that it is immune from parasites and decay, as in the case of the
Sequoias, life seems to be indefinitely prolonged. The growing
branches of one of these trees, whose age has been estimated with
seeming accuracy at six thousand years, are just as fresh and the tree
produces its flowers and fruit in the same degree as a youthful
brother of one thousand years. Nor does old age supervene in the
unicellular organisms. An amoeba assimilates, grows and multiplies
just as long as the environment is favorable.

Old age in itself is seldom a cause of death. In rare cases in the
very old a condition is found in which no change is present to which
death can be attributed, all organs seem to share alike in the
senescence. Death is usually due to some of the accidents of life, a
slight infection to which the less resistant body succumbs, or to the
rupture of a weakened blood vessel in the brain, or to more advanced
decay in some organ whose function is indispensable. The causes and
conditions of age have been a fertile source for speculation. Many of
the hypotheses have been interesting, that of Metschnikoff, for
example, who finds as a dominating influence in causing senescence the
absorption of toxic substances formed in the large intestine by
certain bacteria. He further finds that the cells of the body which
have phagocytic powers turn their activity against cells and tissues
which have become weakened. There may be absorption of injurious
substances from the intestines which the body in a vigorous condition
is able to destroy or to counteract their influence, and these may be
more operative in the weaker condition of the body in the old.
Phagocytes will remove cells which are dead and often cells which are
superfluous in a part, but there is no evidence that this is ever
other than a conservative process. Since it is impossible to single
out any one condition to which old age is due, the hypothesis of