explained on the theory of a secular tradition, untroubled by any external 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 examining churches and other ancient buildings, as also certain rocks inhabited 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 liberty 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 without 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-
