One of the most interesting and significant aspects of the life-history of _Volvox_ is the appearance for the first time of biological death. More elementary organisms are immortal potentially even if not actually, for every portion of the body is capable of passing over into an animal of a succeeding generation. But in _Volvox_ a division of labor has been effected of such a nature that most of the components discharge the tasks of individual value, and with the performance of these they die. Only the reproductive members are immortal in the sense that _Amoeba_ is, for they only have a place in the chain of consecutive generations of _Volvox_ colonies. From the standpoint of the nutritive individual it is better to be relieved of the reproductive task in order that there may be no interruption of its specialized activities for the good of all, but the entailed mortality is certainly disadvantageous to it. It is the higher interest of the colony as a whole that supersedes the welfare of the parts taken singly, and this larger welfare is safeguarded by a differentiation worked out by natural evolution which results in the assignment of personal and racial duties to different individuals, at the cost ultimately of the lives of the former. We now reach the realm of the true many-celled animals, or Metazoa, where the biological units are combined to form an organic association displaying many more resemblances to a human society. The freshwater polyp _Hydra_, like the foregoing illustrations, is one whose structure has already been discussed in the earlier chapters, but now we may use it for an analysis of another series of biological phenomena. Its sac-like body consists of two cell-layers; the outer one is concerned primarily with offense and defense, while the inner layer is made up of digesting or nutritive elements. The essential cells concerned solely with reproduction lie below the outer sheet. Comparing this animal with an association like _Volvox_, we discover the same differentiation into immortal germ-elements and mortal cells, concerned respectively with the _Hydra's_ racial existence and with its individual life; but far-reaching changes have come about in the biological relationships of the second class of cells. In describing the new phenomena it is absolutely necessary to employ the terms of human social organization, because the _Hydra's_ body is a true colony of diverse cells in exactly the same sense that a nation is a body of human beings with more or less dissimilar social functions.