Then there is the Pied Grallina, an Australian bird that makes a nest which resembles a large clay bowl or pan, and another, called the Fairy Martin, belonging to the same country, whose nest, built wholly of clay and mud, has very much the shape of an oil-flask with a rather short neck, which projects forwards and downwards, and has an aperture at the end, by which the bird enters. Like those of other swallows and martins, these nests are built several together, and are fixed to the face of a cliff or the hollow of a large tree. Our own little martin-nests are not quite so remarkable as these, but they are sufficiently curious, and it is interesting that in the swallow family we at last get to birds which make their nests—I mean, of course, the exterior part—entirely of mud, without any straw or grass being mixed up with it. It is interesting, I think, because my own idea is that mud came first to be used in nest-making, through its adhering to the roots of grasses and water-plants, and that in the bits of straw and fibre, mixed up in the pottery of such accomplished mud-builders as, say, the Oven-bird, we see the last traces of the way in which these structures began. It was watching blackbirds build that first gave me this idea, for the blackbird plasters the cup of its nest with mud, as the thrush does with cow-dung and rotten wood; yet this mud is procured in the way indicated, and the plants to which it adheres form the bulk of the burden, and are of more importance than it is in the architecture of the nest. Gradually, as I believe, the mud got more and more, and the vegetable alloy less and less, till, at last, in the nests of some species mud only came to be used. But we reach a further stage where mud has been given up, and something else adopted in its place. Thus the thrush, whose nest, up to a certain point, much resembles that of the blackbird, makes a cup to it, not of mud, but of cow-dung and rotten wood mashed together. That it once used mud, however, but that in civilised lands, rich in cows, the other substance gradually took its place, I have myself little doubt.