§ 356. Tests of the general doctrines set forth in preceding chapters, are afforded by organisms having modes of life which diverge widely from ordinary modes. Here, as elsewhere, aberrant cases yield crucial proofs.
If certain organisms are so circumstanced that highly-nutritive matter is supplied to them without stint, and they have nothing to do but absorb it, we may infer that their powers of propagation will be enormous.
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.
The recently discovered instance of internal metagenesis in the maggots of certain Flies has a like meaning. Incredible as it at first seemed to naturalists, it is now proved that the Cecydomia-larva develops in its interior a brood of larvæ of like structure with itself. In this case, as in the last, abundant food is combined with low expenditure. These larvæ are found in such habitats as the refuse of beet-root-sugar factories—masses of nitrogenous débris remaining after the extraction of the saccharine matter. Each larva has a practically-unlimited supply of sustenance imbedding it on all sides.[65]
It is true that some other maggots, as those of the Flesh-fly, are similarly, or still better, circumstanced; and, it may be said, ought therefore to have the same habit. But this does not necessarily follow. Survival of the fittest will determine whether such specially-favourable conditions result in aggrandizement of the individual or in multiplication of the race. And in the case of the Flesh-fly there is a reason why greater individuation rather than more rapid genesis will occur. For a decomposing animal body lasts so short a time, that were Flesh-fly larvae to multiply agamically, the second generation would die from the disappearance of their food. Hence individuals in which the excessive nutrition led to internal metagenesis, would leave no posterity, and natural selection would establish the variety in which greater growth resulted. All which the argument requires is that when such reversion to agamogenesis does take place, it shall be where the food is unusually abundant and the expenditure unusually small; and this the cases instanced go to show.
§ 360. The physiological lesson taught us by Bees and Ants, not quite harmonizing with the moral lesson they are supposed to teach, is that highly-fed idleness is favourable to fertility, and that excessive industry has barrenness for its concomitant.
The egg of a Bee develops into a small barren female or into a large fertile female, according to the supply of food given to the larva hatched from it. We here see that the germ-producing action is an overflow of the surplus remaining after completion of the individual; and that the lower feeding which the larva of a working Bee has, results in a dwarfing of the adult and an arrested development of the generative organs. Further, we have the fact that the condition under which the perfect female, or mother-Bee, goes on, unlike insects in general, laying eggs continuously, is that she has plenty of food brought to her, is kept warm, and goes through no considerable exertion. While, contrariwise, it is to be noted that the infertility of the workers is associated with the ceaseless labour of bringing materials for the combs and building them, as well as the labour of feeding the queen, the larvæ, and themselves.
Ants also show us these relations, and they are shown in a greatly exaggerated form by what are called white ants—insects belonging to a quite different order. The contrast in bulk between the fecund and infecund females is here immensely greater. The mother-Ant has the reproductive system so enormously developed, that the remainder of her body is relatively insignificant. Entirely incapable of locomotion, she is unable to deposit her eggs in the places where they are to be hatched; so that they have to be carried away by the workers as fast as they are extruded. Her life is thus reduced substantially to that of a parasite—an absorption of abundant food supplied gratis, a total absence of expenditure, and a consequent excessive rate of genesis. “The queen-ant of the African Termites lays 80,000 eggs in twenty-four hours.”
§ 361. It may be needful to say that these exceptional relations cannot be ascribed to the assigned causes acting alone. The extreme fertility which, among parasites and social insects, accompanies extremely high feeding and an expenditure reduced nearly to zero, presupposes typical structures and tendencies of suitable kinds; and these are not directly accounted for. On creatures otherwise organized, unlimited supplies of food and total inactivity are not followed by such results. There of course requires a constitution fitted to the special conditions, and the evolution of this cannot be due simply to plethora joined with rest. These cases are given as illustrating the conditions under which extreme exaltations of fertility become possible. Their meanings, thus limited, are clear, and completely to the point. We see in them that the devotion of nutriment to race-preservation, is carried furthest where the cost of self-preservation is reduced to a minimum; and, conversely, that nothing is devoted directly to race-preservation by individuals on which falls an excessive expenditure for self-preservation and preservation of other’s offspring.
