316 PHYSIOLOGICAL DEVELOPMENT. 



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 intro- 

 verted 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 



