owever, we must glance at the operation of this and
other natural principles in the evolution of the one-celled animals
and plants, which we take to represent the primitive population of
the earth. As there are tens of thousands of different species even of
"microbes," it is clear that we must deal with them in a very summary
way. The evolution of the plant I reserve for a later chapter, and I
must be content to suggest the development of one-celled animals on
very broad lines. When some of the primitive cells began to feed on each
other, and develop mobility, it is probable that at least two distinct
types were evolved, corresponding to the two lowest animal organisms in
nature to-day. One of these is a very minute and very common (in vases
of decaying flowers, for instance) speck of plasm, which moves about by
lashing the water with a single oar (flagellum), or hair-like extension
of its substance. This type, however, which is known as the Flagellate,
may be derived from the next, which we will take as the primitive and
fundamental animal type. It is best seen in the common and familiar
Amoeba, a minute sac of liquid or viscid plasm, often not more than a
hundredth of an inch in diameter. As its "skin" is merely a finer kind
of the viscous plasm, not an impenetrable membrane, it takes in food at
any part of its surface, makes little "stomachs," or temporary cavities,
round the food at any part of its interior, ejects the useless matter at
any point, and thrusts out any part of its body as temporary "arms" or
"feet."

Now it is plain that in an age of increasing microbic cannibalism the
toughening of the skin would be one of the first advantages to secure
survival, and this is, in point of fact, almost the second leading
principle in early development. Naturally, as the skin becomes firmer,
the animal can no longer, like the Amoeba, take food at, or make limbs
of, any part of it. There must be permanent pores in the membrane to
receive food or let out rays of the living substance to act as oars
or arms. Thus we get an immense variety amongst these Protozoa, as the
one-celled animals are called. Some (the Flagellates) have one or two
stout oars; some (the Ciliates) have numbers of fine hairs (or cilia).
Some have a definite mouth-funnel, but no stomach, and cilia drawing
the water into it. Some (Vorticella, etc.), shrinking from the open
battlefield, return to the plant-principle, live on stalks, and have
wreaths of cil