PARASITES. 187 



strong for natural selection to take possession of them, 

 the productive power of the less active portion will gradu- 

 ally decrease, and finally, with the extinction of the 

 physiological character and the function, nothing will 

 be transmitted but the morphological remains, as a mock- 

 ery to the theory of special design or teleology. Here 

 and there only occurs a reversion more or less striking, 

 connected, however, almost exclusively with the adjunc- 

 tive organs, and the secondary sexual characters, by 

 which we mean, not those acquired by either sex, but 

 originally common to both. The tenacity with which 

 these rudiments of sexual organs are inherited is very 

 remarkable. In the class of mammals actual hermaphro- 

 ditism is unheard of, although through the whole period 

 of their development they drag along with them these 

 residues, borne by their unknown ancestry no one can 

 say how long. 



Unless we suppose that parasitic animals were created 

 simultaneously with their hosts from the dust of the 

 earth, — man and his tapeworm, and other disagreeable 

 guests, — and thus put an end to the discussion, this en- 

 tire province has to be explained by descent, with the 

 special co-operation of disuse. The proposition to be 

 demonstrated in the next chapter, that the evolution- 

 ary history of the individual represents the history 

 of the species, will show the influence of the disuse of 

 particular organs on the configuration of the various 

 parasites. The parasitic Crustacea are perhaps the most 

 instructive, as they present the most complete systematic 

 series, exhibiting the gradual atrophy of the organs which 

 accompanies the ever-increasing connection of the para- 

 site with his host. In several orders of intestinal worms, 



