422 Evolution and Adaptation 



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 



