422 Evolution and A daptation 
females. At Banyuls they are males for the first two or three 
years, and then become females; while at Naples some are 
always males, others females, some hermaphrodites, others 
transitional as in the cases just given. In one of the isopod 
crustaceans, Angiostomum, the young individuals are males 
and the older females. In Myzostomum glabrum the young 
animal is at first hermaphroditic, then there is a functional 
male condition, followed by a hermaphroditic condition, and 
finally a functional female phase, during which the male repro- 
ductive organs disappear. 
The flowers of most of the flowering plants have both sta- 
mens and pistils, which contain the two kinds of spores out 
of which the male and female germ-cells are formed. The 
stamens become mature before the pistils, as a rule, but in 
some cases the reverse is the case. This difference in the 
time of ripening of the two organs is often spoken of as an 
adaptation which prevents self-fertilization. The latter is 
supposed to be less advantageous than cross-fertilization. 
This question will be more fully considered later. 
Before we come to an examination of the question of the 
adaptations involved in the cases in which the sexes are 
separate, and the different times at which the sex-cells are 
ripened, it will be profitable first to examine the question as 
to what determines in the egg or young whether a male or a 
female or a hermaphroditic form shall arise. 
THE DETERMINATION OF SEX 
A large number of views have been advanced as to what 
determines whether an egg will give rise to a male or to a female 
individual. The central question is whether the fertilized egg 
has its sex already determined, or whether it is indifferent; 
and if the latter, what external factor or factors determine 
the sex of the embryo. Let us first examine the view that 
some external factor determines the sex of the individual, and 
