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.