HEREDITY OF SEX 275 
still possible to regard this chromosome as representing 
a recessive male determinant, and to suppose that the 
process of sex determination is precisely similar in 
the two cases. On this supposition, the smaller idio- 
chromosome is regarded as being without function so 
far as sex is concerned. 
In a third insect belonging to the same natural group 
both male and female sexes bear alike a pair of idio- 
chromosomes of equal size. Here, again, it is possible 
to apply the same theory of sex determination by. 
simply disregarding one of the idiochromosomes of the 
male as unimportant. We may suppose, in fact, that 
one of these chromosomes corresponds to the smaller 
idiochromosome of the preceding case, and that it 
takes no essential part in these phenomena. The fact 
that this chromosome takes no active part in these 
processes may, indeed, have led to its reduction in the 
second of the three species, and to its final disappear- 
ance in the first. 
Thus, by dint of a good deal of speculation, Wilson 
has arrived at a possible Mendelian description of the 
phenomenon of sex in a species in which the chromo- 
somes of male and female are alike ; and it is a descrip- 
tion which has its basis in actual phenomena observed 
in two other related animals. 
The view here suggested, then, is that the female is 
heterozygous for the female factor, whilst the male is a 
homozygous recessive so far as that factor is concerned. 
Various facts relating to different groups of animals 
can further be adduced in favour of this interpreta- 
