PARASITES. 99 



far from being a cause of sickness, is in this instance 

 a remedy, and no one can foresee all that science has 

 a right to expect from the salutary effects of certain 

 parasitical worms on the system. There are, if we 

 mistake not, many discoveries in store for observers 

 in this order of investigation. 



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 giddi- 

 ness, 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 Trichinae 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, 



