ch look like
precocious senility, due to the poison of the disease. It is no mere
analogy to suppose that human senescence is the result of a slow but
chronic poisoning of the organism. Such poisons, if not completely
destroyed or got rid of, weaken the tissues, the functions of which
become altered or enfeebled in which the latter have the advantage. But
we must make further studies before we can answer the question whether
our senescence can be ameliorated.

The duration of the life of animals varies within very wide limits. As a
general rule, small animals do not live so long as large ones, but there
is no absolute relation between size and longevity, since parrots,
ravens, and geese live much longer than many mammals, and than some much
larger birds. Buffon long ago argued that the total duration of life
bore some definite relation to the length of the period of growth, but
further inquiry shows that such a relation cannot be established.
Nevertheless, there is something intrinsic in each kind of animal which
sets a definite limit to the length of years it can attain. The purely
physiological conditions which determine this limit leave room for a
considerable amount of variation in longevity. Duration of life,
therefore, is a character which can be influenced by the environment.

The duration of life in mammals is relatively shorter than in birds, and
in the so-called cold-blooded vertebrates. No indication as to the cause
of this difference can be found elsewhere than in the organs of
digestion. Mammals are the only group of vertebrate animals in which the
large intestine is much developed. This part of the alimentary canal is
not important, for it fulfils no notable digestive function. On the
other hand, it accommodates among the intestinal flora many microbes
which damage health by poisoning the body with their products. Among the
intestinal flora there are many microbes which are inoffensive, but
others are known to have pernicious properties, and auto-intoxication,
or self-poisoning, is the cause of the ill-health which may be traced to
their activity. It is indubitable that the intestinal microbes or their
poisons may reach the system generally, and bring harm to it. I infer
from the facts that the more the digestive tract is charged with
microbes, the more it is a source of harm capable of shortening life. As
the large intestine not only is that part of the digestive tube most
richly charged with microbes, b