a long time either in the stomach or in the intestines, the
stomach and the remainder of the intestinal canal lose the habit of
being distended in intemperate persons, so also in sedentary persons
and those engaged in mental labor, who are habituated to take but
little food. Gradually and at length their stomach becomes
contracted, and their intestines shortened.

"We are not concerned here with the shrinkage and shortening
produced by a puckering of the parts, which permit ordinary
extension, if instead of a continued emptiness these viscera should
be filled; the shrinkage and shortening in question are real,
considerable, and such that these organs would burst open rather
than yield suddenly to the causes which would require ordinary
extension.

"In circumstances of persons of the same age, compare a man who, in
order to devote himself to habitual study and mental work, which
have rendered his digestion more difficult, has contracted the habit
of eating lightly, with another who habitually takes a good deal of
exercise, walks out often, and eats heartily; the stomach of the
first will be weakened, and a small quantity of food will fill it,
while that of the second will be not only maintained in its ordinary
health but even strengthened.

"We have here the case of an organ much modified in its dimensions
and in its faculties by the single cause of a change in habits
during the life of the individual.

"_The frequent use of an organ become constant by habit increases
the faculties of this organ, even develops it, and enables it to
acquire dimensions and a power of action which it does not possess
in animals which exercise less._

"We have just said that the lack of employment of an organ which
necessarily exists modifies it, impoverishes it, and ends by its
disappearing entirely.

"I shall now demonstrate that the continued employment of an organ,
with the efforts made to draw out its powers under circumstances
where it would be of service, strengthens, extends, and enlarges
this organ, or creates a new one which can exercise the necessary
functions.

"The bird which necessity drives to the water to find there prey
fitted for its sustenance, opens the digits of its feet when it
wishes to strike the water and propel itself along its surface. The
skin which unites these digits at their base, by these acts of
spreading apart being un