136 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, may have come about 

 by the gradual or periodic drying up of fresh-water basins, 

 or by means of temporary excursions to the land, such as we 

 see some water forms capable of enduring today. 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 are the insects, the first group to 

 accomplish the task; a group of reptiles in early geological 

 times; the birds; and a few mammals (as the bats). Animals 

 after passing from one region to another may in their descend- 

 ants reoccupy their old habitat. Thus the whales and seals are 

 air breathing mammals and are probably descended from land 

 forms, but have become aquatic. The same is true of some 

 reptiles. Some birds have lost 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 prin- 

 ciples governing this division are too complicated for our 

 present purposes. 



172. Distribution of Animals in Time. This 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 similar to those we now know 

 have been at work. During all this time, with geographic 



