Parasites are scattered over every region of the globe; they choose their place, and observe, like all living creatures, the laws of geographical distribution. All do not inhabit the animal kingdom; some seek for assistance in vegetable life. Many insects lay their eggs in seeds or fruits, and their progeny, as soon as they are hatched, find abundant nourishment in the sap or in the farina stored up for the young plant; others pass into a state of lethargy while the seed is dry, and recover their activity every time that they receive a little humidity. The female of a coleopterous insect deposits its eggs in the nut, and in proportion as this grows, the young larva devours the kernel. When it is brought to table, it encloses only the skin and the excretions of the larva. A weevil establishes itself in a similar manner in cereal plants, and, small as it is, it may produce great calamity by multiplying in granaries. There are even worms which lodge in 95 certain of the graminaceæ, and get completely dry with the envelope which contains them, without ceasing to live. Their life is suspended till the day when the seed is sufficiently softened in the earth or the water. We have seen that each parasite has its host: we must have a particular name to designate it. But that does not imply that if it find not its dwelling-place it must perish. It may only live some time at the expense of its neighbour, and thus pass for its parasite. Naturalists are occasionally deceived. Thus, they once believed in the passage of the Schistocephalus of the stickleback into the intestines of certain birds which eat them, and in which they are only found accidentally. The Ligulæ of the Cyprinidæ, found in the intestines of the cormorant or the goosander, are not, in our opinion at least, worms peculiar to these birds. They are strangers which must either emigrate again or die. Acari which originally belonged to mammals and birds, have been found living on man, causing prurigo, or even serious maladies, and yet these parasites are not regarded as peculiar to our species. We might cite other examples. Who has not been annoyed by the flea, which abandons for an instant the dog, its natural host?