160 THE DARWINIAN THEORY 



steady and continuous, though slow, progress up- 

 wards. It is then asked, how is it that even at the 

 ( present day we find numerous representatives of the 

 ' N simplest groups of animals living ? And how is it 

 that we find many cases of degeneration — i.e., of 

 animals which, in the early stage of their existence 

 — representing ancestral phases — are more highly 

 organised than in the adult condition ? 



Now, an animal may be placed under conditions 

 in which organs useful to its ancestors, and inherited 

 from them, may be no longer of service. Such 

 organs tend to become degenerate, persisting for a 

 time as vestigial structures, and ultimately perhaps 

 disappearing altogether. Of such cases of degenera- 

 tion we meet with numerous examples, of which the 

 following are the most important : 



(a) An animal fixed in the adult state, but free 

 when young : such as sponges, hydroids, corals, 

 polyzoa, oysters, and barnacles. This involves loss, 

 or modification, of the locomotive organs, and often 

 of the sense-organs as well. 



(i) Parasites which live on or in other animals, 

 and of which Sacculina is a good example. In 

 these animals the whole body often becomes de- 

 generate, the conditions of life rendering locomotor, 

 digestive, sensory, and other organs entirely useless. 

 In such cases, those forms which avoid the waste of 

 energy resulting from the formation and maintenance 

 of these organs will be most in harmony with their 

 surroundings. Parasitic worms, molluscs, &c, show 

 similar wholesale degeneration, and live immersed 

 in the body fluids of their victims. 



