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.