A Theory of Birds' Nests 75 



vaguely guess at, but which must have had an important influence 

 in determining the existing details of nidification. These are — 

 changed conditions of existence, whether internal or external, and 

 the influence of hereditary habit ; the first inducing alterations in 

 accordance with changes of organic structure, of climate, or of the 

 surrounding fauna and flora ; the otlier preserving the peculiarities 

 so produced, even when changed conditions render them no longer 

 necessar}^ There are many facts which shew that Ijirds do adapt 

 their nests to the situations in which they place them. That of 

 the Orchard Oriole of the United States (Icterus spurius), when 

 built among the strong branches of a ft-uit tree, is a very shallow 

 cup, but when suspended from the slender boughs of a weeping 

 willow, it is mucli deeper, to prevent the young birds from being 

 thrown out during violent winds. The common house sparrow 

 buflds a far less careful nest when it is placed in a hole, than when 

 built in a tree ; and the adoption of eaves, chimneys, and boxes, 

 by swallows, wTens, and many other birds, shews that they are 

 always ready to take advantage of changed conditions. It is 

 probable, therefore, that a permanent change of climate would 

 cause many birds to modify the form or materials of their abodes. 

 The introduction qf new enemies to eggs or young birds, might 

 induce many alterations tending to their better concealment. A 

 change in the vegetation of a country, would often necessitate the 

 use of new materials. So, also, we may be sure, that as a species 

 slowly became modified in structure, it would necessarily change 

 in some degree its mode of building. Its work would be more or 

 less delicate, and the materials used more or less fine, according 

 to the modifications its external organs underwent. 



During all these changes however, certain specialities of nest- 

 structure would continue for a shorter or a longer time after the 

 causes which had necessitated them had passed away. Such records 

 of a vanished past meet us everywhere, even in man's works, 

 notwithstanding his boasted reason. All the main features of Greek 

 architecture, for example, are but reproductions in stone of what 

 were originally parts of a wooden building. Our copyists of Gothic 

 architecture often build solid buttresses, capped with weighty pin- 

 nacles, to support a wooden roof which has no outward thrust to 

 render them necessary ; and even think they ornament their build- 

 ings by adding sham spouts of carved stone, while modern water- 

 pipes, stuck on without any attempt at harmony, do the real duty. 

 So, when railways superseded coaches, it was thought necessary to 



