VERTEBRATES — CLARK 291 



Reptiles are highly dependent on abundant light, all of them for 

 heat, and those with heavy skeletons, like most turtles, or with 

 heavy dermal scutes, like the crocodiles, for skeleton formation. 

 Most reptiles are strictly diurnal and are especially active in bright 

 sunlight, but the geckos are mainly nocturnal. The poisonous snakes 

 hunt their food mostly at night, but many, if not most, of them, like 

 our rattlesnakes and copperheads, sun themselves by day. In one 

 lizard from Trinidad the males have a row of luminous organs along 

 each side, recalling certain luminous fishes. 



The senses as a rule are not highly developed, but in lizards and 

 turtles sight is usually good, and in snakes and some lizards there 

 is an acute sense of smell. 



Most reptiles lay eggs with either a tough and flexible or a hard and 

 rigid shell. These may be scattered individually, deposited in batches 

 in a hole and covered up, or, as in the case of the crocodilians, placed 

 in a compost heap to be hatched by the heat of decomposition as is 

 done by some megapodes among the birds. The eggs are usually 

 abandoned, but are sometimes tended. The female python coils about 

 her eggs and incubates them, her body temperature rising during the 

 period in incubation. But few reptiles tend their eggs and these 

 generally lose interest in the young when they appear. Some snakes 

 and a few lizards are viviparous. Sea snakes give birth to their young 

 in the water. Fertilization in the reptiles is always internal. 



All reptiles breathe by means of internal lungs. Some sea turtles 

 have supplementary anal breathing. 



BIRDS 



The first known birds, exhibiting many reptilian features, are from 

 the Jurassic. All birds nest on land, and most birds are terrestrial, 

 the most widely distributed of all terrestrial vertebrates, found on 

 many islets and even on some good-sized islands, as some of the Aleu- 

 tians, where no other vertebrates occur. Many birds are amphibious, 

 obtaining all their food in fresh or salt water, or in both, as the mer- 

 gansers, loons, grebes, many kingfishers, and others, particularly the 

 sea birds. A few birds are oceanic, visiting land only to breed, as do 

 the sea turtles and the fur seals. 



The earliest known birds, which were relatively large and pos- 

 sessed teeth, a long jointed tail, and other reptilian features, were 

 presumably carnivorous. At present the dominant and by far the 

 largest group of birds is that of the perching birds or Passeres which 

 includes at least half of all the known species. As in the case of the 

 modern rodents, the dominant mammals, they are all small, or at least 

 none are very large. They are primarily insectivorous, but many are 

 more or less omnivorous and others are plant feeders, at least as 



