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._122 NATURAL SELECTION vi,^of that group, and we have, therefore, a right to infer that asstructure varies, the nest will vary also in some particularcorresponding to the changes of structure. We have seenalso that birds change the position, the form, and the constructionof their nest whenever the available materials orthe available situations vary naturally or have been alteredby man and we;have, therefore, a right to infer that similarchanges have taken place when, by a natural process, externalconditions have become in any way permanently altered. Wemust remember, however, that all these factors are very stableduring many generations, and only change at a rate commensuratewith those of the great physical features of theearth as revealed to us by geology ;and we may, therefore,infer that the form and construction of nests, which we haveshown to be dependent on them, are equally stable. If,therefore, we find less important and more easily modifiedcharacters than these so correlated with peculiarities ofnidification as to indicate that one is probably the cause ofthe other, we shall be justified in concluding that thesevariable characters are dependent on the mode of nidification,and not that the form of the nest has been determined bythese variable characters. Such a correlation I am nowabout to point out.Classification of NestsFor the purpose of this inquiry it is necessary to groupnests into two great classes, without any regard to their mostobvious differences or resemblances, but solely looking to thefact of whether the contents (eggs, young, or sitting bird) arehidden or exposed to view. In the first class we place allthose in which the eggs and young are completely hidden,no matter whether this is effected by an elaborate coveredstructure, or by depositing the eggs in some hollow tree orburrow underground. In the second, we group all in whichthe eggs, young, and sitting bird are exposed to view, nomatter whether there is the most beautifully formed nest ornone at all.Kingfishers, which build almost invariably inholes in banks; woodpeckers and parrots, which build inhollow trees; the Icteridae of America, which all makebeautiful covered and suspended nests ;and our own wren,which builds a domed nest are examples of the former;