232 A THEORY OF BINDS' NESIS. 



bird's existence, will give a clue, and sometimes a very 

 complete one, to the reason why it builds its nest of 

 certain materials, in a definite situation, and in a more 

 or less elaborate manner. 



I now propose to consider the question from a more 

 general point of view, and to discuss its application 

 to some important problems in the natural history of 

 birds. 



Changed Conditions and persistent Habits as influencing 

 Nidification. 



Besides the causes above alluded to, there are two 

 other factors whose effect in any particular case we can 

 only vaguely guess at, but which must have had an 

 important influence in determining the existing details 

 of nidification. These are changed conditions of ex- 



O 



istence, whether internal or external, and the influence 

 of hereditary or imitative habit ; the first inducing al- 

 terations in accordance with changes of organic struc- 

 ture, of climate, or of the surrounding fauna and 

 flora ; the other preserving the peculiarities so pro- 

 duced, even when changed conditions render them no 

 longer necessary. Many facts have been already given 

 which show that birds do adapt their nests to the situa- 

 tions in which they place them, and the adoption of 

 eaves, chimneys, and boxes, by swallows, wrens, and 

 many other birds, shows that they are always ready 

 to take advantage of changed conditions. It is pro- 

 bable, therefore, that a permanent change of climate 

 would cause many birds to modify the form or 



