THE CLASS BIRDS. 335 



nourishment from a breast. The period of incubation of the 

 young in the egg varies with the species, but is nearly the 

 same in the individuals of a species. For the humming-bird, 



Fig. 321. The Eider (Goose) Duck. 



the smallest of the class, the period is twelve days ; for the 

 canary, it varies from fifteen to eighteen days ; for the domes- 

 tic fowl, twenty-one days ; twenty-five for the duck ; arid 



Fig. 322. Carolina Duck. 



from forty to forty-five for the swan. The heat of the sun 

 suffices for the incubation of some tropical birds, but in 

 general it is quite otherwise, and the eggs require to be 



