THE ANIMAL IN THE WORLD 541 



which are common to them all. Thus, we may divide 

 cecological faunas into those of the land and the water, 

 and it then appears that land animals have means of 

 breathing in air, an absence of special swimming organs, 

 and usually strong skeletons, in correspondence with their 

 life in a medium which does not support them as water 

 would. Differences of colour, clothing, shape of limbs, 

 etc., distinguish the faunas of mountains, deserts, snowfields, 

 marshes, and so forth. Again, the cecological subfaunas of 

 the sea — the shallow-water fauna, the deep-sea fauna, and 

 the free-water or pelagic fauna — have each their own 

 characteristics. The pelagic fauna, for instance, consisting 

 of animals whose life is passed in independence of any 

 solid substratum, usually shows in its members great 

 buoyancy, attained either by extreme delicacy of tissues, as 

 in the jellyfish, or by air-bladders and like arrangements. 

 It has also a curious and unexplained tendency to phos- 

 phorescence, which at times causes it to light up the surface 

 of the sea in a well-known and beautiful manner. The deep- 

 sea fauna, living in darkness or half-darkness, where there 

 is no plant life (p. 532), is blind or has very powerful eyes, 

 and is carnivorous, deriving all its food in the long-run 

 from the falling bodies of pelagic animals. Its members 

 also are often phosphorescent. The study of such faunas 

 and their characteristics is one of the most fascinating 

 chapters of Natural History. 



We have already briefly considered the general relation 



in which living beings stand with their lifeless 

 Living and surroundings. We have seen that living proto- 

 Things! plasm, delicate and unstable though it is, by a 



purposive reaction maintains its existence in the 

 face of the great and unceasing changes of inorganic nature, 

 which would otherwise surely destroy it. In so doing 

 it turns to its own use a part of the forces of nature, 

 and this is the meaning of the circulation of matter and 

 transference of energy through the bodies of organisms. 

 Only by a continual change of its substance can the 

 organism keep in being. It is thus, like a stream or a 

 whirlpool, an object in nature which remains in existence 

 in spite of a continual change of its substance ; but whereas 

 the existence of the stream or the whirlpool is maintained 



