ESTABLISHMENT OF NEW SPECIES 221 



characters of a living creature as having been taken 

 away ? Would there, or would there not, be any 

 residuum ? Upon this knotty point there is a disagree- 

 ment among authorities, and so we may be content 

 to leave it, since the question is hardly one which is 

 capable of a practical solution. . 



A phenomenon to which it is scarcely doubtful 

 that Mendelian principles apply is that of sex. In 

 the male and female sexes of the majority of 

 animals we have a very clear example of a pair ot 

 definite differentiating characters. And the fact that 

 in the majority of forms the two sexes make their 

 appearance in nearly equal numbers, may be thought 

 to point clearly to the conclusion that the separation 

 of the sexes depends upon some quite simple gametic 

 process. Light has recently been thrown upon this 

 question from the side of the study of the minute 

 structure of the gametes, and we shall defer the further 

 discussion of the problem to the chapter which deals 

 with microscopic phenomena within the cell. 



A proper understanding of Mendel's law enables us 

 to escape certain theoretical difficulties which have 

 long been prominent in the minds of students of 

 evolution. Many evolutionists were accustomed to 

 argue that a new form suddenly arising in the midst of 

 an old-established species could not give rise to a new 

 and permanent variety or elementary species, because 

 it would immediately be ' swamped ' by intercrossing 

 with the parent species from which it was derived. 

 If, however, the character distinguishing the new 

 type is allelomorphic to the corresponding character, 



