ity
of form. We see a gradual evolution, on utilitarian principles, as
we run over the thousands of forms; and, when we recollect the
inconceivable numbers in which these little animals have lived and
struggled for life--passively--during tens of millions of years, we are
not surprised at the elaborate protective frames of the higher types.

The Thalamophores, the sister-group of one-celled animals which largely
compose our chalk and much of our limestone, are developed on the same
principle. The earlier forms seem to have lived in a part of the ocean
where silica was scarce, and they absorbed and built their protective
frames of lime. In the simpler types the frame is not unlike a
wide-necked bottle, turned upside-down. In later forms it takes the
shape of a spirally coiled series of chambers, sometimes amounting to
several thousand. These wonderful little houses are not difficult to
understand. The original tiny animal covers itself with a coat of lime.
It feeds, grows, and bulges out of its chamber. The new part of its
flesh must have a fresh coat, and the process goes on until scores, or
hundreds, or even thousands, of these tiny chambers make up the spiral
shell of the morsel of living matter.

With this brief indication of the mechanical principles which have
directed the evolution of two of the most remarkable groups of the
one-celled animals we must be content, or the dimensions of this volume
will not enable us even to reach the higher and more interesting types.
We must advance at once to the larger animals, whose bodies are composed
of myriads of cells.

The social tendency which pervades the animal world, and the evident use
of that tendency, prepare us to understand that the primitive
microbes would naturally come in time to live in clusters. Union means
effectiveness in many ways, even when it does not mean strength. We
have still many loose associations of one-celled animals in nature,
illustrating the approach to a community life. Numbers of the Protozoa
are social; they live either in a common jelly-like matrix, or on a
common stalk. In fact, we have a singularly instructive illustration of
the process in the evolution of the sponges.

It is well known that the horny texture to which we commonly give
the name of sponge is the former tenement and shelter of a colony of
one-celled animals, which are the real Sponges. In other groups the
structure is of lime; in others, again, of flinty material. Now,