To clear up the mystery of a movement whose cause is not apparent at the first glance, let us sprinkle with this impalpable débris,--with this kind of sawdust, or _cheese_-dust,--a little strip of glass, and place it beneath the focus of a microscope. Ah! you exclaim, what a frightful creature! These long sharp ciliæ seem to be so many lancets covering the whole body, and especially the legs; its head, like that of the harvest-bug, protrudes and recedes under a transparent carapace; thus communicating to the animal something of the aspect of a turtle. In all other respects its form exactly resembles the harvest-bug; only its body is more elongated towards the anterior extremity than that of the latter. While the harvest-bug makes us think of a spider, the body of the Acarus has a greater likeness to an insect's. (Fig. 77.) Yet the Acarus has eight legs, like a spider, and the harvest-bug six, like an insect. Attempt, then, to establish your absolute rules! [Illustration: FIG. 77.--The _Acarus domesticus_.] Let us continue our observation of this cheese-worm. The well-defined thorax forms nearly one-third of the fore-part of the body, which is of a shining whitish-red or reddish-white. The proboscis, shaped like a conical tube, is armed with two projecting mandibles, which, like true pincers, can be brought close together, or moved wide apart, thrust forward singly or simultaneously. Our animal, which a small lens makes very distinct, has been more than once confounded with the _Sarcoptes scabiei_. Let us resume. Our cheese-dust, which to all appearance walks alone, encloses legions of mites; the old you may detect by their eight feet, the young by having six. The germs, or eggs, whence they spring, are found mixed among the excrements of the living, and the débris of the dead. It is in this way that a crust of cheese offers us a true, a vivid image of the terrestrial crust. So may we learn to compare small things with great. HOW MANY ANIMAL SPECIES ARE THERE DISTRIBUTED OVER THE SURFACE OF THE GLOBE? In the present condition of scientific knowledge, no satisfactory answer can be given to this important and most interesting question. The truth is, that what we may call Geographical Zoology is as yet in its very infancy. The few works which have been published on the subject have been published within the last eighty or ninety years; and they embrace only the vertebrate animals, notably the mammals, birds, and reptiles, or amphibia. We shall attempt to place before the reader an outline of the results that have so far been obtained. * * * * *