death, according to the extent to which we have conducted the process.
Lastly, if instead of using up unduly the active and reserve force, or
of suppressing the evolution of force by the withdrawal of its
sources, we expose the body to such an external temperature that it is
robbed of its heat faster than it can generate it; if to supply the
waste heat we draw upon the active and reserve forces, we call forth
immediately the same condition as would follow extreme over-exertion,
or suppression of the development of force; we call forth exhaustion
and sleep, and, if we go far enough, death.

We have had in view, in the above description, a man in the prime of
life, in the center of growth, and decay. In regard to the force of
animation in him, let us look at him now retrospectively and
prospectively. In the past his has been a growing, developing body,
and in the course of development he has produced an excess of force
commensurate with the demands of his growth; this has enabled him
gradually to bear more fatigue and more exposure, without exhaustion,
and even with ease, until he has reached his maximum. When he has
stopped in development, when he stands on a fair level with the
external forces that are opposed to him, then his own force, for a
short time balanced, soon stands second in command. He feels cold more
tenderly; if his rest be broken, the demand for artificial heat is
more urgent; if he lose or miss food, he sinks quickly; and, returning
to our facts, as to the influence of the external temperature on
mortality, these are the reasons why a fall in the thermometer sweeps
away our population according to age so ruthlessly and decisively.

If we analyze the facts further by the side of the diseases which kill
the old, we find those diseases to be numerous in name, but all of two
types. They are diseases which of themselves tend either to produce