A THEORY OF BIRDS’ NESTS. 2937 
when, by a natural process, external conditions have 
become in any way permanently altered. We 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 
