sea-gull by keeping it in captivity that it could only
secure a grain diet. The effect was to modify the stomach of the bird,
normally adapted to a fish diet, until in time it came to resemble in
structure the gizzard of an ordinary grain-feeder such as the pigeon.
Holmgren again reversed this experiment by feeding pigeons for a
lengthened period on a meat-diet, with the result that the gizzard
became transformed into the carnivorous stomach. Mr. Alfred Russel
Wallace mentions the case of a Brazilian parrot which changes its color
from green to red or yellow when fed on the fat of certain fishes. Not
only changes of food, however, but changes of climate and of
temperature, changes in surrounding organisms, in the case of marine
animals even changes of pressure, of ocean currents, of light, and of
many other circumstances, are known to exert a powerful modifying
influence upon living organisms. These relations are still being worked
out in many directions, but the influence of Environment as a prime
factor in Variation is now a recognized doctrine of science.[81]

Even the popular mind has been struck with the curious adaptation of
nearly all animals to their _habitat_, for example in the matter of
color. The sandy hue of the sole and flounder, the white of the polar
bear with its suggestion of Arctic snows, the stripes of the Bengal
tiger--as if the actual reeds of its native jungle had nature-printed
themselves on its hide;--these, and a hundred others which will occur to
every one, are marked instances of adaptation to Environment, induced by
Natural Selection or otherwise, for the purpose, obviously in these
cases at least, of protection.

To continue the investigation of the modifying action of Environment
into the moral and spiritual spheres, would be to open a fascinating and
suggestive inquiry. One might show how the moral man is acted upon and
changed continuously by the influences, secret and open, of his
surroundings, by the tone of society, by the company he keeps, by his
occupation, by the books he reads, by Nature, by all, in short, that
constitutes the habitual atmosphere of his thoughts and the little world
of his daily choice. Or one might go deeper still and prove how the
spiritual life also is modified from outside sources--its health or
disease, its growth or decay, all its changes for better or for worse
being determined by the varying and successive circumstances in which
the religious habits are