The flowers of most of the flowering plants have both stamens 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 then the evidence pointing in the opposite direction. Among the different causes suggested as determining the sex of the embryo, that of the condition of the egg itself at the time of fertilization has been imagined to be an important factor in the result. Another similar view holds that the condition of the spermatozoon plays the same rôle. For instance, it has been suggested that if the egg is fertilized soon after it leaves the ovary, it produces a female, but if the fertilization is delayed, a male is produced. It has also been suggested that the relative age of the male and the female parents produces an effect in determining the sex of the young. There is no satisfactory evidence, however, showing that this is really the case. Another view suggested is that the sex is determined by the more vigorous parent; but again there is no proof that this is the case, and it would be a difficult point to establish, since as Geddes and Thompson point out, what is meant by greater vigor is capable of many interpretations. Somewhat similar is the idea that if the conditions are favorable, the embryo develops further, as it were, and becomes a male; but there are several facts indicating that this view is untenable.