484 THE POPULAR SCIENCE MONTHLY. 



energetic than its parents, and this, joined with the fact that the 

 bird's mate may have been reared in a nest of perfect construction, 

 of itself would tend to remedy, in part, the defects its partner might 

 allow ; these facts together would certainly secure an approach to, if 

 not the complete attainment of, a " typical " robin's nest. So, as the 

 years roll by, the nest of the robin would remain substantially the 

 same, while the amount of variation that now exists woidd be per- 

 petuated, and probably very slowly increased. 



Why indeed, a robin should line its nest with mud, and a cat-bird 

 should not, is probably past finding out, but, as changes gradually 

 brought about by man's agency have already effected changes in the 

 habits of some of our birds, so these same changes, ever in progress in 

 the haunts of our robin, may cause these birds to gradually omit this 

 lining of mud in their nests, and so make them more like the nests 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 habits of birds, going hand-in-hand with the 

 undoubted tendency to variation in their anatomical details. Natural 

 selection, or whatever may be the governing impulse that controls it, 

 also indirectly causes the range of variation in the details of the con- 

 struction of their nests, inasmuch as these variations of habit are the 

 necessary result of changes wrought in the physical construction of 

 the creatures themselves, for, stripped of the haze that metaphysics has 

 gathered about it, as a bewildering gloom, we can see in the opera- 

 tions of the mind, in man and bird, only the curious results of the 

 workings of those fatty atoms, intimately combined, we call the brain ; 

 and no argumentation can separate this brain and mind. They are 

 just as interdependent, and 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 our hearts. 



II. 



A nest of totally different character, that of the well-known Balti- 

 more oriole {Icterus Baltimore), was more carefully studied by the 

 writer, inasmuch as it afforded more marked variations from what may 

 be considered a " typical " form of the structure. 



Mr. Wallace has shown that, where a nest is so constructed as 

 to conceal the sitting bird, in all such cases, the birds are of bright, 

 showy plumage, and would be easily detected by birds of prey, if not 

 concealed when occupying their nests. Of the family Icteridce, to 

 which our Baltimore oriole belongs, Mr. Wallace says, " The red or 

 yellow-and-black plumage of most of these birds is very conspicuous, 

 and is exactly alike in both sexes. (This is not true of the Baltimore 

 oriole, the female of which is much less brightly colored.) They are 

 celebrated for their fine, purse-shaped, pensile nests." There are now 

 two considerations worthy of attention, with reference to this bird 



