Each of these one-celled individuals thus lives on and grows, till its surface, through which all nutritive substance is absorbed and which increases at a less rapid rate than its cubic content, becomes relatively too small, so that the creature can no longer be nourished through it and the mature cell faces the alternative either to die or divide into two halves; and, accepting the latter, becomes, by division, two smaller daughter cells that, as the food-absorbing surface becomes now relatively greater, are rejuvenated, although their combined substance is exactly the same as that of the mother cell of which they are simply bifurcations. As, thus, the substance of the parent cells all goes over into the offspring resulting from the fission, nothing is lost in the process, not even an envelope or membrane, as Götte, Weismann’s chief earlier critic, thought was the case in encystment. Thus there is no vestige or rudiment of a corpse. Nothing is sloughed off. It is in this sense that such creatures are immortal. They have gone on growing and dividing thus ever since life began and will continue to do so until it ceases. Thus there is a direct continuity from first to last that is unbroken by anything that can be called death. If once and so long as these single-celled creatures were the only or highest forms of life, death or anything like it was unknown. In all this process, of course, nothing like conjugation, mating, or fertilization occurs. That this is not mere theory the experiments and observations of many subsequent investigators, especially Woodruff and his pupils, have shown. In thirteen and a half years he found paramecia had divided some 8,500 times, and the process is still going on as actively as at first. Of these results Raymond Pearl[166] says, “If in 8,500 generations--a duration of healthy reproductive existence which, if the generations were of the same length as in man, would represent roughly a quarter of a million years in absolute time--natural death has not occurred, we may, with reasonable assurance, conclude that the animal is immortal.” Thus it is that larger, older cells are constantly being regenerated by spontaneous division and natural death does not occur among most protozoa.