388 



often takes up in succession, harping for a longer or shorter time 

 first on one and then on another.* 



All these facts are of importance, especially when we take into 

 consideration the case of those birds, in which we not only find the 

 song never varying, but in which the peculiar note is characteristic 

 of the species, as distinguished from others, in almost all respects 

 except the note, most closely allied to it. Thus the willow wren 

 and chip chop, even in the living state but still more in the 

 dead, are so extremely similar as regards plumage, size, and pro- 

 portions, that it is difficult to believe they have not sprung originally 

 from one stock. Yet supposing this to have been the case, there 

 must have been a time when the song of the two species, which is 

 now quite different, was similar also ; and there must have been a 

 gradually increasing divergence in this particular until the difference 

 became what it is. 



And as with the song of birds, so with the nests. These last 

 are not always alike in the same species, nor constructed of the 

 same materials, nor placed in the same kind of situations. Allusion 

 has already been made to the difference in the nest of the house- 

 sparrow, according as it is placed in trees or buildings. The jack- 

 daw's nest is found in various situations, most often perhaps in 

 steeples or hoUow trees ; in Cambridgeshii-e, they build much in 

 chimneys ; in some open places, where there are neither trees nor 

 towers, according to White, in rabbit-burrows, t White also long 

 ago remarked that the form and material of the nest of the same 

 species of bird, in cei-tain cases he noticed, were adapted to the 

 "circumstances of place and convenience." t Quite recent observers, 

 comparing the nest of the house-martin as constructed at the 

 present day with the way in which it was constructed formerly, 

 have come to the conclusion that they have undergone with the 

 lapse of time "certain progressive modifications of structure." § 

 This if correct would be an alteration not arising directly from any 



* See Knapp's Joum. of Naturalist, 3rd ed., p. 164. f Nat. Hist. 



Selb. "Lett. xxi. and xxii. to Pennant. j Id. Lett. Ivi. to Daines 



Barrington. ^ " Nature," vol. i., p. 522. See also Wallace, Nat 



Select., p. 228a. 



