_Vorticella_, provide characteristic
examples of these primitive compound protozoa. Here the assemblage is made
up of one-celled individuals essentially similar to one another in
structure and in physiological activities; in the latter respect each one
of them is like _Amoeba_ as well. They may remain together for a longer
or shorter period, or during their whole existence until the time of
reproduction. Like the solitary protozooen, each member leads a complete
life in and by itself, equivalent to that of every biological unit. It
obeys the two great laws already laid down, but in addition it seems to be
required to remain with the others for some mutual good. The biological
value of the association which imposes this additional obligation may be
found perhaps in the fact that a large group is not so readily eaten by an
enemy as an individual cell; but it is clearer that the process of
reproduction, which consists of the fusion of small "gametes," or
nucleated fragments produced by diverse or similar parents, must be
greatly facilitated by the occurrence of gamete-forming individuals in one
and the same colony. "_To remain together_" is the new duty imposed by
nature for the good of all and for the welfare of each member of the
group. Some biological advantage accrues to the several components, just
as the banding of wolves enables the pack to accomplish something which
the single wolf is unable to do, although in the latter case it is not so
much a reproductive alliance that is formed as an offensive and defensive
union.

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