Great division of the Living World
If we imagine this simple principle at work for ages among the
primitive microbes, we understand the first great division of the
living world, into plants and animals. There must have been a
long series of earlier stages below the plant and animal. In
fact, some writers insist that the first organisms were animal in
nature, feeding on the more elementary stages of living matter.
At last one type develops chlorophyll (the green matter in
leaves), and is able to build up plasm out of inorganic matter;
another type develops mobility, and becomes a parasite on the
plant world. There is no rigid distinction of the two worlds.
Many microscopic plants move about just as animals do, and many
animals live on fixed stalks; while many plants feed on organic
matter. There is so little "difference of nature" between the
plant and the animal that the experts differ in classifying some
of these minute creatures. In fact, we shall often find plants
and animals crossing the line of division. We shall find animals
rooting themselves to the floor, like plants, though they will
generally develop arms or streamers for bringing the food to
them; and we shall find plants becoming insect-catchers. All this
merely shows that the difference is a natural tendency, which
special circumstances may overrule. It remains true that the
great division of the organic world is due to a simple principle
of development; difference of diet leads to difference of
mobility.
But this simple principle will have further consequences of a
most important character. It will lead to the development of mind
in one half of living nature and leave it undeveloped in the
other. Mind, as we know it in the lower levels of life, is not
confined to the animal at all. Many even of the higher plants are
very delicately sensitive to stimulation, and at the lowest level
many plants behave just like animals. In other words, this
sensitiveness to stimuli, which is the first form of mind, is
distributed according to mobility. To the motionless organism it
is no advantage; to the pursuing and pursued organism it is an
immense advantage, and is one of the chief qualities for natural
selection to foster.
For the moment, however, 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."
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