_Animal Colonies._—These examples, as well as those furnished to zoologists by the consideration of the polypi which have produced by their slow growth the reefs, or _atolls_, of the Polynesian seas, do not, however, prove the perennity of living beings. The argument is valueless, for it is founded upon a confusion. It turns on the difficulty that biologists experience in defining the individual. The oak and the polypus are not simple individuals, but associations of individuals, or, to use Hegel’s expression, the nations of which we see the successive generations. We give to this succession of generations a unique existence, and our reasoning comes to this, that we confer on each present citizen of this social body the antiquity which belongs to the whole. _Destruction of the Social Individual due to Extrinsic Causes._—As for the destruction, the death of this social individual, of this hundred-year-old tree, it seems indeed that there is no ground for considering it a natural necessity. We find the sufficient reason of its usual end in the repercussion on the individual of external and contingent circumstances. The cause of the death of a tree, of an oak many centuries old, is to be found in the ambient conditions, and not in some internal condition. Cold and heat, damp and dryness, the weight of the snow, the mechanical action of the rain, of hail, of winds unchained, of lightning; the ravages of insects and parasites—these are what really work its ruin. And further, the new branches, appearing every year and increasing the load the trunk has to bear, increase the pressure of the parts, and make more difficult the motion of the sap. But for these obstacles, external, so to speak, to the vegetable being itself, it would continue indefinitely to bloom, to fructify, and as each spring returned to show fresh buds. _Difficulty of Interpretation._—In this as in all other examples we must know the nature of the beings that we see lasting on and braving the centuries. Is it the individual? Is it the species? Is it a living being, properly so called, having its unity and its individuality, or is it a series of generations succeeding one another in time and extending in space? In a word, the question is one of knowing if we have to do with a real tree or with a genealogical tree. We are just as uncertain when we deal with animals. What is the being that lasts on—a series of generations or an individual? This doubt forbids us to draw any conclusion from the observation of complex beings. We must therefore return from them to the _elementary being_; and we must examine it from the point of view of perennity or of vital decay. Let us then ask the questions that we have already examined with reference to animals high in organization and to man himself. Is the death of the cell an inevitable characteristic? Are there any cells, protophytes, protozoa, which are immortal? CHAPTER V. THE IMMORTALITY OF THE PROTOZOA.