672 - Heredity and Evolution 



girdles, and appendicular parts of the skele- 

 ton are very well ossified. Copulatory organs 

 have been developed so that fertilization is 

 generally internal. And last but not least, 

 reptilian eggs can develop in dry localities 

 on land. Typically, each is covered by a 

 tough moisture-proofed shell and richly sup- 

 plied with yolk. Moreover, the embryo pos- 

 sesses an efficient set of extraembryonic mem- 

 branes (p. 389) with which it meets its 

 respiratory, excretory, and other nutritive 

 needs during development. 



The fossil and other evidence strongly in- 

 dicates that both birds and mammals orig- 

 inated from reptilian stocks during the Trias- 

 sic period. Fossils of ancient birds, however, 

 are relatively scarce and the bridge to the 

 ancient bircllike reptiles (thecodonts) is not 

 complete. 



The Birds: Class Aves. Feathers, which repre- 

 sent highly modified reptilian scales, are 

 unique possessions of the birds. All birds 

 have them and no other feathered creature 

 has ever appeared upon the evolutionary 

 scene. Moreover, these light, plane-surfaced 

 integumental structures have played a domi- 

 nant role in determining the biological suc- 

 cess of the many species of modern birds. 



The feathered wings and tail of the typical 

 bird are highly efficient as flying mechanisms; 

 and the ability of birds to fly and to migrate 

 long distances has enormously extended and 

 diversified the range of habitats available to 

 the numerous and highly varied species. 

 Moreover, the feathered skin of the bird is 

 even more effective than the hairy skin of 

 the mammal in providing the body with a 

 thermal insulation. Such an insulation makes 

 it possible for the birds (and mammals) to 

 maintain a "constant" body temperature 

 (with very slight variation) in the range of 

 35° to 40° C, which is well above the usual 

 environmental temperature. The homoio- 

 thermic animals (p. 582), consequently, are 

 able to maintain their metabolic activities at 

 a high pitch of efficiency despite climate and 

 season. They can "stay in business" winter 

 and summer, even in truly arctic regions. 



This is in sharp contrast to all other ani- 

 mals, including lower vertebrates, in which 

 the body temperature varies according to the 

 environment. Such poikilothermic animals 

 may be very active under warm conditions, 

 but they tend to become sluggish when it 

 gets colder. And finally, most poikilotherms 

 are forced to go into a dormant (hibernating) 

 state if the temperature falls to a critical 

 degree (usually in the range between 0° and 

 10° C). 



In addition to feathers and to forelimbs 

 that are modified into wings, birds display 

 some other distinctive characteristics. These, 

 all in all, give birds, compared to reptiles, a 

 higher status. The typically four-toed hind- 

 limbs are adapted for perching, walking, or 

 running; or they may be webbed for swim- 

 ming. The skeleton is light, but strong and 

 fully ossified. The mouth is equipped with a 

 horny projecting beak, but teeth are lacking, 

 except in a few ancient species, all extinct. 

 The pelvic girdle, although open ventrally, is 

 firmly attached dorsally to a long series of 

 the vertebrae, and the large sternum possesses 

 a prominent keel, from which the powerful 

 wing muscles take origin. The heart is four- 

 chambered with completely separate right 

 and left ventricles, and the persistent aortic 

 arch, which becomes the main systemic ar- 

 tery, is on the right side, rather than on the 

 left (as in mammals). The lungs are compact 

 and efficient. However, respiration is aug- 

 mented by a number of air sacs, which orig- 

 inate from the lungs and extend out into the 

 spaces between the other organs and even, 

 to some extent, into the bones. The "voice 

 box" is not a single larynx (modified upper 

 part of the trachea) but rather a double 

 structure, the syrinx, situated at the fork 

 where the bronchi take origin from the tra- 

 chea. Fertilization is internal; the eggs are 

 heavily yolked and covered with a calcareous 

 shell; and well-developed embryonic mem- 

 branes (amnion, chorion, allantois, and yolk 

 sac) provide for the nutritional necessities 

 during development. 



Ornithology (Gr. ornes, "bird") has al- 



