488 THE POPULAR SCIENCE MONTHLY. 



explained on the theory of a secular tradition, untroubled by any ex- 

 ternal influences. 



But why not apply the same reasoning to the facts presented to us 

 by the animal kingdom ? The processes of nidification are determined 

 by the physical circumstances, as well as by the conformation of the 

 nest, and by the tools supplied by Nature, and they are modified in 

 accordance with external conditions. An alteration of climate, any 

 sensible change in the vegetation of a country, the introduction of new 

 enemies, bring about architectural variations more or less marked. 

 Several birds prefer the ends of threads which they pick up on the 

 streets to the vegetable fibres used by them before, and of their own 

 accord take up their quarters in boxes or hollow gourds arranged for 

 their use, thus saving a part of their labor. The common sparrow 

 readily adapts himself to circumstances : he takes far less pains with 

 his work when he can avail himself of a nook in a wall, than when he 

 is obliged to build in the open air, on the branch of a tree, for then his 

 nest must be solidly built and well covered. The orchard oriole or 

 bobolink, of the United States, builds his nest almost flat when he can 

 fasten it on a stout, stiff branch, but far deeper when he has to hang 

 it on the slender branches* of the weeping- willow, where it may be 

 swayed by the wind, and the chicks thrown out. Finally, M. J. A. 

 Pouchet published, in 1870, some very curious observations on the 

 progressive improvement of martins' nests. He kept for 40 years in 

 the Rouen Museum some of these nests, which he had himself detached 

 from the walls of old buildings in that city. Having one day got some 

 new nests, he was amazed, on comparing them with the old, to perceive 

 considerable differences. The new-style nests all came from a new 

 quarter of the town, and were all built on the one plan ; but on ex- 

 amining churches and other ancient buildings, as also certain rocks in- 

 habited by martins, he found several nests of the old pattern, together 

 with others constructed according to the more recent model. The 

 figures and descriptions given by old naturalists portray only the 

 primitive type, which is a quarter-hemisphere, having a very small 

 circular orifice. The modern nest, on the contrary, has a width greater 

 than its depth, and forms a segment of an oblate spheroid, the orifice 

 being very wide. Here we see an evident progress, the new type being 

 larger, more comfortable. The wider bed gives the chicks greater lib- 

 erty of movement than they had in the deep and contracted nest of 

 former times ; the wider opening allows them to look out and take the 

 air ; in short, it is a sort of balcony, where two chicks find room with- 

 out being in the way of the old birds. Nor is this all. Being situated 

 nearer to the top of the nest, the opening is less exposed to rain and 

 wind. One well-proved case of this kind is enough to show that the 

 architecture of birds is susceptible of progress ; and this would seem 

 to overturn the hypothesis of blind instinct. Then, too, the evident 

 imperfections observed in the nests of some species, and the awkward- 



