nt 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. The farmer kept the ram,
reasoning that his short legs would prevent him from leading the
flock over the farm-walls and fences. From this ram was descended
the breed of ancon, or otter, sheep. Now the stimulus which had
excited this variation must have been applied early in embryonic
life, or perhaps during the formation or maturing of the germ-cells
themselves. Such a variation we call a congenital variation.

These cases are merely illustrations of the general truth that in
every variation there are two factors concerned: the living being
with its constitution and inherent tendencies and the external
stimulus.

The courses of the different balls in a charge of grape-shot, hurled
from a cannon, are evidently due to two sets of forces--1, their
initial energy and the direction of their aim; 2, the deflecting
power of resisting objects or forces--or the different balls might
roll with great velocity down a precipitous mountain-side. In the
first case velocity and direction of course would be determined
largely by initial impulse; in the second, by the attraction of the
earth and by the inequalities of its surface.

In evolution, environment, roughly speaking, corresponds to these
deflecting or attracting external objects or forces; inherent
tendencies to initial impulse. If we lay great weight on initial
tendencies, inherent in protoplasm from the very beginning, we shall
probably lay less stress on natural selection as a guiding,
directing process.

Th