Let us begin with a simple case of change in the adult body. The
workman grasps his tools day after day, and his hands become horny.
The skin has evidently thickened, somewhat as on the soles of the
feet. This is no mere mechanical result of pressure alone.
Continuous pressure would produce the opposite result. But under the
stimulus of intermittent pressure the capillaries, or smallest blood
vessels, furnish more nutriment to the cells composing the lowest
layer of the outer skin or epidermis. These cells, being better
nourished, reproduce by division more rapidly, and the epidermis,
becoming composed of a greater number of layers of cells, thickens.
The outer-most layers, being farthest from the blood supply, dry up
and are packed together into a horny mass.
If I go out into the sunshine I become tanned. This again is not a
direct and purely chemical or physical result of the sun's rays, but
these have stimulated the cells of the skin to undergo certain
modifications. Any change in the living body under changed
conditions is not passive, but an active reaction to a stimulus
furnished by the surroundings. The same stimulus may excite very
different reactions in different individuals or species.
Early in this century a farmer, Seth Wright, found among his lambs a
young ram with short legs and long body.
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