QUOTE : concrete example will explain this more clearly. It is known that the atoms of radium, and of other radio-active substances, disintegrate into atoms of lead and helium with the mere passage of time, so that a mass of radium continually diminishes in amount, being replaced by lead and helium. The law which governs the rate of diminution is very remarkable. The amount of radium decreases in precisely the same way as a population would if there were no births, and a uniform death-rate which was the same for every individual, regardless of his age. Or again, it decreases in the same way as the numbers of a battalion of soldiers who are exposed to absolutely random undirected fire. In brief, old age appears to mean nothing to the individual radium atom; it does not die because it has lived its life, but rather because in some way fate knocks at the door.