294 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1952 



eggs of all birds are provided with a rigid calcareous shell. Nearly 

 all birds incubate their eggs. The young are hatched in widely differ- 

 ent stages of development in the different types, and are usually tended 

 and fed for some time by both parents or by the female, in a few only 

 by the male. A few birds, as the megapodes, deposit their eggs in 

 compost heaps as do alligators and crocodiles, or in warm sand, like 

 the sea turtles, and leave them to be incubated by the heat of decom- 

 position or of the sun. 



MAMMALS 



The first definitely known mammals, showing certain resemblances 

 to reptiles, are from the Jurassic. Although mammals are primarily 

 terrestrial, living on or above the surface of the ground, a number, 

 such as water rats, muskrats, beavers, otters, the hippopotamus, and 

 the platypus, are amphibious, the seals are aquatic but bear their young 

 on land or on ice floes, and the cetaceans are wholly aquatic, bearing 

 their young in the water. A few mammals, such as the moles, marsu- 

 pial moles, mole rats and some other rodents, are largely or almost 

 wholly subterranean. 



The earliest known mammals were small and presumably insectiv- 

 orous, and many of the later types such as the present-day insectivores 

 and bats were and are primarily insectivorous, with the larger carni- 

 vores feeding chiefly on vertebrates. 



At present, by far the largest mammalian group is that including 

 the rodents, very nearly all of which are exclusively, or almost exclu- 

 sively, vegetarian. With these should be considered the ecologically 

 similar but very much smaller group including the rabbits, hares, and 

 pikas. Most of the very large terrestrial mammals such as the ele- 

 phants, rhinoceroses, hippopotamus, tapirs, horses, the giant panda, 

 the great apes, kangaroos, and manatees, and all the hoofed animals 

 except for a few omnivorous pigs are exclusively vegetarian. These 

 large vegetarians are very delicately adjusted to their environment. 

 They require a constant and abundant supply of food and water under 

 a limited range of physical conditions, and are very sensitive to en- 

 vironmental changes greater than those that can be met by limited 

 migrations. A large number of rodents and the pikas store food to 

 tide them over unfavorable seasons, and many rodents hibernate. 

 Some rodents and some lemurs estivate. These characteristics, to- 

 gether with their much smaller size and vastly superior reproductive 

 powers, give the rodents a great advantage over the larger herbivores. 

 Strictly vegetarian and more or less omnivorous species are found in 

 greater or less numbers in all the other mammalian groups except the 

 seals and cetaceans. 



All mammals are covered with hair, or at least have traces of hair, 

 although the elephants, rhinoceroses, some pigs, and manatees and 



