NATURAL HISTORY. 



BIRDS. 



THE numerous Class of vertebrated animals 

 which we are about to consider, differs from that 

 of the Mammalia, in that the young are not born 

 alive, but are produced from eggs; which consist 

 of a living point, attached to a globular sac of 

 nutriment, called the yolk, surrounded by a layer 

 of albumen, the glaire, and inclosed in two series 

 of membrane, and a hard calcareous shell. For 

 the development of the vital point into a living 

 and active chick, it is needful that it should 

 receive the stimulus of warmth ; and this is, in 

 general, supplied from the body of the parent 

 bird, during the process of brooding or incuba- 

 tion ; while it is retained, in most cases, by means 

 of nests, in which the eggs are deposited, and 

 which are composed of substances more or less 

 calculated to resist the rapid abstraction of heat 

 by the surrounding atmosphere. During the pro- 

 cess of incubation, which lasts only a few weeks, 

 the yolk is gradually absorbed into the body of 

 the inclosed chick, forming its sustenance, until 



