122 NATURAL SELECTION vi 
of that group, and we have, therefore, a right to infer that as 
structure varies, the nest will vary also in some particular 
corresponding to the changes of structure. We have seen 
also that birds change the position, the form, and the con- 
struction of their nest whenever the available materials or 
the available situations vary naturally or have been altered 
by man ; and we have, therefore, a right to infer that similar 
changes have taken place 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 com- 
mensurate 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 nidification, 
and not that the form of the nest has been determined by 
these variable characters. Such a correlation 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 regard 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 exposed to view, no 
matter whether there is the most beautifully formed nest or 
none at all. Kingfishers, which build almost invariably in 
holes in banks; woodpeckers and parrots, which build in 
hollow trees; the Icteride of America, which all make 
beautiful covered and suspended nests; and our own wren, 
which builds a domed nest—are examples of the former ; 
