FACTS OF INHERITANCE 167 



The Neo-Darwinians, on the other hand, have to 

 prove that the phenomena in question cannot 

 be the results of modification-inheritance. And 

 this is, in most cases, impossible. Thus we seem 

 to reach a logical dead-lock. What we need are 

 more facts. 



CASES WHERE THE THEORY OF MODIFICATION- 

 INHERITANCE is INAPPLICABLE. It is true, how- 

 ever, that there are certain characters of particular 

 organisms in regard to which it may be said with 

 some security that they could not have arisen by 

 the inheritance of acquired modifications. Thus 

 many insects, and the like, have adaptive characters 

 in their cuticular structures knobs for crushing, 

 saws suited for cutting, gimlets suited for boring, 

 and so on. But these cuticular structures are 

 non-cellular, non-living parts of the external 

 investment of the body ; they are made and 

 remade (after moulting) by the underlying, living 

 skin. How, then, can they be interpreted in 

 terms of modification-inheritance ? The matter 

 becomes even more difficult when we consider 

 cases in which the adaptiveness is in the colour 

 or markings of these inert cuticular parts. Weis- 

 mann has argued that, since there are some adaptive 

 characters which cannot be interpreted in terms 

 of modification-inheritance, this hypothetical factor 

 need not be assumed in attempting to interpret 

 the origin of other adaptations, similar to the 

 former, except that the factor in question is not 

 by the nature of the case apparently excluded 

 from having any connection with them. 



But it cannot be said that this application of 

 the " law of parsimony " is altogether successful. 

 It may recoil on those who use it, It might be 



