10 RETROGRESSIVE DEVELOPMENT IN NATURE. [IX. 



retrogression either stops just short'of actual elimination, as in 

 the case of the wings of the ostrich, or so alters and transforms 

 the structure as to fit it for new functions, like the wings of the 

 penguin, which aid it in swimming. 



The far-reaching effects, on the development of species, of 

 retrogression consequent upon disuse are nowhere to be seen 

 more clearly than among parasitic animals. 



Many groups of animals contain certain genera, families, 

 or even whole orders, which live at the expense of other 

 animals, feeding on their blood or tissues, yet not killing them 

 after the manner of beasts of prey. Such are the parasites, 

 some of which only seek their unwilling host when impelled 

 by hunger, and leave it as soon as they are satisfied ; while 

 others take up their abode in or upon it, only to be driven 

 thence by its death. The great group of worms includes very 

 many parasites, and they are almost as numerous among the 

 Crustacea. Most crustaceans are free-swimming or actively 

 running inhabitants of the water, especially of the sea, and 

 their food is partly of a vegetable nature and partly consists of 

 living or dead animals ; but nearly every order includes some 

 parasitic form, in which the effects of disuse resulting from 

 parasitism are plainly traceable. 



A visit to the fish-market at any European sea-port, and an 

 examination of some of the larger fish, will generally lead to 

 the discovery of certain segmented animals firmly attached to 

 the integument, and bearing some resemblance to wood-lice. 

 These parasites, called fish-lice, suck the blood of the fish. 

 They are not permanently fixed, but leave their host from 

 time to time and seek a fresh one. Now these animals exhibit 

 with great clearness the effects of parasitic habits : their legs 

 are short, being no longer required for swimming, but chiefly 

 for holding on by, and the organs of sense also are somewhat 

 degenerate, for parasites scarcely need them. It is, of course, 

 necessary for predaceous crustaceans to be able to distinguish 

 their prey at a distance, and for this purpose they require keen 

 sight and a delicate sense of touch in their antennae ; but para- 

 sitic forms, when once attached to their host, do not readily 

 leave it, or if they do so, a new host is easily found, since fish 

 are mostly gregarious. Hence in these fish-lice the eyes and 

 antennae have become small and insignificant. 



