122 NATURAL SELECTION vi 



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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 Icteridae of America, which all make 

 beautiful covered and suspended nests ; and our own wren, 

 which builds a domed nest are examples of the former; 



