It is quite otherwise with parasites which live singly within the body of a host: for these it was indispensably necessary that they should not only produce both kinds of germ-cells, but that they should unite the two kinds in fertilization, and they therefore possess the power of self-fertilization. Thus, in the urinary bladder of the frog, there occurs a flat-worm (Polystomum integerrimum) which possesses special organs for pairing with another individual, but which is also capable of self-fertilization when, as frequently occurs, it has no companion in its place of abode. But this self-fertilization is always liable to be interrupted by cross-fertilization, for not infrequently there are two, three, or even four such parasites within the bladder of a single frog.
In the tape-worms, too, cross-fertilization is not excluded, for there are often two or more of these animals together in the intestine of a host at the same time. But even where there is only one, self-fertilization on the part of the joints, that is, the sexual individuals, is prevented, and by the same device, metaphorically speaking, as in the case of the oyster, for in each joint the male elements mature first and the female elements afterwards. In certain parasitic Isopods of the genus Anilocra and related forms close inbreeding is prevented in the same way—by a difference in the period at which the two sets of gonads in the hermaphrodite individual become mature (dichogamy).
This is secured in a different way in Crustaceans which have grown to maturity in a sedentary state, like the Cirrhipeds. These animals, known as 'acorn-shells' and 'barnacles,' are sedentary, sometimes on rocks and stones, sometimes on a movable object, the keel of a ship, floating pieces of wood, cork, or cane, or sometimes attached to turtles or whales, and although they generally occur in great numbers together, they are probably only able to fertilize each other occasionally, and are therefore essentially dependent upon self-fertilization. But Charles Darwin discovered long ago that many of them, notwithstanding their hermaphroditism, have males which are small, dwarf-like, and very mobile organisms, destined only for a very brief life. These seemed quite superfluous in association with hermaphrodite animals, and they have therefore long been regarded as vestigial males, as the last remnant, so to speak, of a past stage of the modern Cirrhipeds, in which the sexes were separate. It is obvious, however, that we must now attribute to them a deeper significance, for these so-called 'primordial males,' although extremely transitory creatures without mouth or intestine, represent a means of securing the cross-fertilization of the species. What importance nature attaches to their preservation is shown especially by the parasitic Cirrhipeds which have been so carefully studied by Fritz Müller and Yves Delage—those sac-like Rhizocephalidæ or root-crustaceans which are altogether disfigured by parasitism. The fully developed animals are hermaphrodite and live partly in, partly upon crabs and hermit-crabs (Fig. 112, C, Sacc). These hermaphrodites indeed fertilize themselves, but in their youth they are of distinct sexes, and the females are so constituted that they lay eggs for the first time just when the males of the current year are appearing. Thus the first batch of eggs liberated by the females are fertilized by the minute free-swimming 'primordial males,' but after that the females themselves develop testes, and then fertilize themselves; the males die very soon after copulation, and only appear the following year in a new generation. They are therefore far from being mere historic reminiscences, vestiges of the early history of the modern species, for they are the instruments of a regular cross-fertilization of the species, and therefore of a constant mingling of new ids in the germ-plasm. This is not the place to discuss the marvellous life-history of these parasites in detail; I can only say that when we inquire into the whole story, and appreciate the difficulties associated with the persistence of these 'primordial males,' we can no longer doubt that crossing is an indispensable feature of amphimixis—a feature which must at least occasionally occur if amphimixis is to retain its significance. This is shown, it seems to me, especially by these numerous instances of what we may call compulsory retention of ephemeral males in hermaphrodite, self-fertilizing animals; it follows also from the theory, for with continued self-fertilization all the ids in the germ-plasm of an individual would tend to become identical, and the mingling of two germ-plasms which contained identical ids would, at least according to the germ-plasm theory, have no meaning at all.
