ORIGIN OF PARASITES II 



only the progeny of flies, and never appear in the flesh of slaughtered 

 animals when fully developed flies are prevented from approaching 

 and depositing their eggs on it. Swammerdam likewise knew 

 that the " worms " living in the caterpillars of butterflies were the 

 larvae of other insects (Ichneumon flies) which had laid their eggs in 

 their bodies ; he also discovered the ova of lice. The two authors 

 mentioned were, however, unwilling to see the experience they had 

 gained regarding insects applied to the helminthes, while Leeuwen- 

 hoek vehemently opposed the theory of a spontaneous generation, 

 maintaining that, on a basis of common sense, eggs, or at all events 

 germs, must exist, even though they could not be seen. 



The use of the microscope also revealed a large number of very 

 small organisms in the water and moist soil, some of which undoubtedly 

 resembled helminthes. Considering the wide dissemination of these 

 minute organisms, it was natural to conjecture that after their almost 

 unavoidable introduction into the human system they should grow 

 into helminthes (Boerhave, Hoffmann). Linnaeus went even further, 

 for he traced the descent of the liver-fluke of sheep from a free-living 

 planaria (Dendroccelum lacteum), the Oxyuris vermicularis, from free- 

 living nematodes, and the Tcenia lata (i.e., Bothriocephalus latus) 

 from a tapeworm (Schistocephalus solidus) found free in the water. 

 Linnaeus' statements met with general approval. However, we 

 must bear in mind that at that time the number of helminthes 

 known was very small, and many of the forms that we have long 

 ago learned to differentiate as specific were then regarded as of ONE 

 SPECIES. Linnaeus' statements were partly supported by similar dis- 

 coveries by other investigators, such as Unzer, and partly also by 

 the discovery of eggs in many helminthes. It was believed that the 

 eggs hatched out in the outer world gave birth to free-living creatures, 

 and that these, after their introduction into the intestine, were trans- 

 formed into helminthes. By means of these eggs the old investigators 

 tried to explain the HEREDITARY TRANSMISSION of the intestinal 

 worms, which was universally believed until the commencement of 

 the last century ; some authors went so far as to regard the intes- 

 tinal worms as congenital or inherited ; they maintained the possi- 

 bility of direct transmission, as in suckling, and denied that the eggs 

 reaching the external world had anything to do with the propagation 

 of the parasites. 



The more minute comparison between the supposed free-living 

 stages of the helminthes and their adult forms, and the impossibility 

 of finding corresponding free forms for the ever-increasing number 

 of parasitic species, revealed the improbability of Linnaeus' state- 



