INDIVIDUAL DIFFERENTIATION AND ADAPTATION. 129 



spread in all directions, unless kept back by adequate barriers. 

 Thus we should expect all animals to be found all over the 

 earth if all the conditions were equally suitable and all animals 

 were equally adaptable to varying conditions. This however 

 is not so. Species have unequal powers of adaptation to the 

 different conditions and thus it comes tq be that certain groups 

 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. 



164. The Original Home of Animals, and the Sea- 

 faunas. 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 animals suited to its conditions. 

 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 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 dark- 

 ness and the great pressure of the water. Many of the forms 

 are highly modified, differing markedly from the correspond- 

 ing 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 broken up by the waves, and the great variety of phys- 

 ical conditions at the meeting of land and water. 



165. Library Exercises. What are the special conditions, of each of 

 the regions indicated in the preceding section, which are likely to be favor- 

 able or unfavorable to life? Illustrate more fully the typical forms char- 

 acterizing each region? Find instances of the special adaptations which 

 seem peculiarly advantageous to some of the animals frequenting each 

 region. 



1 66. Fresh-water Faunas. From the littoral regions of 

 the sea, animals doubtless originally migrated into the brackish 



