THE ORIGIN OF VARIATION 57 



there is a process by which the hereditary basis handed on to 

 the next generation may be gradually altered, as opposed to the 

 apparently sudden induction of mutants. The actual experi- 

 mental evidence is not very conclusive, except in so far as it 

 shows that lesions and mutilations are not inherited. The 

 problem of the degeneration of disused organs requires further 

 consideration. There is no positive evidence that disuse has 

 a direct effect, but the alternative selectionist explanations are 

 equally unsatisfactory. 



In a few cases there is experimental evidence which suggests 

 that induced modifications are inherited, but confirmatory 

 experiments are much to be desired. There is also a con- 

 siderable body of indirect evidence which may be held to 

 support the experiments. In a number of instances alternative 

 adaptational explanations of the data have been (or could be) 

 put forward. Such explanations depend on very large as- 

 sumptions as to the closeness of the adaptation of the organism 

 to its environment. The prime difficulty of the assumption 

 that induced modifications are inherited lies in explaining how 

 the modified character comes ultimately to be represented in 

 the germ cells. 



