128 OF THE COVERING OF ANIMALS.

notice so it is of all others the first which presents to us the evidence of design and adaptation. We tell the climate an animal inhabits, with moderate certainty, by looking at the skin; and if we occasionally meet with apparent exceptions, further examination usually shows that they exemplify the more strikingly the unity of plan. Thus we may find animals who inhabit hot regions furnished with a somewhat warm covering of the skin, as the tiger, for example; but when we examine the eye, and inquire into the habits of the animal, we find that he preys or feeds at night, when the atmosphere is charged with damp and cold.

We know that the animals whence we obtain our furs inhabit cold regions. The changes in the same animal are not less instructive. Animals placed in certain circumstances, in which they require greater warmth, have increase of covering, and vice versa. Again, the tendency to become white, in those inhabiting cold regions, is a very interesting adaptation, although I am not aware that it has been satis

COLOUR OF ANIMALS.

129

factorily explained. Two things, however, are certain, that they are placed in different circumstances as regards the relation to heat, and would reflect great quantity of light, which, in its intensity in snowy regions, might be prejudicial, as there is no doubt of the influence of this principle in animals. Again, it is a very common arrangement that animals should take the colour of the ground they occupy; and this is sometimes very curiously exemplified. I have observed in the common hunting-spiders which inhabit some palings in a garden in the country, that they are of different shades, but they all more or less resemble that part of the old paling on which they are found. Those which we see on the ground are generally of some darker colour. Birds exemplify in a very remarkable manner the adaptation of their external coverings to the requisitions which their habits establish. All animals may be said to be surrounded by an atmosphere of their own, and they are not therefore, strictly speaking, in contact with the atmosphere; but when they are exposed to air in motion, this stratum is blown aside, and the atmosphere is brought

VOL. I.

K

130

CLOTHING OF BIRDS.

in contact with the surface. Its refrigerating influence is now felt; and just as a boy cools his broth by blowing on it, a fresh stratum of cold air is constantly brought to the surface.

the

This is variously met 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 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

OF VARIETY OF COLOUR.

131

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 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 hedgerow 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 crane's-bill, or wild geranium.

132

OF RESPIRATION BY THE SKIN.

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 anything, 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.

But all animals may be said to breathe quite as much by their skin as by their lungs. Leaves, too, are the breathing surfaces of vegetables; and therefore to ascertain the facts in the one without inquiring into those observable in the other, would be likely to fog our reasoning and falsify our conclusions. The first impression

we obtain from all animals is from external form and appearance-from, in fact, its outward covering. It was the first organ to which Abernethy devoted his most particular attention, and here again his investigations show how little those knew of his mind who imagined that his thoughts were restricted to any one set of organs.