its validity can only be admitted when it has 

 positively been shown to be a fact. If the Lamarckian 

 position is still held so widely, it is to a great extent due 

 to a confusion of thought with regard to the issues involved. 



(a) EVOLUTION AND DEVELOPMENT. 



One of the fundamental mistakes made by beginners in 

 this question is that they confuse Evolution with Develop- 

 ment. Though a thorough-going adherent of Weismann 

 refuses to recognize use-inheritance as a factor in the 

 development of the race, he nevertheless admits, as every 

 scientist necessarily must do, the effect of use and environ- 

 ment on the individual. The individual organism, though 

 endowed with a given heritage, cannot bring this potenti- 

 ality of its being into existence unless proper conditions 

 favour its development. The seed may be there, but if 

 the soil and sunshine are missing it will not spring forth. 

 Normal development is possible only under the appropriate 

 stimulus. If the limb of an infant becomes paralyzed, and 

 can no longer be moved, we find it remains stunted in growth. 

 Nay, more, the reaction of a part may vary according to the 

 kind of stimulus falling on it. Thus the ivy grows leaves 



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