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.

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