In this watchglass of water we have several Cercariæ swimming about. To them we add three or four of those darting, twittering insects which you have seen in every vase of pond-water, and have learned to be the larvæ, or early forms, of the Ephemeron. The Cercariæ cease flapping the water with their impatient tails, and commence a severe scrutiny of the strangers. When Odry, in the riotous farce, Les Saltimbanques, finds a portmanteau, he exclaims, “Une malle! ce doit être à moi!” (“Surely this must belong to me!”) This seems to be the theory of property adopted by the Cercaria: “An insect! surely this belongs to me!” Accordingly every one begins creeping over the bodies of the Ephemera, giving an interrogatory poke with the spine, which will pierce the first soft place it can detect. Between the segments of the insect’s armour a soft and pierceable spot is found; and now, lads, to work! Onwards they bore, never relaxing in their efforts till a hole is made large enough for them to slip in by elongating their bodies. Once in, they dismiss their tails as useless appendages; and begin what is called the process of encysting—that is, of rolling themselves up into a ball, and secreting a mucus from their surface, which hardens round them like a shell. Thus they remain snugly ensconced in the body of the insect, which in time develops into a fly, hovers over the pond, and is swallowed by some bird. The fly is digested, and the liberated Cercaria finds itself in comfortable quarters, its shell is broken, and its progress to maturity is rapid.
Von Siebold’s description of another form of emigration he has observed in parasites will be read with interest. “For a long time,” he says, “the origin of the threadworm, known as Filaria insectorum, that lives in the cavity of the bodies of adult and larval insects, could not be accounted for. Shut up within the abdominal cavity of caterpillars, grasshoppers, beetles, and other insects, these parasites were supposed to originate by spontaneous generation, under the influence of wet weather or from decayed food. Helminthologists (students of parasitic worms) were obliged to content themselves with this explanation, since they were unable to find a better. Those who dissected these threadworms and submitted them to a careful inspection, could not deny the probability, since it was clear that they contained no trace of sexual organs. But on directing my attention to these entozoa, I became aware of the fact that they were not true Filariæ at all, but belonged to a peculiar family of threadworms, embracing the genera of Gordius and Mermis. Furthermore, I convinced myself that these parasites wander away when full-grown, boring their way from within through any soft place in the body of their host, and creeping out through the opening. These parasites do not emigrate because they are uneasy, or because the caterpillar is sickly; but from that same internal necessity which constrains the horsefly to leave the stomach of the horse where he has been reared, or which moves the gadfly to work its way out through the skin of the oxen. The larvæ of both these insects creep forth in order to become chrysalises, and thence to proceed to their higher and perfect condition. I have demonstrated that the perfect, full-grown, but sexless threadworms of insects are in like manner moved by their desire to wander out of their previous homes, in order to enter upon a new period of their lives, which ends in the development of their sex. As they leave the bodies of their hosts they fall to the ground, and crawl away into the deeper and moister parts of the soil. Threadworms found in the damp earth, in digging up gardens and cutting ditches, have often been brought to me, which presented no external distinctions from the threadworms of insects. This suggested to me that the wandering threadworms of insects might instinctively bury themselves in damp ground, and I therefore instituted a series of experiments by placing the newly-emigrated worms in flower-pots filled with damp earth. To my delight I soon perceived that they began to bore with their heads into the earth, and by degrees drew themselves entirely in. For many months I kept the earth in the flower-pots moderately moist, and on examining the worms from time to time I found they had gradually attained their sex-development, and eggs were deposited in hundreds. Towards the conclusion of winter I could succeed in detecting the commencing development of the embryos in these eggs. By the end of spring they were fully formed, and many of them having left their shells were to be seen creeping about the earth. I now conjectured that these young worms would be impelled by their instincts to pursue a parasitic existence, and to seek out an animal to inhabit and to grow to maturity in; and it seemed not improbable that the brood I had reared would, like their parents, thrive best in the caterpillar. In order, therefore, to induce my young brood to immigrate, I procured a number of very small caterpillars which the first spring sunshine had just called into life. For the purpose of my experiment I filled a watch-glass with damp earth, taking it from amongst the flower-pots where the threadworms had wintered. Upon this I placed several of the young caterpillars.” The result was as he expected; the caterpillars were soon bored into by the worms, and served them at once as food and home.[30]
Frogs and parasites, worms and infusoria—are these worth the attention of a serious man? They have a less imposing appearance than planets and asteroids, I admit, but they are nearer to us, and admit of being more intimately known; and because they are thus accessible, they become more important to us. The life that stirs within us is also the life within them. It is for this reason, as I said at the outset, that although man’s noblest study must always be man, there are other studies less noble, yet not therefore ignoble, which must be pursued, even if only with a view to the perfection of the noblest. Many men, and those not always the ignorant, whose scorn of what they do not understand is always ready, despise the labours which do not obviously and directly tend to moral or political advancement. Others there are, who, fascinated by the grandeur of Astronomy and Geology, or by the immediate practical results of Physics and Chemistry, disregard all microscopic research as little better than dilettante curiosity. But I cannot think any serious study is without its serious value to the human race; and I know that the great problem of Life can never be solved while we are in ignorance of its simpler forms. Nor can anything be more unwise than the attempt to limit the sphere of human inquiry, especially by applying the test of immediate utility. All truths are related; and however remote from our daily needs some particular truth may seem, the time will surely come when its value will be felt. To the majority of our countrymen during the Revolution, when the conduct of James seemed of incalculable importance, there would have seemed something ludicrously absurd in the assertion that the newly-discovered differential calculus was infinitely more important to England and to Europe than the fate of all the dynasties; and few things could have seemed more remote from any useful end than this product of mathematical genius; yet it is now clear to every one that the conduct of James was supremely insignificant in comparison with this discovery. I do not say that men were unwise to throw themselves body and soul into the Revolution; I only say they would have been unwise to condemn the researches of mathematicians.
Let all who have a longing to study Nature in any of her manifold aspects, do so without regard to the sneers or objections of men whose tastes and faculties are directed elsewhere. From the illumination of many minds on many points, Truth must finally emerge. Man is, in Bacon’s noble phrase, the minister and interpreter of Nature; let him be careful lest he suffer this ministry to sink into a priesthood, and this interpretation to degenerate into an immovable dogma. The suggestions of apathy, and the prejudices of ignorance, have at all times inspired the wish to close the temple against new comers. Let us be vigilant against such suggestions, and keep the door of the temple ever open.
FOOTNOTES:
[21] The needful term Biology (from Bios, life, and logos, discourse) is now becoming generally adopted in England, as in Germany. It embraces all the separate sciences of Botany, Zoology, Comparative Anatomy, and Physiology.
[22] See Ehrenberg: Microgeologie: das Erden und Felsen schaffende Wirken des unsichtbar kleinen selbstständigen Lebens auf der Erde. 1854.
[23] Charles Robin: Histoire Naturelle des Végétaux Parasites qui croissent sur l’Homme et sur les Animaux Vivants. 1853.
[24] From cilium, a hair.