re
almost the sole difference between plants and animals is in the
fluid or solid character of their food, a change from the one form
into the other is not as difficult or improbable as one might
naturally think. And plants and animals are here so near together,
and travelling by roads so nearly parallel, that, even if volvox
never was an animal, it might still serve very well to illustrate a
stage through which animals must have passed.

The cells of volvox do not form a solid mass, but have arranged
themselves in a single layer on the outer surface of the sphere. For
a time, under favorable circumstances, volvox reproduces very much
like magosphaera, and each cell can give rise to a new, many-celled
individual. But after a time, especially under unfavorable
circumstances, a new mode of reproduction appears. Certain cells
withdraw from the outer layer into the interior of the colony. Here
they are nourished by the other cells and develop into true
reproductive elements, eggs and spermatozoa. Fertilization, that is,
the union of egg and spermatozoon, or mainly of their nuclei, takes
place; and the fertilized egg develops into a new organism. But the
other cells, which have been all the time nourishing these, seem now
to lack nutriment, strength, or vitality to give rise to a new
colony. They die.

We find thus in volvox division of labor and corresponding
difference of structure or differentiation; certain cells retain the
power of fusing with other corresponding cells, and thus of
rejuvenescence and of giving rise to a new organism. And these
cells, forming a series through all generations, are evidently
immortal like the protozoa. Natural death cannot touch them. These
are the reproductive cells. The other cells nourish and transport
them and carry on the work of excretion and respiration. These
latter correspond practically to our whole body. We call them
somatic cells. In volvox they are entirely subservient to, and exist
for, the reproductive cells, and die when they have completed their
service of these. The body is here only a vehicle for ova.
Furthermore, in volvox there has arisen such an interdependence of
cells that we can no longer speak of it as a colony. The colony has
become an individual by division of labor and the resulting
differentiation in structure.

But hydra gives us but a poor idea of the coelenterata, to which
kingdom it belongs. The higher coelenterata have nearly or quite
all the tissue