^1^ THE NEW EVOLUTION 



the germ plasm. An individual is marked as a 

 mutant at the time when the body is composed of only 

 a single cell. But sometimes mutations may appear 

 during the course of development. 



Mutants — individuals differing from the normal as 

 a result of mutation — may show only a very slight 

 departure from the usual form of the animal con- 

 cerned, or they may depart so very widely that the 

 individual is incapable of development beyond the 

 earliest embryonic stages, or they may be of any inter- 

 mediate degree. 



The mutations with which we are familiar among 

 the wild and domestic animals and in the laboratory 

 are all relatively slight, showing no very wide depar- 

 tures from the normal form. How, then, is it possible 

 to assume that mutation could account for such dif- 

 ferences as those between flies, butterflies and bees, 

 or between insects and crustaceans, or between the 

 crustaceans and the mollusks? 



The more specialized an animal type becomes the 

 more inflexible and unchangeable does it become, the 

 more closely dependent upon the maintenance of con- 

 ditions as they are, and hence the more liable to 

 extinction if conditions change. 



The reason for this is that specialization is a func- 

 tion of progressive subtraction. The more an animal 

 type has lost through this process of progressive sub- 

 traction, the less there remains for the production of 

 mutants which will be capable of existence. For all 

 mutants arise through the subtraction of something 

 from the usual form. In a very highly specialized 



