If there are classes of creatures which expend very little for self-support in comparison with allied creatures, a relatively-extreme prolificness may be expected of them.
Or if, again, we find species presenting the peculiarity that while some of their individuals have much to do and little to eat, others of their individuals have much to eat and little to do, we may look for great fertility in these last and comparative infertility or barrenness in the first.
These several anticipations we shall find completely verified.
§ 357. Plants which, like the Rafflesiaceæ, carry their parasitism to the extent of living on the juices they absorb from other plants, exhibit one of these relations in the vegetal kingdom. In them the organs for self-support being needless, are rudimentary; and the parts directly or indirectly concerned in the production and distribution of germs, constitute the mass of the organism. That small ratio which the race-preserving structures bear to the self-preserving structures in ordinary Phænogams, is, in these Phænogams, inverted. A like relation occurs in the common Dodder.
There may be added a kindred piece of evidence which the Fungi present. Those of them which grow on living plants, repeat the above connection completely; and those of them which, though not parasitic, nevertheless subsist on organized materials previously elaborated by other plants, substantially repeat it. The spore-producing part is relatively enormous; and the fertility is far greater than that of Cryptogams of like sizes, which have to form for themselves the organic compounds of which they and their germs consist.
§ 358. The same lesson is taught us by animal-parasites. Along with the decreased cost of Individuation, they similarly show us an increased expenditure for Genesis; and they show us this in the most striking manner where the deviation from ordinary conditions of life is the greatest.
Take, among the Epizoa, such an instance as Chondracanthus gibbosus. Belonging to the Entomostraca, both males and females of this species are, in their early days, similar to their allies; and the males, practically parasitic, though they become greatly degraded, continue throughout life to show by their segmentation and other external traits their original nature. The female, however, having fixed herself where she can suck the juices of her host, the Lophius, grows to twelve times the length of the male and probably a thousand times its bulk, and becomes utterly transformed by loss of the organs of animal life and enormous development of the organs of reproduction. “No heart is discoverable, and the nervous system and organs of sense (if any) are equally undistinguishable. The interspace between the alimentary canal and the walls of the body is almost wholly occupied by the ovarium.”[63] And then beyond this there are appended ovi-sacs twice the length of the body. So that the germ-producing organs and their contents, eventually acquire a total bulk many times that of all the other organs put together. Numerous species of this type and habit, repeat this relation between a life of inaction with high feeding, and an enormous rate of genesis. Parasites belonging to another great division of the animal kingdom, the Platyhelminthes, supply an example of an epizoon in which the rate of multiplication is made great not so much by immense development of the egg-producing organs as by the rapidity with which generations succeed one another—a rapidity such that each generation partially develops the next before it is itself anything like ready for independent life. This is the Gyrodactylus elegans, of which it is said that “its most remarkable feature is that it is viviparous, and its embryos before they leave the body of their mother have already developed their embryos inside them; and the latter may contain their embryos, so that four generations may be included under the cuticle of the sexually mature animal.”[64]
Entozoa yield us many examples of this causal relation, raised to a still higher degree. The Gordius, or Hair-worm, is a creature which, finding its way when young into the body of an insect which is afterwards swallowed by a fish, there grows rapidly, and then emerging to breed, lays as many as 8,000,000 eggs in less than a day. Similarly with those larger types infesting the higher animals. It has been calculated by Dr. Eschricht, as quoted by Professor Owen, that there are “64,000,000 of ova in the mature female Ascaris lumbricoides.” Very many of the Entozoa belong to the Platyhelminthes, and among them occur examples of fertility caused not only by great numbers of ova, but by rapid succession of partially-developed individuals and also examples of fertility caused by production of ova almost exceeding numeration. Among the first the Liver-fluke may be named. Of the half-million eggs it produces each yields a free-swimming ciliated embryo, and any one of these, which finds its way into a water-snail, becomes a sporocyst—a bag, presently occupied exclusively by masses of cells: each mass by and by becoming a Redia, which makes its way out. Like all its fellows which develop in succession, this, with the exception of a small space occupied by the stomach, devotes the whole of its interior partly to the formation of other Rediæ (which presently escape and become similarly transformed), and partly to the development of Cercariæ, into which the internal substance of all the Rediæ is eventually transformed: Cercariæ which, escaping from the host, become agents for infecting other creatures. So that each ovum thus gives rise to a number of forms which severally subserve multiplication in different ways. Of the other division of Platyhelminthes referred to as carrying on its multiplication by production of ova only, the commonest of the Cestoidea furnishes the best example. Immersed as a Tape-worm is in nutritive liquid, which it absorbs through its integument, it requires no digestive apparatus. The room which one would occupy, and the materials it would use up, are therefore available for germ-producing organs, which nearly fill each segment: each segment, sexually complete in itself, is little else than an enormous reproductive system, with just enough of other structures to bind it together. Remembering that the Tape-worm, retaining its hold, continues to bud out such segments as fast as the fully-developed ones are cast off, and goes on doing this as long as the infested individual lives; we see that here, where there is no expenditure, where the cost of individuation is reduced to the greatest extent while the nutrition is the highest possible, the degree of fertility reaches its extreme. These Entozoa yield us further interesting evidence. Of their various species, most if not all undergo passive migration from animal to animal before they become mature. Usually, the form assumed in the body of the first host is devoid of all that part in which the reproductive structures take their rise; and this part grows and develops reproductive structures, only in some predatory animal to which its first host falls a sacrifice. Occasionally, however, the egg gives origin to the sexual form in the animal that originally swallowed it, but the development remains incomplete—there is no sexual genesis, no formation of eggs in the rudimentary segments. That these may become fertile it is needful, as before, for the containing animal to be devoured; so that the imperfect Tape-worm may find its way into the intestine of a higher animal. Thus the Bothriocephalus solidus, found in the abdominal cavity of the Stickleback, is barren while it remains there; but if the Stickleback be eaten by a Water-fowl, the reproductive system of the transferred Bothriocephalus (then known as B. nodosus) becomes developed and active. So, too, a kind of Tape-worm which remains infertile while in the intestine of a Mouse, becomes fertile in the intestine of a Cat that devours the mouse. May we not regard these facts as again showing the dependence of fertility on nutrition? Barrenness here accompanies conditions unfavourable to the absorption of nutriment; and it gives way to fecundity where nutriment is large in quantity and superior in quality.
§ 359. Extremely significant are those cases of partial reversion to primitive forms of genesis, which occur under special conditions in some of the higher Annulosa. I refer to the pseudo-parthenogenesis and metagenesis in Insects.
Under what conditions do the Aphides exhibit this strange deviation from the habits of their order? Why among them should imperfect females produce, agamically, others like themselves, generation after generation, with great rapidity? There is the obvious explanation that they get plenty of easily-assimilated food without exertion. Piercing the tender coats of young shoots, they sit and suck—appropriating the nitrogenous elements of the sap and ejecting its saccharine matter as “honey dew.” Along with a sluggishness strongly contrasted with the activity of most insects—along with a very low rate of consumption and a correlative degradation of structure; we have here a retrogression to asexual genesis, and a greatly-increased rate of multiplication.