The preceding examples will suffice to give a general idea of what is meant by adaptation in organisms. That the term includes a large number of phenomena of very different kinds is apparent. When we have examined these phenomena further we shall find, I think, that it will be necessary to put some of them into different categories and treat them differently. It is probably incorrect to suppose that all processes useful to the organism have been acquired in the same way, nevertheless, for the present the term adaptation is sufficiently general, even if vague, to cover these different groups of cases. It may be asked, in what respects are these structures and processes of adaptation different from the ordinary structures and changes that go on in the organism? Why is the leg of the mole more of an adaptation than that of a dog? The one is of as much use as the other to its possessor. What reason can we give for citing the poison of the snake, and not mentioning in the same connection the other glands of the body? In fact, the poison gland of the snake is supposed to be a modified superior labial gland. Why, in short, are not the processes of digestion, excretion, secretion, the beating of the heart, the ordinary reflex acts of the nervous system, and the action of the sense-organs, as truly adaptations as the special cases that have been selected for illustration. The answer is simply that we are more impressed by those cases of adaptation that are more unusual, as when an animal departs in the use of certain structures from the rest of the group to which it belongs. For example, if all mammals lived underground, ourselves included, and the fore-legs or arms were used for burrowing, we should not think this unusual; but if we found an animal using all four legs to support the body and for purposes of progression, we should, most likely, think this was an excellent illustration of adaptation. In other instances the condition is somewhat different. The color of certain animals may unquestionably be of use to them in concealing them from their enemies. In other cases the color may not serve this purpose, or any purpose at all. Thus while in the former case we speak of the color as an adaptation to the surroundings, in the latter we do not think of it as having any connection at all with the environment. Even in the same animal the color of different parts of the body may appear under this twofold relation. For example, the green color of the skin of the frog renders it less conspicuous amongst the green plants on the edge of the stream, but the brilliant orange and black pigment in the body-cavity cannot be regarded as of any use to the animal. Adaptations for the Good of the Species