114 THE ORIGIN OF PARASITES. 



does not bring the intestinal worm to complete development, but to a 

 certain more or less advanced stage, after which the parasite attains 

 maturity only after transference into its definitive host.^ The 

 intestinal worms undergo, for the most part, as has been shown at 

 length above, a change of hosts, and in consequence their life-history 

 and development are spread over two or more hosts. 



Of this change of hosts we have hitherto taken no account in our 

 discussion, and yet it is clear that it is a process which not only com- 

 plicates, in an unexpected manner, the phenomena of parasitism, but 

 also requires an interpretation from a genetic standpoint before we 

 can obtain a complete insight into the nature of parasitic life. 



At the outset only an ambiguous answer can be given to the 

 question of the significance and mode of origin of the so-called " inter- 

 mediate hosts," provided that we do not wish to forsake the point of 

 view we have hitherto occupied. The intermediate hosts have either 

 been interpolated subsequently into the life-history of the parasites, 

 or they were originally true definitive carriers, which formerly brought 

 their intestinal worms to sexual maturity, but have since become 

 merely intermediate, because the history of development of the in- 

 mates has extended itself over a greater number of stages by means 

 of further formation and differentiation. That we have in both cases 

 to do with a far-reaching adaptation needs scarcely to be expressly 

 mentioned. 



If I express myself unconditionally in favour of the second of 

 these possibilities, it is chiefly in consideration of the fact that the fully 

 formed and sexually mature stages of the Entozoa are found, with few 

 exceptions, in the vertebrates — that is, in creatures which have 

 relatively only recently originated. The Invertebrata, of course, are 

 not free from Helminths, but all the hundreds and thousands of 

 species which they shelter are, with few exceptions, young forms, 

 which require transference into a vertebrate in order to complete the 

 cycle of their development. If this do not imply that the intestinal 

 worms have arisen along with the Vertebrata, or that they became 

 extinct in their oldest representatives, with the exception of a few 

 remnants — and both seem unlikely upon unprejudiced consideration 

 — then the only possible conclusion is that the Helminths of the 

 Invertebrata have in course of time changed their character, and have, 

 during their further development in the Vertebrata, become mere larval 

 forms instead of sexually mature animals. In view of these facts, we 

 cannot doubt that the vertebrates afford a much more favourable soil 

 for the development of the Helminths than the invertebrates. We 

 must even admit that numerous forms have originated after the Verte- 



1 See 1 a and b, 2 h, and 3, in the short review at the commencement of this chapter. 



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