Fig. 112 (repeated). Development of the parasitic Crustacean Sacculina carcini, after Delage. A, Nauplius stage. Au, eye. I, II, III, the three pairs of appendages. B, Cypris stage. VI-XI, the swimming appendages. C, mature animal (Sacc), attached to its host, the shore-crab (Carcinus mænas), with a feltwork of fine root-processes enveloping the crab's viscera. S, stalk. Sacc, body of the parasite. oe, aperture of the brood-cavity. Abd, abdomen of the crab with the anus (a).
Thus we see that in the animal kingdom hermaphroditism is always associated with cross-fertilization in some way or other, even though the latter may occur rarely, being usually periodically interpolated, and thus bringing new ids into the germ-plasm which is rapidly becoming monotonous or uniform. Adaptations quite analogous to these are found in relation to parthenogenesis, and it will repay us to give a brief summary of these.
Parthenogenesis effects a very considerable increase in the fertility of a species, and in this increase the reason for its introduction among natural phenomena obviously lies. By the occurrence of parthenogenesis, the number of ova produced by a particular colony of animals may be doubled, because each individual is a female, and as the multiplication increases in geometrical ratio a few parthenogenetic generations result in a number of descendants enormously in excess of those produced by bi-sexual reproduction. We can therefore understand why parthenogenesis should obtain among animals whose conditions of life are favourable only for a short time, and are then uncertain and dangerous for a long period. This is the case with the water-fleas, the Daphnids (see Figs. [57] and [58]), whose habitats—pools, ponds, and marshes—often dry up altogether in summer, or freeze in winter, so that it becomes almost if not quite impossible for the colonies to go on living, and the preservation of the species can only be secured by the production of hard-shelled 'lasting' eggs, which sink to the bottom, dry up in the mud, or become frozen, or at least remain latent in a sort of slumber. As soon as the favourable conditions reappear, young animals which emerge from the eggs are all females and reproduce parthenogenetically, so that after a few days there is a numerous progeny swimming freely about, which in their turn are all females, and reproduce after the same manner. In many Daphnids this goes on for a series of generations, and there thus arises an enormous number of animals, which may fill a marsh so densely that, by drawing a fine net a few times through the water, one can draw out a veritable animal soup. In our ponds and lakes these little Crustaceans form the fundamental food of numerous fishes. But notwithstanding the enormous havoc wrought among them by enemies, large numbers remain at the end of a favourable season, and these produce the lasting eggs, after fertilization. For shortly before the end of the season males appear among the progeny of the hitherto purely parthenogenetic females. Although each female will only produce a few of these 'lasting' eggs, which require fertilization and are richly supplied with yolk, the whole number in each colony is a very large one, because the number of individuals is very large; and it must be so, since the eggs, though secure against cold and desiccation, are very imperfectly protected against the numerous enemies which may do them injury.
Of course the number of individuals which form a colony may vary greatly in the different species, and the same is true of the number of parthenogenetic generations which precede the bi-sexual generation. I have already shown in detail that this depends precisely on the average duration of the favourable conditions, so that, for instance, a species which lives in large lake-basins will produce many purely parthenogenetic generations before the bi-sexual one, which only appears towards autumn, while species which live in quickly-drying pools have only a few parthenogenetic generations, and the true puddle-dwellers give rise to males and sexual females along with the parthenogenetic females as early as the second generation.
We thus find in the Daphnids an alternation, regulated and made normal by natural selection, of purely parthenogenetic with bi-sexual generations, and the result is that the uniformity of the germ-plasm, which is the necessary consequence of pure parthenogenesis, is interrupted after a longer or shorter series of generations by the occurrence of amphimixis. That the number of parthenogenetic generations may be so varied, though with a definite norm for each species, indicates again that amphimixis is not an absolute condition of the maintenance of life, not an indispensable rejuvenation, designed to counteract the exhaustion of vital force—whether this be meant in a transcendental sense or otherwise—but that it is an important advantage calculated to keep the species at its highest level, and that its influence appears whether it occurs in the species regularly, or frequently, or only rarely.