is so resistant to changes in environment as is man, and to this
resistance he owes his supremacy. By means of his intelligence he can
change the environment. He is able to resist the action of cold by
means of houses, fire and clothing; without such power of intelligent
creation of the immediate environment the climatic area in which man
could live would be very narrow. Just as disease can be acquired by an
unfavorable environment, man can so adjust his environment to an
injury that harmony will result in spite of the injury. The
environment which is necessary to compensate for an injury may become
very narrow. For an individual with a badly working heart more and
more restriction of the free life is necessary, until finally the only
environment in which life is even tolerably harmonious is between
blankets and within the walls of a room.

The various conditions which may act on an organism producing the
changes which are necessary for disease are manifold. Lack of
resistance to injury, incapacity for adaptation, whether it be due to
a congenital defect or to an acquired condition, is not in itself a
disease, but the disease is produced by the action on such an
individual of external conditions which may be nothing more than those
to which the individuals of the species are constantly subject and
which produce no harm.

[Illustration: FIG. 3.--A SECTION OF THE SKIN. 1. A hair. Notice there
is a deep depression of the surface to form a small bulb from which
the hair grows. 2. The superficial or horny layer of the skin; the
cells here are joined to form a dense, smooth, compact layer
impervious to moisture. 3. The lower layer of cells. In this layer new
cells are continually being formed to supply those which as thin
scales are cast off from the surface. 4. Section of a small vein. 9.