ESTABLISHMENT OF NEW SPECIES = 221 
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 will ultimately be found to apply, 
although as yet the precise proof is wanting, is that of 
sex. In the male and female sexes of the majority of 
animals we have a very clear example of a pair of 
definite differentiating characters. And the fact that 
in the majority of forms the two sexes make their 
appearance in nearly equa] 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, 
or absence of a character, shown by the parent form, 
