IV. The question as to whether any advantage is gained by having each new individual produced by the union of two germ-cells, or by having the germ-cells carried by two kinds of individuals.
While our main problem is concerned with the last of these topics, yet there would be little hope of giving a complete answer to it unless we could get some answer to the first three questions.
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.