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.
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