Suppose a small pit to be formed on the previously flat skin; and suppose that the growth and casting off of horny cells which goes on over the skin in general, continues to go on at the usual rate over the depressed surface of this pit. Clearly the quantity of horny matter produced within this hollow, will be greater than that produced on a level portion of the skin subtending an equal area of the animal’s outside. Suppose such a pit to be deepened until it becomes a small sac. If the exfoliation goes on as before, the result will be that the horny matter, expelled, as it must be, through the mouth of the sac, which now bears a small proportion to the internal surface of the sac, will be large in quantity compared with that exfoliated from a portion of the skin equal in area to the mouth of the sac: there will be a conspicuous thrusting forth of horny matter. Suppose once more that the sac, instead of remaining simple, has its bottom pushed up into its interior, like the bottom of a wine-bottle--the introversion being carried so far that the introverted part reaches nearly to the external opening, and leaves scarcely any space between the introverted part and the walls of the sac. It is easy to see that the exfoliation continuing from the surface of the introverted part, as well as from the inside of the sac generally, the horny matter cast off will form a double layer; and will come out of the sac in the shape of a tube having within its lower end the introverted part, as the core on which it is moulded, and from the apex of which is cast off the substance filling, less densely, its interior. The structure resulting will be what we know as a hair. Manifestly by progressive enlargement of the sac, and further complication of that introverted part on which the excreted substance is moulded, the protruding growth may be rendered larger and more involved, as we see it in quills and feathers. So that insensible steps, thus indicated in principle, carry us from the exfoliation of epidermis by a flat surface, to the exfoliation of it by a hollow simple sac, an introverted sac, and a sac further complicated; each of which produces its modified kind of tegumentary appendage. But now, after contemplating this typical illustration, we return to the general question. What are the agencies which have been operative in developing these skin-structures? Indirect equilibration must have worked almost alone in producing them. No direct incidence of forces can have developed the enamelled armour of the _Lepidosteus_ or the tesselated plates of the _Glyptodon_ and its modern allies. Survival of the fittest must here and in multitudinous other cases be regarded as the sole cause. § 295. Among many other differentiations of the outer tissues, the most worthy to be noticed in the space that remains, are those by which organs of sense are formed. We will begin with the simplest and most closely-allied to the foregoing.