210 MENTAL EVOLUTION IN ANIMALS. 



show this, it will be sufficient to state the following in- 

 stances. 



Thread and worsted are now habitually used by sundry 

 species of birds in building their nests, instead of wool and 

 horsehair, which in turn were no doubt originally substitutes 

 for vegetable fibres and grasses ; this is specially noticeable 

 in the case of the tailor-bird and Baltimore oriole, and 

 Wilson believes that the latter improves in nest-building by 

 practice — the older birds making the better nests. The com- 

 mon house-sparrow furnishes another instance of intelligent 

 adaptation of nest-building to circumstances ; for in trees it 

 builds a domed nest (presumably, therefore, the ancestral 

 type), but in towns avails itself by preference of sheltered 

 holes in buildings, where it can afford to save time and 

 trouble by constructing a loosely formed nest. A similar 

 case is furnished by the gold-crested warbler, which builds 

 an open cup-shaped nest where foliage is thick, but makes a 

 more elaborate domed nest with a side entrance, where the 

 site chosen is more exposed. Moreover, the chimney and 

 house-swallows have taken to building in chimneys and 

 under the roofs of houses by way of an intelligent or plastic 

 change of instinct, and in America this change has taken 

 place within the last three centuries or less. Indeed, accord- 

 ing to Captain Elliott Coues, all the species of swallow on 

 the American continent (with one possible exception) have 

 modified the structure of their nests in accordance with the 

 novel facilities afforded by the settlement of the country ; for 

 he writes : — 



" Various species, indeed, now regularly accept the arti- 

 ficial nesting-places man provides, whether by design or 

 otherwise. Such is notably the case with several kinds of 

 Wrens, with at least one kind of Owl, with one Bluebird, the 

 Pewit Flycatcher, and especially the House-sparrow. Various 

 other birds occasionally avail themselves of like privileges, 

 still retaining in the main their original habits. But in no 

 other case than that of the Swallows is the modification of 

 habit so profound, or so nearly without exception throughout 

 the entire family. . . . All of our Swallows have been 

 modified by human agency, excepting the Bank Swallow. 

 . . . . Some of them, like the Purple Martin and the 

 Violet-green Swallow, are still surviving their apprenticeship 

 under the new regime, which the settlement of the country 



