758 GEOGRAPHICAL DISTRIBUTION OF ANIMALS. 



of animals. The temperature is very equable, varying little more than 

 a degree throughout the year ; it is, of course, dark ; and there are no 

 plants other than a few Fungi. Thus the conditions present some 

 analogy with those of the deep sea. The fauna is of much interest to 

 the evolutionists, for we wonder how far the peculiarities of the cave- 

 animals, e.g., absence of coloration and frequent blindness, are due to 

 the cumulative effect of the environment and disuse, or how far they 

 represent the survival of fortuitous variations, and the result of the 

 cessation of natural selection along certain lines. Have the seeing 

 animals found their way out, leaving only the blind sports, which crop 

 up even in daylight ? or is the loss of eyes the result of disuse and 

 absence of stimulus ? Or again, if it be granted that pigment is an 

 organic constitutional necessity, e.g., a waste-product, while coloration 

 is explicable as an adaptation wrought out in the course of natural 

 elimination, then the question arises whether the cessation of natural 

 selection a condition awkwardly called "panmixia" which might 

 account for the disappearance of the coloration when there is no 

 premium set upon it, can also account for the loss of pigment, that is of 

 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, 

 nor 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 larvoe, and Arachnids. 



The parasitic habit is a common one, illustrated by many different 

 types. It is associated with degeneration, varying according to the 

 degree of dependence, with great nutritive security and prolific repro- 

 duction, but with 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., barnacle on Norway lobster ; (I)} from com- 

 mensals (p. 1 60), 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. 114), e.g., 

 Radiolarians and Algae. But between these habits there are many grada- 

 tions, and from close association there is always an easy transition to 

 parasitism. 



Terrestrial Fauna. 



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 Protozoa, e.g.. Amoeba terricola, 



