A SHORT STUDY OF BTRDS* NESTS. 129 



and its near relatives, the wood-thrush and cat-bird, 

 should not, is not known ; but as changes gradually 

 brought about by man's agency have already effected 

 changes in the habits of some of our birds, ko these same 

 changes, ever in progress in the haunts of the robin, may 

 cause these birds to gradually omit this lining of mud, 

 and so make their habitations more like those of other 

 thrushes ; just as the cliff-swallow, with us, no longer 

 places a " bottle-neck " opening to its mud-built nests. 



There is an instability in the whole range of the hab- 

 its of birds going hand-in-hand with the undoubted ten- 

 dency to variation in their anatomical structure, exces- 

 sively slow as this is. Natural selection, or whatever may 

 be the determining influence that governs it, controls 

 as surely the range of variation in the details of the con- 

 struction of their nests, inasmuch as these variations are 

 the inevitable results of changes wrought in the physical 

 onstruction of the creatures themselves. Stripped of 

 the haze that metaphysics has gathered about it, the op- 

 erations of the mind, whether in man or bird, are only 

 the curious results of the working of those fatty atoms, 

 intimately combined, which we call the brain, and by no 

 argumentation can the two be separated. They are just 

 as interdependent, and as much parts of a single whole, as 

 the eye and sight, the nose and smell, hearing and the 

 ear, the circulation of the blood and the beating of the 

 heart. 



A nest of a totally different character, that of the 

 Baltimore oriole, was more carefully studied, inasmuch 

 as it afforded more marked variations from what may be 

 considered the typical form of such a structure. 



In the essays by Mr. Wallace (" On Natural Selec- 

 tion," by A. K. Wallace, London, 18TO, p. 211 



