SUGGESTIONS FOE KELIABLE HYPOTHESES 185 



8T. 



srn 



human intestine if by chance their eggs find their way 



there 9 as, for instance, the larvae of the flies Eristalis 



tenax and Anthomyia canicularis. 1 



The worm Leptodera appendiculata 



lives mostly in rotting material 



in the soil ; if it finds entry into 



the hiding-place of a snail in close 



vicinity (which thus shares its 



habitat) it can also flourish there 



very well. 



Certainly by such examples 

 nothing is shown to demonstrate 

 how a parasite by opportunity 

 only may become a true one which 

 can only exist in a definite foreign 

 organism. We can, in the first 

 place, only say that frequent asso- 

 ciation at close quarters or under 

 similar life conditions, can often 

 present the most favourable op- 

 portunity, and that, with the ovary ; m ' mouth ; sb ' 

 change of the mode of life, the 



te 



FIG. 33. DISTOMTJM. 



Plan of an entoparasitic 

 Distomum. D, anus ; Dl, 

 lett, and Dr, right main 

 intestine ; g, brain ; ge, 



ventral attachment ; sm, 

 mouth attachment ; te, 

 testicles ; vi, yolk bag. 



instinct of the animals and of their There should be noted 



the 



offspring, in conjunction with 



feeble development 

 of the intestinal system 

 . . (minus outlet) and the 



their conformation, may become strength of the genital 



. n -. one. The sucker is a 



influenced. positive adaptation. 



The best example how shape- 

 less forms may be associated with normals i.e. those 

 which agree with a definite type is that of the 



1 Graff : Wissenschaft und Bildung, p. 9. 



