causes of age. In the ordinary course of life slight injuries are
constantly being received and more or less perfectly repaired. An
infection which may but slightly affect the ordinary well-being of the
individual may produce a considerable damage. Excess or deficiency or
improper food, occasional or continued use of alcohol and other
poisons may lead to very definite lesions. Repair after injury is
rarely perfect, the repaired tissue is more susceptible to injury, and
with advancing age there is constant diminution in the ease and
perfection of repair. The effect of the sum of all these changes
becomes operative: a vicious circle is established in which injury
becomes progressively easier to acquire and repair constantly less
perfect. There is some adjustment, however, in that the range of
activities is diminished, the environment becomes narrower and the
organism adapts its life to that environment which makes the least
demands upon it.

Whether there is, entirely apart from all conditions affecting
nutrition and the effect of injuries which disturb the usual cell
activities, an actual senescence of the cells of the body is
uncertain. In the presence of the many factors which influence the
obvious diminution of cell activity in the old, it is impossible to
say whether the loss of cell activity is intrinsic or extrinsic. The
life of the plant cell seems to be immortal; it does not grow old.
Trees die owing to accidents or because the tree acquires in the
course of its growth a mass of tissue in which there is little or no
life, and which becomes the prey of parasites. The growing tissue of a
tree is comprised in a thin layer below the bark, and the life of this
may seemingly be indefinitely prolonged by placing it in a situation
in which it escapes the action of accidental injuries and decay, as by
grafting on young trees. Where the nature of the dead wood is such