A BIRD’S OUTER WRAP, aT 
vertebre. The nervous system began superficially, 
Spencer thinks, and the spinal marrow begins on the 
surface in embryos now as a flat strap. The eye and 
the ear (and doubtless other organs) began superficially 
in some creatures, and yet show traces often of the 
path by which ee have gone inward. 
All this great work is possible to the skin because 
of its exterior position exposing it so much to the ef- 
fects of environment. It was “in touch” with every- 
thing and plastic to every demand of the organism. 
After the inside skeleton was formed, it demanded 
so much of the lime and magnesia of the body that 
the skin seems to have had at first little material out 
of which to build an outside skeleton or covering, 
though, so to speak, it seems to have turned its atten- 
tion that way. The first efforts seem to have been by 
folds or thickenings of the skin itself. These folds 
formed pockets into which a hornlike substance—a 
new material which appeared to dawn with the back- 
bone—formed, and it finally grew out and pierced the 
outer layer, or epidermis, as in the fish scales. The 
reptiles have in most cases kept the primitive skin 
fold, rendered horny in places, as best suited to their 
haunts and habits, but many amphibians and some 
fishes have lost them nearly altogether, or had them 
sink again beneath the surface. 
Perhaps some of the reptiles, especially the bird- 
like, upright walkers, may have burst their epidermis 
by the great growth of their scales, as the feathers of 
the birds do now. But it is astonishing, even yet, to 
note in birds how resisting the outer skin is, as though 
4 
