14 INTKODUOTION. II. 



and contains little nutriment, are rarely free from parasites. This is the case 

 Avith the coleopterous insect Passalus, and the myriapod Julus. 



Cooking food is of advantage in destroying the germs of parasites ; and hence 

 man, notwithstanding his liability to the latter, is less infested than most other 

 mammalia. Did instinct originally lead him to cook his food, to avoid the intro- 

 duction of parasites ? 



Entozoa are more abundant than entophyta, because the power of voluntary 

 movement favors them in their transmigrations, and renders them less liable to 

 expulsion from the intestinal canal. 



Influence of Parasites in the Prodiiction of Disease. — In many animals entozoa and 

 entophyta are almost never absent, and probably when in their natural habitation, 

 and few in number, or not of excessive size, are harmless, as observed by Dujardin 

 in the introduction of his excellent work on Intestinal Worms : " Les helmiuthes 

 se developpent dans un site qui leur convient, sans nuire plus que les lichens sur 

 r^corce d'un arbre vigoureux. lis ne peuvent devenir nuisibles, gen^ralement, que 

 par suite d'une multiplication excessive, laquelle semble alors etre une des conse- 

 quences d'un affaiblissement provenant d'une tout autre cause, d'une mauvaise ali- 

 mentation, du sejour dans un lieu froid et humide, etc. : sans cela, les helminthes 

 naissent et meurent dans le corps de leurs hfites, et peuvent paraitre et disparaitre 

 alternativement sans inconv6nients." 



Many important diseases have been supposed to originate from parasitic animals 

 and vegetables. The former are not the true entozoa, for these are too large, and 

 may be detected by the naked eye, but they are considered to be animalcule so 

 small that they cannot be discovered even with the highest powers of the micro- 

 scope. But, independent of the fact that the existence of such entities is a mere 

 suspicion, none of the well-known animalcula3 are poisonous. At various times, I 

 have purposely swallowed large draughts of water containing myriads of Monas, 

 Vibrio, Euglenia, Volvox, Leuco^lirys, Paramecium, VorticeUa, etc., without ever 

 having perceived any subsequent effect. 



The production of certain diseases, however, through the agency of entophyta, is 

 no longer a subject of doubt; as in the case of Muscardine in the Silk-worm, the 

 Mycoderm of Porrigo favosa in Man, etc. ; but that malarial and epidemic fevers 

 have their origin in cryptogamic vegetables or spores requires yet a single proof.^ 

 If such were the case, these minute vegetables and spores, conveyed through the 

 air, and introduced into the body in respiration, could be detected. The minutest 

 of all known living beings is the Vibrio lineola of Miiller, measuring only the 

 36,000th of an inch, and the smallest known vegetable spore is very much larger 

 than this, whilst particles of inorganic matter can be distinguished the 200,000th 

 of an inch in size. 



I have frequently examined the rains and dews of localities in which intermit- 

 tents were epidemic upon the Schuylkill and Susquehanna Rivers, but without 

 being able to detect animalculge, spores, or even any solid particles whatever. I 



* See an ingenious little work by my distinguished friend Dr. J. K. Mitchell, "On the Cryptogamous 



Origin of Malarious and Epidemic Fevers." 



