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vi A THEORY OF BIRDS' NESTS 119Changed Conditions and persistent Habits as influencingNidificationBesides the causes above alluded to, there are two otherfactors whose effect in any particular case we can only vaguelyguess at, but which must have had an important influence indetermining the existing details of nidification. These arechanged conditions of existence, whether internal or external,and the influence of hereditary or imitative habit the first;inducing alterations in accordance with changes of organicstructure, of climate, or of the surrounding fauna and flora ;the other preserving the peculiarities so produced, even whenchanged conditions render them no longer necessary. Manyfacts have been already given which show that birds do adapttheir nests to the situations in which they place them, and theadoption of eaves, chimneys, and boxes by swallows, wrens,and many other birds, shows that they are always ready totake advantage of changed conditions. It is probable, therefore,that a permanent change of climate would cause manybirds to modify the form or materials of their abodes, so asbetter to protect their young. The introduction of newenemies to eggs or young birds might introduce many alterationstending to their better concealment. A change in thevegetation of a country would often necessitate the use ofnew materials. So, also, we may be sure, that as a speciesslowly became modified in any external or internal characters,it would necessarily change in some degree its mode of building.This effect would be produced by modifications of themost varied nature ;such as the power and rapidity of flight,which must often determine the distance to which a bird willgo to obtain materials for its nest the; capacity of sustainingitself almost motionless in the air, which must sometimesdetermine the position in which a nest can be built; thestrength and grasping power of the foot in relation to theweight of the bird, a power absolutely essential to the constructorof a delicately -woven and well -finished nest; thelength and fineness of the beak, which has to be used likea needle in building the best textile nests ;the length andmobility of the neck, which is needful for the same purpose ;the possession of a salivary secretion like that used in the