One step higher in the scale stands the plant-form called _Volvox_, near the border-line between the one-celled and the many-celled organisms. This aquatic type, about the size of the head of an ordinary pin, is a hollow spherical colony, with a wall composed of closely set cellular components. These elements are not all alike, as in the case of colonial protozoa like _Vorticella_, for they fall into two classes which are distinguished by certain structural and functional characteristics. Most of them are simple feeding individuals which absorb nourishment for themselves primarily, but they pass on their surplus supplies to less favored neighbors if occasion demands. The other members begin life like the first-named, but later they become specialized to serve as reproductive individuals solely. Every member of the colony must obey the first precept of nature, otherwise it would be unable to play its part in the life of the whole community. But the discharge of the second natural obligation, namely to preserve the race, is here assigned to some, and to some only, of the whole group of cell individuals. It follows therefore that the division of the tasks necessary for the maintenance of a complete biological individual, and the differentiation of the members of the group into two kinds, leads to the establishment of an individuality of a higher order than the cell. Neither the purely nutritive nor the reproducing member is complete in itself; the two kinds must be combined to make a perfect organism. The life of any member can be selfish no longer, for if it is to exist itself, it must help others for the mutual advantage of all. A clear social relation is thus established; and the reflex conduct of the units of a _Volvox_ colony can be justly denoted altruistic, even though in this case, as before, there can be no conscious recognition of the reasons why mutual interests are best served by what is actually done.
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.