28 INTRODUCTION. 



of their eggs would lead one to suppose. This inclines one 

 more readily to the belief that nature, seeing how full of diffi- 

 culties is the way of these parasites to sexual development, has 

 endowed them with the power of generating millions and 

 billions of eggs, when once they have overcome these obstacles 

 and developed the necessary sexual organs. Through the un- 

 ceasing spread of cultivation, the decrease and extirpation of 

 certain animals, on the one hand, and the taming and increase of 

 domestic animals on the other, the conditions of life of many of 

 the intestinal worms have become so changed, and so widely 

 different from their original state, that, with their inherent 

 tendency to wander, many of these parasites must often go 

 astray. 



The Trichina spiralis, which is found in human beings, and 

 which, as I have already shown, must be regarded as an encysted 

 sexless nematoid worm, can hardly have found its way into the 

 muscular substance of man, except by having gone astray ; so also 

 the Cysticercus cellulose, which not unfrequently appears in the 

 muscles and other organs of man, and which, as I shall hereafter 

 show, is an asexual tsenioid agamozooid. The Cysticercus celluloses 

 changes to a sexual tape- worm in the intestinal canal of certain 

 mammals ; the Trichina spiralis, after transportation to another 

 and more favorable situation, will also become sexually de- 

 veloped. That these two parasites should have been originally 

 intended to pass into and establish themselves in, human beings, 

 waiting for the opportunity to emigrate, which could only occur 

 when the person who harboured the sexless parasite should be 

 devoured by some appointed beast of prey, is an idea insufferable 

 to the dignity of man, 1 which every reader of these lines must of 

 necessity reject ; and admit, instead, that the appearance of these 

 parasites in the interior of man can only be accounted for by 

 the fact of their having gone astray. 



Many of the young of the intestinal worms which only attain 

 the last stage of their development in the digestive canal of the 

 Vertebrata, chance, in the course of their wanderings, to pass 

 into the wrong organs ; for instance, into the muscular substance, 

 the liver, or the peritoneum; here they remain undeveloped, 



1 To appeal to the " dignity of man" in a zoological argument appears a little out of 

 place. Nature seems to have had small respect for our " dignity" when she created the 

 fleas, lice, and hugs which annoy us ; the Ascaris, which reduces us below the level of 

 the beast ; the Strongylus, and the Echinococcus, which destroy us outright. [Eo.] 



