24 THEORY OF THE ORIGIN OF PARASITES. 



chiefly wished to prevent it from being thought that intestinal worms 

 were derived from insects and other free-living animals ; nevertheless, 

 he does not deny the theory that they originate from the eggs of 

 species " that have already existed in the intestines of other animals." 



But in spite of the anathemas which Swammerdam hurled against 

 the theory of the heterogeneous development of the Entozoa, this 

 theory shortly after was very generally accepted. 



While, on the one hand, the existence of sexual generation in 

 animals was being shown to be more and more universal, and became 

 more definite, the microscope, newly applied to scientific researches, 

 revealed a whole host of minute creatures, which, in spite of their wide 

 distribution, had hitherto escaped attention on account of their small 

 size. Animalcules were found in drinking water and in food, in 

 the earth, and were supposed to exist even in the air: was it not 

 natural that under the influence of these discoveries the theory of the 

 heterogeny of Entozoa fell upon a fruitful soil ? The introduction of 

 these creatures into the human body appeared almost inevitably to 

 lead to the conclusion that, when acted upon by the warmth and 

 abundant nutriment in the body, they increased in size and became 

 veritable Entozoa. It is not surprising, therefore, that men like 

 Boerhaave * and Hoffmann 2 traced back the Cestodes and Nematodes 

 to animals which, when existing in the free state, were totally 

 different in appearance. The creatures that were supposed to be the 

 progenitors of the Entozoa were by no means the Infusoria alone, but 

 sometimes other larger creatures, such as free-living worms, and 

 specially such worms as possessed a superficial resemblance to the 

 Entozoa. Although a theory of this kind appears to us now entirely 

 unscientific, we must not forget that at that time discoveries in the 

 metamorphosis of animals were too recent and incomplete to allow of 

 a just appreciation of the law of stability of species and their cyclical 

 development. 



The actual nature of parasitic worms did not long remain unknown. 

 Not only did naturalists gradually come to see that the occasional 

 change of free-living animals into Entozoa was in entire contradic- 

 tion to the common phenomena of generation and development, but 

 they learnt to recognise the Entozoa as sexual animals, whose organic 

 structure marked them out as representatives of special classes of 

 animals. 



At the same time, however, it appeared that these creatures did 

 not exist exclusively as Entozoa, but that they were also capable of a 

 free existence. By a careful and systematic examination of our rivers 

 and streams, a number of animal forms were discovered that appeared 



1 " Aphorism.," 1360. a " Opera," t. iii., p. 490. 



