182 EVOLUTION AND GENETICS 



with the welfare of the individual, or which can he 

 corrected by modern appliances or operations. We 

 have far less information concerning the inheritance 

 of these blemishes, and at present can seldom be 

 certain that they are inherited or how they are in- 

 herited. Only by comparison with the better-known 

 cases can we surmise that inheritance may play a 

 role in many of them. Since the disadvantages that 

 follow are slight, or can be corrected, these defects 

 have little practical importance, which need not, 

 however, detract from their theoretical interest. 

 Other ways than elimination by means of artificial 

 selection have been found to standardize individuals 

 that show these slight dej^artures. Corrective surger}^ 

 has proven a more efficacious remedy in man than 

 the slow process of selective breeding. 



In genetic work each mutant type (defective or 

 otherwise ) is contrasted with the original type from 

 which it came, sometimes called the normal type. In 

 man and in some of the domesticated animals there 

 is no standard or original type with which to make 

 such a comparison ; and opinions may even differ as 

 to what is to be regarded as a normal type, each race 

 having probably a standard of its own. The situation 

 is the same in several domestic races of animals and 

 garden plants. In some, the new mutant genes have 

 entirely replaced the original genes, i.e., the race has 

 become pure for certain mutant genes. Whenever 

 the original genes have been replaced byinutant genes 