[Note.—Among specialities of these relations may be fitly added here a very strange one, for a description of which I am indebted to M. Charles Julin, Professor of Comparative Anatomy in the University of Liège. In the Revue Générale des Sciences for 30th August, 1894, in an account of certain investigations of M. Giard, he describes what he calls “la castration parasitaire”—a castration not of a literal kind but one effected by the arrest of development which follows from the depletion caused by a parasite. The Sacculina is an amazingly transformed type belonging to the Cirrhipedia—a type without segments or appendages and without mouth and alimentary canal. Fixing itself, during its early locomotive stage, under the abdomen of a decapodous crustacean, and leaving behind its exo-skeleton, it makes its way into the interior, and there becoming a mere bag containing the reproductive organs, obtains the needful nutriment by developing what are practically roots and rootlets which run everywhere among the viscera and absorb nutriment from the surrounding tissues. Here we are concerned merely with the effect produced upon the host by this physiological robbery. This effect is to arrest the development not only of the primary sexual organs devoted to the production of germs, but also of those secondary sexual organs which characterize the male. M. Julin writes:—
“Il convient cependant de dire, pour être plus exact, que, dans les cas des Crabes infesté par des Sacculines, il n’y a pas, en réalité, apparition de caractères femelles chez le sexe mâle, mais plutôt absence de développement des caractères mâles. En fait, l’animal reste à un stade jeune, non différencié sexuellement, tout en prenant une taille plus considérable. Cela nous porte à attribuer les modifications dont nous avons parlé à un simple arrêt de développement, qui est plus sensible chez le mâle, parce que chez lui les caractères sexuels secondaires sont à l’état normal plus développés que chez la femelle.
D’une manière générale, nous croyons, avec M. Giard, qu’il faut assimiler les modifications dues à la castration parasitaire à celles qui sont le résultat de la progenèse ou qui engendrent le dimorphisme saisonnier.
Il y a progenèse lorsque, chez un animal, la reproduction sexuée s’opère d’une façon plus ou moins précoce, c’est-à-dire lorsque les produits sexuels (œufs ou spermatozoïdes) se forment et mûrissent avant que l’être n’ait atteint son complet développement. On peut citer comme exemples les Axolotls et les larves de Tritons qui, les uns normalement, les autres accidentellement, pondent en ayant encore leurs branchies.
Très souvent la progenèse n’affecte qu’un seul sexe. Tantôt, c’est le sexe femelle qui mûrit à l’état larvaire comme chez les pucerons, les Stylops, etc.... Tantôt c’est le sexe mâle, comme chez la Bonellie, les mâles complémentaires de Cirripèdes, les mâles pygmées des Rotifères, le mâle de l’Anguille, etc. D’autres fois, enfin, l’animal présente successivement les deux sexes avec progenèse pour l’un d’entre eux. C’est ainsi qu’il y a progenèse protandrique chez les Crustacés cymothoadiens, et, parmi les Vertébrés, chez les Myxines, qui, mâles dans le jeune âge, deviennent femelles en vieillissant et en achevant de prendre leur développement. Le cas des vieilles femelles de Gallinacés à plumage et à instincts masculins semble être, au contraire, un exemple imparfait de progenèse protogynique, puisque ces femelles ont pondu lorsqu’elles avaient encore la livrée des jeunes et qu’elles ont continué plus tard leur développement, et présentent le caractère des mâles sans que, cependant, l’on ait constaté la production de spermatozoïdes.
Dans les cas extrêmes de progenèse femelle, la reproduction se fait même sans le concours de l’élément mâle, revenant ainsi à la forme agamique primordiale. Ces cas sont connus depuis longtemps sous le nom de pédogenèse. On les a observé chez les larves de Miastor, de Chironomus et chez certains pucerons.
Chaque fois qu’il y a progenèse dans un type déterminé, on constate soit momentanément, soit d’une façon définitive, un arrêt de croissance et de développement: l’animal progénétique a, par suite, l’aspect d’une larve sexuée, lorsqu’on le compare soit à l’autre sexe, soit aux formes voisines, qui ne présentent pas le phénomène de la progenèse.
Cela est en parfaite harmonie avec le principe, si bien mis en lumière par Herbert Spencer, de l’antagonisme entre la genèse et la croissance et entre la genèse et le développement. Cet antagonisme s’explique facilement si l’on songe que les matériaux employés pour la reproduction ne peuvent servir à l’accroissement de l’individu. S’il est avantageux pour un organisme de se reproduire sans acquérir des organes inutiles, la sélection naturelle déterminera bientôt une progenèse de plus en plus complète. Les animaux parasites, outre qu’ils tirent de leur hôte une nourriture abondante, n’ont guère besoin d’une foule d’organes qui servent à leurs congénères libres dans la vie de relation. Aussi voyons-nous qu’un très grand nombre d’animaux parasites sont progénétiques. Les mâles progénétiques de la Bonellie et des Cirripèdes vivent en parasites dans leurs femelles. Chez certains types, les pucerons, la progenèse cesse dès que, la nourriture devenant moins abondante, un déplacement pourra être nécessaire.
En résumé, l’arrêt de développement dû à la progenèse résulte d’une dérivation des principes nourriciers au détriment de l’animal progénétique. Dans les exemples de castration parasitaire que nous avons examinés, le parasite joue, par rapport à son hôte, absolument le même rôle que la glande génitale d’un type progénétique. Il détourne, pour sa propre subsistance, une partie des principes qui auraient servi au développement de l’animal. Aussi les effets produits sont-ils tout à fait de même ordre.”
A phenomenon so anomalous as this, explicable upon the hypothesis set forth but not otherwise explicable, furnishes striking verification.]