Evolution and Adaptation

by Thomas Hunt Morgan

There is a peculiarity in the development of the parthenogenetic eggs of animals that will be more fully discussed later, but may be mentioned here. Ordinarily an egg that becomes fertilized gives off two polar bodies, but in a number of cases in which parthenogenetic development occurs it has been found that only one polar body is given off. It is supposed that in such cases one polar body is retained, and that it plays the same part as the entrance of the spermatozoon of the male.

4. Exceptional Cases.—Occasionally in a species that is unisexual an individual is found that is bisexual. The male of the toad, Pelobates fuscus, has frequently a rudimentary ovary in front of the testis. The same thing has been found in several species of fish. In Serranus, a testis is present in the wall of the ovary, and the eggs are said to be fertilized by the spermatozoa of the same individual. In frogs it has been occasionally found that ovary and testis may be associated in the same individual, or a testis may be present on one side, and a testis with an anterior ovarian portion on the other. Cases like these lead up to those in which the body itself may also show a mosaic of sex-characters, and it is noticeable that when this occurs there is nearly always a change in the reproductive organs also. Thus butterflies have been found with the wings and the body of one side colored like the male and the other side like the female. Similar cases have also been found in bees and ants. Bees have been found with the anterior part of the body of one sex and posterior part of another!

The preceding cases illustrate, in different ways, the fact that in the same individual both kinds of reproductive organs may suddenly appear, although it is the rule in such species that only one set develops. Conversely, there are cases known, especially amongst plants, in which individuals, that usually produce male and female organs (or more strictly spores of two kinds from which these organs develop), produce under special conditions only one or the other kind. Facts like these have led to the belief that each individual is potentially bisexual, but in all unisexual forms one sex predominates, and the other remains latent. This idea has been the starting-point for nearly all modern theories of sex.

An excellent illustration of this theory is found in those cases in which the same individual may be male at one time and female at another. For instance, it is said that in one of the species of starfish (Asterina gibbosa) the individuals at Roscoff are males for one or two years, and then become 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 reproductive organs disappear.

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.

Düsing maintains that several of these factors may play a part in determining the sex of the embryo, and if this be true, the problem becomes a very complex one. He also suggests that there are self-regulative influences of such a kind that, when one sex becomes less numerous, the conditions imposed in consequence on the other sex are such as to bring the number back to the normal condition; but this idea is far from being established. The fact that in some species there are generally more individuals of one sex than of the other shows that this balance is not equally adjusted in such forms.