PARASITES. 187 



given in the manuals of comparative anatomy. We 

 shall merely indicate the manner in which the theory 

 of selection is here borne out. It is self-evident that 

 in hermaphrodite animals, fluctuations in the sexual 

 sphere must take place, in which one half or the other 

 will predominate. Should these fluctuations be suffi- 

 ciently strong for natural selection to take possession 

 of them, the productive power of the less active portion 

 will gradually decrease, and finally, with the extinction 

 of the physiological character and the function, nothing 

 will be transmitted but the morphological remains, as a 

 mockery 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 adjunctive organs, and the secondary sexual cha- 

 racters, 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 in- 

 herited is very remarkable. In the class of mammals 

 actual hermaphroditism is unheard of, although through 

 the whole period of their development they drag along 

 with them these residues, borne by their unknown an- 

 cestry 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 

 entire 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 evolu- 

 tionary history of the individual represents the history 

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



