The power of resisting or limiting this refrigerating influence is somewhat differently conferred in different animals: in the healthy human subject, by increased activity of the vessels of the skin, which induces greater heat. Birds, in their rapid flight, and especially in the more elevated regions of the atmosphere, are exposed to intensely refrigerating influences. These are met by the surface being clothed first by fine feathers, the worst of all conductors of heat, and these are overlapped, where they meet the atmosphere, in such a way that the bad-conducting property of the feathers is increased by the mechanical arrangement of them. Again, the respiration of birds, which (_as we contend_) is a refrigerating process, is very restricted; although, for want of due consideration of all the circumstances, and especially of certain analogies afforded by insects, very opposite views have been entertained. Domestic animals (birds inclusive) impressively suggest the refined adaptation of colour even, of the whole surface, to the altered position of the individual. Nothing is more striking than the general uniformity of colour in wild animals—few things more familiar than their infinitely varied hues when domesticated. Now it is certain that these differences have a meaning, and that their relations are important; but when we extend these thoughts from the coverings of animals to the consideration of surface, whether of animals or vegetables, what wonderful things occur to us. Every variety of colouring which we observe in domestic animals, every spot on an insect's wing, every pencilling on a a flower, places the individual in a different relation, so far, to light, heat, and other powerful agents in nature. Or if we look from another point of view—we cannot walk by a hedge-row in summer without observing how very small the differences of light and aspect are, which seem on the same soil to confer on the same species of flowers such numerous varieties of colour. I have most frequently observed this in the common cranes-bill, or wild geranium. In order to estimate correctly the value of these surfaces to the animal or vegetable, it is obviously of great importance to us to know what they do; and if they give off any thing, to ascertain its nature. That either animal or vegetable may be healthy, the processes of nature, whatever they are, must be carried on; and we may be assured, that the fragrance of the rose is just as necessary an exhalation _from_ the plant, as it is an agreeable impression _to_ us.