ways been a popular study. Birds are every- 

 where and the beauty of their colors and 

 songs and the diversity of their behavior pro- 

 vide an endless fascination. Bird migrations, 

 particularly, have received intensive study, 

 but still they are not completely understood. 

 Some birds, like the quail, are strictly local, 

 while others, like the robin, migrate just a 

 few hundred miles, toward the tropics in the 

 fall and in the opposite direction in the 

 spring. But the arctic tern annually migrates 

 back and forth over a distance of more than 

 20,000 miles from Alaska to Patagonia. Just 

 how it is guided between its winter and sum- 

 mer quarters is still quite problematical. 



The Mammals. If the Mesozoic era can be 

 regarded as the Age of Reptiles, certainly 

 the Cenozoic deserves to be called the Age 

 of Mammals. The oldest mammalian fossils 

 were formed much earlier, in early Triassic 

 formations. These indicate that mammals 

 originated from an ancient reptilian group, 

 the Therapsida. Slowly the mammals in- 

 creased in abundance and diversity, reaching 

 a climax toward the end of the Tertiary 

 period. Subsequently they have shown a small 

 decline but today they are still dominant 

 among land animals. The bony structure of 

 the mammal appears to have been well 

 adapted to fossilization and the lines of con- 

 nection between the modern and the ancient 

 mammals are quite clear. 



Mammals are distinguished by their hairy 

 skin and by mammary glands that produce 

 milk in the female. Typically also there are 

 sebaceous and perspiration glands in the 

 skin, and generally the teeth are of three 

 kinds — incisors, canines, and molars (p. 299). 

 All higher mammals also possess a placenta 

 (p. 389). This temporary reproductive organ, 

 in which the capillary circulations of mother 

 and embryo lie in intimate juxtaposition, 

 permits the mother to nourish the embryo 

 and to keep it in the uterus until it reaches 

 a relatively advanced state of development. 

 However, a few primitive mammals (Mono- 

 tremata) still lay eggs; and another small 

 group (Marsupialia), also lacking a placenta, 



The Animal Kingdom - 673 



deliver the young from the uterus while they 

 are still exceedingly immature (Fig. 32-42). 



Only mammals have a diaphragm (p. 297) 

 separating the abdominal from the thoracic 

 and pericardial cavities; and the mammalian 

 voice box is a single structure, the larynx, 

 in contrast to its avian counterpart, the 

 double syrinx. 



Mammals resemble birds in that they are 

 homoiothermic and because they have a 

 four-chambered heart, which does not per- 

 mit any mixing of aerated and nonaerated 

 blood. However, in mammals the main sys- 

 temic artery (aorta) is derived from a left 

 aortic arch of the embryo, rather than from 

 a right one, as it is in birds. 



Typically the mammal has four limbs, with 

 five or less clawed, hoofed, or nailed toes, 

 although the hindlimbs are vestigial or 

 absent in a few aquatic forms (for example, 

 whales). Usually the limbs are adapted for 

 walking, running, or climbing. However, a 

 few forms have limbs modified for burrow- 

 ing (moles), for swimming (seals), or for fly- 

 ing (bats). 



Modern mammals occur in three main 

 groups: (1) the Prototheria (egg-laying mam- 

 mals); (2) Metatheria (pouched mammals); 

 and (3) Eutheria (placental mammals). 



The Prototheria, or Monotremata. 

 These are represented solely by the duck- 

 billed platypus (Fig. 21-13) and a very few 

 species of spiny anteaters, all from Australia 

 and neighboring islands. All are very primi- 

 tive forms, which have retained the egg-lay- 

 ing habits of their reptilian ancestors. The 

 very small, "semiembryonic" young, on 

 hatching from the eggs, feed upon milk 

 lapped up from numerous scattered mam- 

 mary glands present on the belly of the 

 mother. 



The Metatheria. These mammals are 

 best represented by marsupials, such as the 

 kangaroo. Marsupials are considerably less 

 primitive than monotremes, but still they 

 do not possess a placenta. Fertilization is 

 internal, but the embryos are kept in utero 

 for only a short time. Upon delivery the very 



