A THEORY OF BIBDS' NESTS. 237 



when, by a natural process, external conditions have 

 become in any way permanently altered. AVe must 

 remember, however, that all these factors are very 

 stable during many generations, and only change at 

 a rate commensurate with those of the great physical 

 features of the earth as revealed to us by geology ; and 

 we may, therefore, infer that the form and construction 

 of nests, which we have shown to be dependent on 

 them, are equally stable. If, therefore, we find less 

 important and more easily modified characters than 

 these, so correlated with peculiarities of nidification as 

 to indicate that one is probably the cause of the 

 other, we shall be justified in concluding that these 

 variable characters are dependent on the mode of nidi- 

 fication, and not that the form of the nest has been 

 determined by these variable characters. Such a corre- 

 lation I am now about to point out. 



Classification of Nests. 



For the purpose of this inquiry it is necessary to 

 group nests into two great classes, without any re- 

 gard to their most obvious differences or resemblances, 

 but solely looking to the fact of whether the contents 

 (eggs, young, or sitting bird) are hidden or exposed 

 to view. In the first class we place all those in 

 which the eggs and young are completely hidden, no 

 matter whether this is effected by an elaborate covered 

 structure, or by depositing the eggs in some hollow 

 tree or burrow underground. In the second, we group 

 all in which the eggs, young, and sitting bird are 



