VERTEBRATES — CLARK 299 



same food as certain birds, and both often hunt together in the evening. 

 But except for the large fruit bats, which have good eyes, the bats 

 are guided by hearing and not by sight and are active on very dark 

 nights when the so-called nocturnal birds are wholly inactive. 



The power of flight enables the birds to populate, permanently or 

 temporarily, many regions where mammals are few or absent such as 

 swamps, marshes, offshore rocks and islands, and in the summer the 

 rich marshy tundras of the far north. In such regions they are often 

 incredibly abundant. The necessity for incubating their eggs for a 

 considerable time forces most birds to scatter in more or less widely 

 separated pairs during the breeding eason, though they may be highly 

 gregarious at other seasons. But in areas where there are few or no 

 other creatures that would feed on the eggs or young, such as inacces- 

 sible marshes or swamps, on isolated rocks and islands, on steep cliffs 

 or gravel banks, or on the Arctic tundra, birds, especially the larger 

 birds, often live in colonies which frequently are of immense size. 

 Some of the smaller tree-living birds also nest in colonies usually in 

 specialized nests, as the apartment nests of some weaverbirds or the 

 long pendent nests of the oropendolas. Some birds nest under the 

 protection of bees or termites, and a number in widely different groups 

 make no nests but deposit their eggs in the nests of other birds. 



The geological history of birds appears to be parallel with that of the 

 mammals, and the geographical distribution of the two classes is 

 roughly similar. Both groups show a high degree of adaptation to 

 present conditions with many plant feeders. In both groups there 

 are few ancient types still persisting. 



Reptiles differ from mammals and birds in being cold-blooded. 

 They agree with birds in having a dry skin and in being highly de- 

 pendent on sunlight. More dependent on sunlight than birds, nearly 

 all live in very sunny regions, especially more or less arid regions. 

 Nearly all reptiles lay eggs as do the birds, but the young emerge from 

 the eggs completely developed, except sexually, and are not tended or 

 fed by the parents. Nearly all modern reptiles are carnivorous, 

 largely insect feeders, like most birds, at least when young. Except 

 for being cold-blooded the reptiles in the broader aspects of their 

 ecology are more similar to the birds than to any other vertebrates. 

 Morphologically they have been united with birds under the term 

 Sauropsida. But some reptiles are viviparous and some are wholly 

 aquatic, like the sea snakes. Some have horny beaks like birds, others 

 uniform or more or less diversified teeth. In the past a number had 

 the power of flight, their wings being essentially like those of the mam- 

 malian bats, not like those of birds. 



Whereas since the Eocene the mammals and birds have undergone 

 continuous and great diversification with adaptations for the con- 



