28 THEORY OF THE ORIGIN OF PARASITES. 



know well, from transmitted germs, and only in consequence of a 

 process of generation, similar to that which exists in the rest of the 

 animal kingdom. 



In spite of the accordance between our present knowledge and 

 the theories of van Doeveren and Pallas, the one is not the direct 

 outcome of the other. The path of science strays now on one side, 

 now on another side of the direct line of truth, and we ought not, 

 therefore, to be surprised that the theory just quoted was pushed 

 out of the way by other theories before it had time to take root. 



With Pallas, Bloch, and Goze began a long list of helmintholo- 

 gists, of whom the most eminent were Eudolphi and Bremser. 

 Thousands of animals were examined for their parasites, and with 

 such success, that the number of the known Entozoa was soon esti- 

 mated at many hundreds. As the material got larger, the science of 

 helminthology became gradually more and more separated from 

 zoology, and treated as a distinct specialty. This distinction had its 

 evil effects. It caused helminthology to become a mere descriptive 

 enumeration, hardly at all concerned with the life-histories and de- 

 velopment of the animals so carefully registered. This one-sided 

 way of looking at parasites was hardly suitable for solving the ques- 

 tions concerning their origin by careful and unprejudiced experiment. 

 That all previous attempts to explain the presence of these remark- 

 able creatures by the theory of their introduction into the body of 

 their host from without were more or less conspicuously faulty was 

 never at any time doubtful, and perhaps least of all at the present 

 moment. Instead of increasing the number of known facts by the 

 empirical method, and getting, where possible, fresh support on which 

 to base some theory, which might, although not completely proved, 

 furnish numerous and weighty arguments for induction, helmin- 

 thologists were content to point out the insufficiency of earlier investi- 

 gations, and return again to the almost forgotten theory of spontaneous 

 generation, 1 which was at any rate a convenient and simple method 

 of cutting the knot. 



Those were the times when the all-powerful " vital force," governed 

 the organism. And it seemed an easy thing for this " vitality :> to 

 organize a mass of mucus, a villus of the intestine, or a fragment of con- 

 nective tissue, perhaps by an intensified abnormal process of develop- 

 ment, into a bladder- worm instead of a simple hydatid. The structure 

 of the Entozoa was regarded as comparatively simple, and it appeared, 

 therefore, that from this point of view no great difficulties stood in 



1 See specially the excellent work of Bremser, ' ' Lebende Warmer im lebenden 

 Menschen," pp. 1-16 : Wien, 1819. 



