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.
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