BRANCH CHORDATA; CLASS AYES: THE BIRDS 339 



developed than in batrachians and reptiles, but the cere- 

 brum has no convolutions as in the mammals. Of the 

 special senses the organs of touch and taste are apparently 

 not keen; those of smell, hearing, and sight are well 

 developed. The optic lobes of the brain are of great size, 

 relatively, compared with those of other vertebrate brains, 

 and there is no doubt that the sight of birds is keen and 

 effective. The power of accommodation or of quickly 

 changing the focus of the eye is highly perfected. The 

 structure of the ear is comparatively simple, there being 

 ordinarily no external ear, other than a simple opening. 

 The organs of the inner ear, however, are well developed, 

 and birds undoubtedly have excellent hearing. The 

 nostrils open upon the beak, and the nasal chambers are 

 not at all complex, the smelling surface being not very 

 extensive. It is probable that the sense of smell is not, 

 as a rule, especially keen. 



Development and life-history. All birds are hatched 

 from eggs, which undergo a longer or shorter period of 

 incubation outside the body of the mother, and which are, 

 in most cases, laid in a nest and incubated by the parents. 

 The eggs are fertilized within the body of the female, the 

 mating time of most birds being in the spring or early 

 summer. Some kinds, the English sparrow, for example, 

 rear numerous broods each year, but most species have 

 only one or at most two. The eggs vary greatly in size 

 and color-markings, and in number from one, as with 

 many of the Arctic ocean birds, to six or ten, as with most 

 of the familiar song-birds, or from ten to twenty, as with 

 some of the pheasants and grouse. The duration of in- 

 cubation (outside the body) varies from ten to thirty days 

 among the more familiar birds, to nearly fifty among the 

 ostriches. ' The temperature necessary for incubation is 

 about 40 C. (i 00 F.). Among polygamous birds 

 (species in which a male mates with several or many 



