74 BASIS OF THE PEOBLEM 



in which mutations originate, it must be supjjosed that changes in 

 and additions to the complex of factors can and do occur. And 

 we may note that certain distinguishing characters of domestic 

 varieties of fowls and of pigeons appear to have arisen by the 

 addition of factors ; thus the ' single comb ' of fowls is the 

 original character of the wild stock which has been modified by 

 the apparent addition of other factors and not by the dropping 

 out of one or more existing factors. It is again not necessary 

 to go into the reasons which have led to this conclusion. The 

 conclusion is a deduction from the analysis of crosses between 

 different breeds of fowls. 



7. For the most part we are quite ignorant as to the causes 

 which have led to the apparent losses and apparent additions 

 of factors, though a few observations seem to indicate certain 

 circumstances under which the dropping out of factors may take 

 place. In this connexion some reference must be made to the 

 problem as to the inheritance of acquired characters, though it 

 follows from what has been said above regarding terminology, 

 that we should more correctly speak of this problem as the 

 question whether modifications in any one direction tend to be 

 followed by mutations in the same direction. As the result of 

 prolonged discussion and controversy it is now generally held 

 that nothing of the kind takes place. It is almost universally 

 agreed, for instance, that such modifications as are induced 

 among men at the present day do not lead to mutations in the 

 same direction. Therefore we may for the purposes of this book, 

 so far as man is concerned, take it that acquired characters are 

 not inherited. But looking at the problem as a whole it cannot be 

 regarded as settled. There is, for example, some evidence of the' 

 parallel induction of modifications and of mutations ; but the 

 question may be raised whether such cases if substantiated fall 

 under the heading of the inheritance of acquired characters. 

 Again, some biologists regarding the problems of evolution generally 

 find difficulty in arriving at an explanation unless under certain 

 circumstances adaptive variations are followed by mutations. 



We are thus left with the fact that mutations arise, and may 

 be either large or small. Outwardly, variations in characters are 

 usually continuous, because the environmental stimuli vary con- 

 tinuously, smooth over and obscure the differences due to mutation, 

 as in the case of the beans mentioned above. But, discounting 



