GENERAL CONCLUSIONS 315 



and commonly spoken of as variations. Today we know 

 that there is no such sharp contrast, but that sports and 

 variations may have the same kind of origin, and are 

 inherited according to the same laws. 



It is true that many of the small individual differences 

 are due to the environmental conditions under which the 

 development takes place, and superficial examination 

 fails often to distinguish between this sort of variability 

 and that due to minor changes brought about by genetic 

 factors. One of the most important results of modern 

 genetics is the recognition of this fact, and the invention 

 of methods by which the smaller differences may be re- 

 ferred to one or to the other of these factors. If, as Dar- 

 win supposed, and if, as is generally accepted today, the 

 process of evolution has taken place by the slow process 

 of accumulation of small variations, it follows that it 

 must be the genetic variations that are utilized, since 

 these, and not those due to environmental effects, are 

 inherited. 



It must not be supposed, however, from what has just 

 been said, that mutant changes produce only a single 

 striking or even a single small change in one particular 

 part of the body. On the contrary, the evidence from the 

 Drosophila work, which is in accord with that from all 

 other forms that have been critically studied, shows that 

 even in those cases where one part is especially modified, 

 other effects are commonly present in several or in all 

 parts of the body. The subsidiary effects not only involve 

 structural modifications, but physiological effects also, 

 if one may judge by the activity, the fertility, and the 

 length of life of the mutants. For example, the loss of 

 positive phototropism, characteristic of Drosophila, ac- 

 companied a change involving a very slight alteration 

 in the general color of the body. 



The converse of this relation must also hold. Slight 



