Again, the Baltimore Oriole, which inhabits both the Northern and Southern States of North America, knows very well how to adapt his manner of work to the external circumstances in which he lives. Thus, in the Southern States the nest is woven of delicate materials united in a rather loose fashion, so that the air can circulate freely and keep the interior fresh; it is lined with no warm substance, and the entrance is turned to the west so that the sun only sends into it the oblique evening rays. In the north, on the contrary, the nest is oriented to the south to profit by all the warm sunshine; the walls are thick, without interstices, and the dwelling is carpeted in the warmest and softest manner. Even in the same region there is great diversity in the style, neatness, and finish of the nests, as well as in the materials used. Skeins of silk and hanks of thread have frequently been found in the Baltimore Bird's nest, so woven up and entangled that they could not be withdrawn. As such materials could not be obtained before the introduction of Europeans, it is evident that this bird, with the sagacity of a good architect, knows how to select the strongest and best materials for his work. Many other facts might be quoted, but these suffice to show that the species is not animated by an inevitable instinct, but that each individual, skilful no doubt by heredity, can modify the methods transmitted to him by his ancestors, according to his own experience and his own judgment. _Built dwellings._--The built dwelling, the expression of the highest civilisation, still remains to be studied. Man has only known how to construct this kind of shelter at a comparatively late period in his evolution; and among animals we do not find it widely spread, much less so, certainly, than the two foregoing methods, especially the first. The difficulty of this work is greater, and it only arrives at considerable development among very sociable species, since the united efforts of a great number of individuals are needed to carry it on. There are, however, masons who operate separately; but their constructions are rudimentary. The characteristic of all these works is that they are manufactured with some substance to which the animal gives a determined form while it is still soft, and that in drying it preserves this form and acquires solidity. The matter most usually employed is softened and tempered earth--mortar; but there are animals who use with success more delicate bodies. Two examples will suffice to indicate the nature of these exceptions: the labours of Wasps and those of certain Swallows. [Illustration: FIG. 33.]