762 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



a character which was not acquired in the course of natural selection ? 

 (see Beddard's "Animal Coloration"). Our only answer at present is 

 that there is need for experiment. 



(c) Parasitic fauna. — It seems legitimate to rank together those 

 animals whose habitat is in or on other organisms, from which they 

 derive subsistence, without in most cases killing them quickly, if at all, 

 or, on the other hand, rendering them any service. Among ectopara- 

 sites there are such forms as fish-lice and many other Crustaceans, 

 numerous insects such as lice and fleas, and Arachnids such as mites. 

 Among endoparasites there are Gregarines, some Mesozoa, many 

 Nematodes, most Trematodes, all the Cestodes, many Crustaceans, 

 insect larvae, and Arachnids. 



The parasitic habit implies degeneration, varying according to the 

 degree of dependence, great nutritive security, prolific reproduction, and 

 enormous hazards in the fulfilment of the life history. 



Parasitic animals must be distinguished — {a) from epiphytic or epizoic 

 animals which live attached to plants or animals, but are in no way 

 dependent upon them, e.g. acorn-shells on Norway lobster ; (b) from 

 commensals (p. 1 54), who live in some degree of partnership, but without 

 in any way preying upon one another, e.g. crab and sea-anemone ; and 

 (c) from symbions, who live in close partnership, or symbiosis (p. no), 

 e.g. Radiolarians and Algse. But between these habits there are many 

 gradations, and from close association there is always an easy transition 

 to parasitism. 



Terrestrial. — The colonising of dry land has doubtless 

 been a gradual process, as different types wandered inland 

 from the shore, or became able to survive the drying up 

 of fresh-water basins. The fauna includes some Pro- 

 tozoa, e.g. Amxba terricola, which lives in moist earth, 

 some of the Planarians, Nematodes, Leeches, Chsetopods, 

 and other " worms," a few Crustaceans like the wood-lice 

 (Oniscus), many insects and Arachnids, a legion of slugs 

 and snails, most adult Amphibians, most Reptiles, many 

 Birds, and most Mammals. Among Vertebrates certain 

 fishes are of interest in having learned to gulp mouthfuls 

 of air at the surface of the water, to clamber on the roots 

 of the mangrove trees, or to lie dormant through seasons of 

 drought. But among Vertebrates, Amphibians were the 

 first successfully to make the transition from water to dry 

 land. 



It is important to bear in mind that many a stock may, in the course 

 of its evolution, have passed through a variety of environments. Thus 

 the thoroughly aquatic Cetaceans were probably derived from a land 

 stock common to them and to the Ungulates, and may have passed 

 through a fresh-water stage. Without going further back, we have 

 here an illustration of the zigzag course of evolution. 



