DURATION OF INCUBATION. 243 



occupies about one month in the task ; the ma- 

 jority of the smaller birds are from twelve to four- 

 teen days. The swan occupies six weeks ; the 

 eider-duck, one month; and the guillemot and 

 puffin, the same period. It appears that the 

 aquatic birds occupy a longer period than others, 

 as a general rule ; which is explained by the fact 

 that during this time the young chick is being 

 perfected, so that immediately on being hatched it 

 may be able to take care of itself, with less exer- 

 cise of caution on the part of the mother. The 

 chicks of the smaller birds are sooner hatched ; but 

 they are quite helpless when they leave the shell, 

 and some time must elapse before they will be 

 able to quit the nest. In the case of the aquatic 

 birds, it is surprising to notice how early the 

 young run about, and swim, and help themselves 

 to food. Thus it is evident that, as a general 

 rule, the period of incubation is determined by 

 the habits of the future chick ; and the more nearly 

 it is required to be perfected previous to quitting 

 the egg, the greater the duration of incubation, 

 and vice versa. There are exceptions, however, 

 to this rule, as to most others. 



