Evolution and Adaptation

by Thomas Hunt Morgan

The Different Kinds of Sexual Individuals

Amongst the unicellular animals and plants the fusion of two (or more) individuals into a single one is generally regarded as the simplest, and possibly also the most primitive, method of sexual reproduction. Two amœbas, or amœba-like bodies, thus flow together, as it were, to produce a new individual.

In the more highly specialized unicellular animals, the processes are different. Thus in vorticella, a small, active individual unites with a larger fixed individual. The protoplasm fuses into a common mass, and a very complicated series of changes is passed through by the nucleus. In paramœcium, a free-swimming form very much like vorticella, two individuals that are alike unite only temporarily, and after an interchange of nuclear material they separate.

In the lower plants, and more especially in some of the simple aggregates or colonial forms, there are found a number of stages between species in which the uniting individuals are alike, and those in which they are different. There are several species whose individuals appear to be exactly alike; and other species in which the only apparent difference between the individuals that fuse together is one of size; and still other species in which there are larger resting or passive individuals, and smaller active individuals that unite with the larger ones. In several of the higher groups, including the green algæ and seaweeds, we find similar series, which give evidence of having arisen independently of each other. If we are really justified in arranging the members of these groups in series, beginning with the simpler cases and ending with those showing a complete differentiation into two kinds of germ-cells, we seem to get some light as to the way in which the change has come about. It should not be forgotten, however, that it does not follow because we can arrange such a series without any large gaps in its continuity, that the more complex conditions have been gradually formed in exactly this way from the simplest conditions.

So far we have spoken mainly of those cases in which the forms are unicellular, or of many-celled species in which all the cells of the individual resolve themselves into one or the other kind of germ-cells. This occurs, however, only in the lowest forms. A step higher we find that only a part of the cells of the colony are set aside for purposes of reproduction. The cells surrounding these germ-cells may form distinct organs, which may show certain differences according to whether they contain male or female germ-cells. When these two kinds of cells are produced by two separate individuals, the individuals themselves may be different in other parts of the body, as well as in the reproductive organs.

When this condition is reached, we have individuals that we call males and females, because, although they do not themselves unite to form new individuals, they produce one or the other kind of germ-cell. It is the germ-cells alone that now combine to form the new individual.

Amongst living groups of animals we find no such complete series of forms as exist in plants, and the transition from the one-celled to the many-celled forms is also more abrupt. On the other hand, we find an astonishing variety of ways in which the reproduction is accomplished, and several ways in which the germ-cells are carried by the sexual individuals. Let us examine some of the more typical conditions under the following headings: (1) sexes separate; (2) sexes united in the same individual; (3) parthenogenetic forms; (4) exceptional methods of propagation.

1. Sexes Separate; Unisexual Forms.[^[34]]—Although the animals with which we are more familiar have the sexes separate, this is far from being universal amongst animals and plants; and, in fact, can scarcely be said to be even the rule. When the sexes are separate they may be externally alike, and this is especially true for those species that do not unite, but set free their eggs and spermatozoa in the water, as fish, frogs, corals, starfish, jellyfish, and many other forms. In other animals there are sometimes other secondary differences in the sexes besides those connected with the organs of reproduction. Such differences are found, as we have seen, in insects, in some spiders, crustaceans, and in many birds and mammals. In a few cases the difference between the sexes is very great, especially when the female is parasitic and the male free, as in some of the crustaceans. In some other cases the male is parasitic on the female. Thus in Bonellia the male is microscopic in size, being in length only one-hundredth part of the female. In Hydatina senta the male is only about a third as large as the female. It has no digestive tract, and lives only a few days. In another rotifer the males are mere sacs enclosing the male reproductive organs.

[34]. Geddes and Thompson’s “The Evolution of Sex” has been freely used in the preparation of this part of this chapter.

2. Hermaphroditic Forms.—There are many species of animals and plants in which each individual contains both the male and the female organs of reproduction, and there are whole groups in which only these hermaphroditic forms occur. Thus in the ctenophors the eggs develop along one side of each radial canal and spermatozoa along the other. The group of flatworms is almost exclusively hermaphroditic. The earthworms and the leeches have only these bisexual forms, and in the mollusks, while a few groups have separate sexes, yet certain groups of gasteropods and of bivalve forms are entirely hermaphroditic.

