[Pg 338] A considerable part of a frog’s breathing is carried on, in these first stages, through the thin membrane forming the roof of the mouth-cavity. This is richly supplied with blood capillaries, and is therefore admirably adapted for the exchange of gases which constitutes respiration. Moreover, the skin covering the general surface of the body has also a very abundant blood-supply, and forms yet a third respiratory organ; so that it is practically impossible to drown a frog in ordinary water—in water, that is, which contains dissolved air. This power of breathing through the skin is of great importance to a frog during the winter sleep at the bottom of a pond. The frog’s skin.—The frog’s skin is kept moist by a slimy fluid which is continually being discharged from small glands in its substance. The moisture not only facilitates skin-breathing as described above, but its evaporation keeps the body cool even in hot weather—a matter of vital importance to an animal to which a temperature of 40° C. or so is fatal. A third most interesting property of the frog’s skin is its power of changing somewhat in colour to match the colour of its surroundings. The change depends upon an alteration in form and size of certain small brown specks imbedded in the thickness of the skin. When the animal finds itself in dark-coloured surroundings these specks enlarge, and the skin as a whole takes on a darker hue. On a light background the reverse change takes place. A case of evolution.—The advantage which accrues to a frog from being thus rendered less conspicuous to enemies and prey alike is obvious, and there is no difficulty in picturing to ourselves the probable manner in which the advantage was developed. Widely different races of animals have colour-specks in the skin, and we may assume that frogs and their ancestors have possessed such spots for countless generations. Now suppose that, ages ago, a frog happened to be born [Pg 339] with the power of altering very slightly the size of his colour-specks. If this power rendered him less conspicuous, in however slight a degree, than his neighbour frogs he would, other things being equal, be more likely than they to escape from enemies, grow to maturity, and in due course have sons and daughters. All his offspring, and they might number thousands, would tend to inherit, to varying extents, the power of changing colour. In some—perhaps in most—the power would be practically absent; in others it might be equal to that of the parent; while in a few instances it would probably be greater. In any case, those frogs of the second generation which had the greatest power of changing their colour would be the most likely to survive the keen struggle for existence and therefore to leave offspring. The survival of the “fittest” frogs of each generation, and the transmission to the next generation, in ever-increasing intensity, of their favouring