SEX INHERITANCE 135 
random fertilization occurs, and that the resulting 
zygotes are equally viable. There is evidence 
bearing on all these possible sources of disturbances 
of the normal sex-ratios. 
In the males of aphids and phylloxerans all of 
the no-X gametes degenerate; in the male bee 
three of the four gametes fail to develop; in the 
hornet two of the four; in Hydatina one of the 
primary spermatocytes gives rise to two female- 
producing gametes, the other, which would give rise 
to male-producing gametes, fails to divide and then 
degenerates. In all of these cases fertilization 
produces only females. The imperfect pollen grains 
that are found in many plants, do not appear to 
come under this heading, since they are not involved 
in sex-determination. 
Much of the genetic work goes to show that 
selective fertilization does not take place. There is 
one set of direct observations, on the unisexual 
mollusk, Cumingia, that shows that the first sperm 
that meets the egg, head on, enters the egg and 
fertilizes it. It has been suggested that under 
environmental conditions unfavorable to the sper- 
matozoa, one kind may be affected more seriously, or 
sooner, than the other kind, and thus bring about a 
selective result; but no direct evidence has ever 
been brought forward to substantiate such a view. 
That one class of spermatozoa may travel faster 
and hence may more often succeed in reaching the 
egg first is a suggestion that is perhaps less hypo- 
thetical than the last. Zeleny and Faust’s careful 