In the common garden snail, although there are two sets of sexual ducts closely united, yet from the same reproductive sac both eggs and sperm are produced. The barnacles and the ascidians are for the most part hermaphroditic forms. Many other examples might be cited, but these will suffice to show that it is by no means unusual in the animal kingdom for the same individual to produce both male and female germ-cells. However, one of the most striking facts in this connection is that self-fertilization seldom takes place, so that the result is the same in certain respects as though separate sexes existed. This point will come up later for further consideration.

3. Parthenogenetic Reproduction.—It has long been known that, in some cases, eggs that are not fertilized will begin to develop and may even produce new individuals. Tichomiroff showed that by rubbing with a brush the unfertilized eggs of the silkworm moth, a larger percentage would produce caterpillars than if they were not rubbed. During the last few years it has been shown that the development of a non-fertilized egg may be started in a number of ways. Such, for example, as by certain solutions of salt or of sugar, by subjecting the eggs to cold, or by simply shaking them.

There are certain groups of animals in which the males appear only at regular (in others at irregular) intervals. In their absence the females produce eggs that develop without being fertilized, i.e. parthenogenetically. The following examples will serve to show some of the principal ways in which this “virgin reproduction” takes place. In the group of rotifers the males are generally smaller than the females and are usually also degenerate. In some species, although degenerate males are present, they are unnecessary, since parthenogenesis is the rule. In still other species no males exist and the eggs develop, therefore, without being fertilized. In some of the lower crustaceans parthenogenesis occurs in varying degrees. In Apus males may be entirely absent at times in certain localities, and at other times a few, or even very many, males may appear. Some species of ostracod crustaceans seem to be purely parthenogenetic; others reproduce by means of fertilized eggs; and others by an alternation of the two processes. The crustaceans of the genus Daphnia produce two kinds of eggs. The summer eggs are small, and have a thin shell. These eggs develop without being fertilized, but in the autumn both male and female individuals develop from these unfertilized eggs, and the eggs of the female, the so-called winter eggs, are fertilized. These are also larger than the summer eggs, have thicker shells, and are much more resistant to unfavorable conditions. They give rise in the following spring to females only, and these are the parthenogenetic individuals that continue to produce during the summer new parthenogenetic eggs.

It is within the group of insects that some of the most remarkable cases of parthenogenesis that we know are found. In the moth, Psyche helix, only females are present, as a rule, but rarely males have been found. In another moth, Solenobia trinquetrella, the female reproduces by parthenogenesis, but at times males appear and may then be even more numerous than the females. In the gall-wasps parthenogenetic generations may alternate with a sexual generation, and it is interesting to note that the sexual and the parthenogenetic generations are so different that they were supposed to belong to separate species, until it was found that they were only alternate generations of the same species.

The aphids or plant-lice reproduce during the summer by parthenogenesis, but in the autumn winged males and females appear, and fertilized winter eggs are produced. From these eggs there develop, in the following spring, the wingless parthenogenetic summer forms, which produce the successive generations of the wingless forms. As many as fourteen summer broods may be produced. By keeping the aphids in a warm temperature and supplying them with plenty of moist food, it has been possible to continue the parthenogenetic reproduction of the wingless forms for years. As many as fifty successive broods have been produced in this way. It has not been entirely determined whether it is the temperature or a change in the amount, or kind, of food that causes the appearance of the winged males and females, although it seems fairly certain that diminution in the food, or in the amount of water contained in it, is the chief cause of the change.

In the honey-bee the remarkable fact has been well established that fertilized eggs give rise only to females (queens and workers), while unfertilized eggs develop into males. Whether a fertilized egg becomes a queen or a worker (sterile female) depends solely on the kind of food that is given to the young larva, and this is determined, in a sense, entirely by the bees themselves.

In plants also there are many cases of parthenogenesis known. Some species of Chara when kept under certain conditions produce only female organs, and seem to produce new plants parthenogenetically. In this case it appears that the same conditions that caused the plants to produce only female organs may also lead to the development of the egg-cells without fertilization. In fact it is only by a combination of this kind that parthenogenesis could arise. The result is similar when the eggs of insects produce only females whose eggs are capable of parthenogenetic development. If a case should arise in which only females appeared whose eggs did not possess the power of parthenogenetic development, the species would die out.

In the green alga, Spirogyra, it has been found that if conjugation of two cells is prevented, a single cell may become a parthenogenetic cell. In a number of parasitic fungi the male organs appear to be degenerate, and from the female organs parthenogenetic development takes place. A small number of flowering plants are also capable of parthenogenetic reproduction.

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.