But here, as in all things, excess is hurtful. Certain organisms, developing themselves immoderately, may break the harmony necessary between the parasites and the host which they frequent. It has been found recently that many morbid affections, as the potato and vine diseases, have for their origin only the abnormal development of certain microscopic beings hidden in the organism. It is found, that in Egypt, a distoma is developed in the blood, and occasions a very severe malady, scarcely known to physicians. In Iceland, a cestode causes the death of a third part of the population. Worms develop themselves in the eye, and may even cause blindness; the _Coenurus_ of the sheep causes giddiness, and becomes fatal to the animal which harbours it. The chlorosis observed in Egypt and Brazil must, it appears, be attributed to a considerable development of a nematode worm, which lives in the small intestines, and which naturalists know under the name of _Dochmius duodenalis_; and lately the Trichinæ set all Europe in a state of excitement, and trichinosis was for a time more dreaded than cholera. In spite of all these accidental circumstances we think that the animal which possesses its ordinary parasites, far from being ill, is in a normal physiological condition. When we consider these animal parasites in general, one would think 100 that their tenacity of life is very feeble, and that the slightest derangement would be sufficient to kill them. It is not so; on the contrary, some of them can be entirely dried up, and return to life every time that they are moistened; and the eggs of some of them resist the most violent reagents. We have known eggs preserved for years in alcohol, in chromic acid, and in other agents which destroy life everywhere else; and then give birth to embryos directly they are placed in pure water or damp earth. Some years ago they had no idea of the migration of animals from one body to another. As we have said elsewhere, Abildgard, half a century ago, made experiments on the worms of fishes which he caused ducks to swallow, but these experiments had no result, and formed rather an obstacle to ulterior progress, than an approach to truth. The worms of fishes have been known to live in birds; but these worms were only there as adventitious parasites. Liguli live some days in the goosander, but they do not maintain their position.