138 ZOOLOGY 



species of the sponges, the jelly-fish group, and none of the 

 starfish group have left the salt water. Some species of 

 animals, as the salmon and eels, pass back and forth from fresh 

 to salt water in obedience to their spawning or other instincts, 

 but these are not very numerous. 



171. From the fresh-water fauna or from the ocean shore 

 fauna have come those species which have acquired the power 

 of breathing by means of the air. These embrace some worms 

 and mollusks, the insects, and the vertebrates above the fishes. 

 This adaptation, which is one of the most important acquired 

 in the history of animal life on the earth, is assumed to have 

 come about through successive mutations making possible the 

 increased use of oxygen from the air as well as from the water. 

 Progressive mutations of this sort were doubtless associated 

 with periodic drying up of fresh-water basins, and gradual 

 migrations to land. Several types of these terrestrial animals 

 have achieved a more or less complete mastery over the air 

 (aerial fauna) by means of flight. Chief among these ar-e the 

 insects, the first group to accomplish the task; a group of rep- 

 tiles in early geological times; the birds; and a few mammals 

 (as the bats) . Animals after passing from one region to another 

 may in their descendants re-occupy their old habitat. Thus the 

 whales and seals are air breathing mammals and are probably 

 descended from land forms, but have now become aquatic. 

 The same is true of some reptiles. Some birds have lost all 

 their powers of flight and have become purely terrestrial. 



Other divisions of the continental and oceanic faunas into 

 geographical faunas are made, depending on the climatic con- 

 ditions and the geological history of the regions. The facts 

 governing this division are too complicated for our present 

 purposes. 



172. Distribution of Animals in Time. — This geographical 

 distribution of animals on the surface of the earth does not 

 come from modern conditions merely. Throughout all the 

 millions of years that the earth has been reaching its present 

 conditions and has been inhabited by living things, forces sim- 

 ilar to those we now know have been at work. During all this 



