148 THE PROBLEMS OF EVOLUTION 



environment which either stimulates it to a new 

 response or forces upon it a change of structure. 

 In addition the change must fall in one of two 

 classes, it must be either adaptive or incidental, 

 related in the future activity of the individual to 

 the stimulus producing it or merely a product of 

 that stimulus acting with the previously existing 

 heritage. Unless we consider these possibilities 

 we have failed to recognize all of the factors in the 

 problem of evolution. 



Again, granting that change may be brought 

 about in genes as a response to environmental 

 conditions, are the characters so produced to be 

 regarded as mutations.'^ The concept of mutation, 

 or saltation, was originally that of a sudden change 

 of considerable degree. Gradually it has been re- 

 vised to include slighter and slighter degrees of 

 modification until Sumner, in a very able discus- 

 sion of the problem, is able to write as follows: 

 "Mutation, according to current usage, is nothing 

 more than hereditary variation regardless of mag- 

 nitude. It may be defined as hereditary (trans- 

 missible) modification of the germinal substance; 

 conversely, any modification of the germinal sub- 

 stance, however produced, would by definition be 

 a mutation." ^ 



We have already considered a mutation in 

 Drosophila whose appearance was governed by 



^Scientific Monthly, Vol. XL (1), 1929. 



