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 suff- 
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 
