the Origin of Intestinal JVorms. 316 



Chap. I. — Historical Introduction. 



Sect. 1. Intestinal Worms regarded as identical with Common 

 Worms ; the theory of Equivocal Generation adopted, rejected, 

 and again adopted. — The viscera, and other parts of man and 

 animals, occasionally abound with living worms, and such ques- 

 tions as the following very naturally occur ; — How are they 

 introduced into these situations ? — do they originate sponta- 

 neously ? — or are they introduced from without ? — and in what 

 manner ? The answer to these inquiries must be deemed of 

 the highest importance, by the philosopher and physician. 

 They have, however, been little attended to in the earlier as 

 well as the later periods of the history of science. During the 

 infancy of zoology, when the Ascarides were considered iden- 

 tical with common worms, it was readily concluded that they 

 might be introduced into the body with the water which was 

 drunk, or that their ova or young might be mixed with our 

 food; and thus, at a time when plants, and the lower, and even 

 vertebrate animals, were considered as originating from the 

 decomposition of organic substances, and some of them, for 

 example eels, always in this way. — This theory was scarcely 

 applied to intestinal worms, so easily was their introduction in- 

 to the body accounted for. In the first centuries after the re- 

 vival of science, the whole theory of equivocal generation was 

 generally discredited. Many instances occurred in which its 

 admission was proved to be erroneous, and especially on the 

 discovery of the metamorphosis of insects in the 17th century. 

 Single facts speedily led to a general conclusion, and the Har- 

 veian maxim, " omne vivum ex ovo,^' became a favourite axiom 

 in many physiological schools, in opposition to the theory of 

 spontaneous generation. The supposition, that intestinal worms 

 were identical with those without the body, being prevalent, 

 the question concerning their origin offered no particulai' dif- 

 ficulty. 



About the middle of the 18th century, however, the an- 

 cient theory of equivocal generation again revived ; for the 

 maxim *' omne vivum ex ovo**' was more easily announced, than 

 applied in every individual case. After the discovery of the 

 infysory animalcules by Leuwenhoeck at the close of the 17th 



