DIFFEEENTIATION OF INDIVIDUALS AND ADAPTATION I37 



of species adapted to some special environment will be found 

 together in certain regions, but will be absent from others. 

 The total animal life of any region is known as its fauna. 



168. The Original Home of Animals, and the Sea-fatmas. — 



There can be no reasonable doubt that animal life began in the 

 sea and close to its surface, and probably not close to the shore. 

 From this region the various nooks and crannies of the earth 

 have been occupied, until now it seems that there is no place 

 which does not have at least a few animals suited to its condi- 

 tions. The fauna of the surface of the mid-ocean is known as 

 the pelagic fauna. It is made up largely of protozoa; certain 

 more or less transparent, free-swimming types of invertebrates, 

 as worms, jelly-fishes, tunicates; many minute Crustacea, and 

 fishes. The abyssal or deep-sea fauna contains representatives 

 of all types of animals from protozoa to fishes, notwithstanding 

 the darkness and the great pressure of the water. Many of 

 the forms are highly modified, differing markedly from the 

 corresponding species found in other life-regions. The littoral 

 or shore fauna is the most varied, abundant, and interesting of 

 the sea-faunas. Indeed there is no place on the earth where life 

 is more abundant. This is true because of the wonderful food 

 supply brought from sea and land and broken up by the 

 waves, and the great variety of physical conditions at the 

 meeting of land and water. 



169. Library Exercises. — What are the special conditions of each of the 

 regions indicated in the preceding section, which are likely to be favorable or un- 

 favorable to life? Illustrate more fully the typical forms characterizing each 

 region? Find instances of the special adaptations which seem peculiarly advan- 

 tageous to some of the animals frequenting each region. 



170. Fresh-water Faunas. — From the littoral regions of the 

 sea, animals doubtless originally migrated into the brackish 

 water of the mouths of rivers. Thus certain types came to 

 inhabit the fresh waters of the streams, and as the restilt of 

 the adaptations thus made necessary new species arose dis- 

 tinctly different from their relatives which remained in the. 

 sea. The most of the branches or phyla of the animal king- 

 dom have their fresh-water representatives, but very few 



