First Principle of Evolution
For the moment it will be enough to state two leading principles.
The first is that there is no such thing as a "law of evolution"
in the sense in which many people understand that phrase. It is
now sufficiently well known that, when science speaks of a law,
it does not mean that there is some rule that things MUST act in
such and such a way. The law is a mere general expression of the
fact that they DO act in that way. But many imagine that there is
some principle within the living organism which impels it onward
to a higher level of organisation. That is entirely an error.
There is no "law of progress." If an animal is fitted to secure
its livelihood and breed posterity in certain surroundings, it
may remain unchanged indefinitely if these surroundings do not
materially change. So the duckmole of Australia and the tuatara
of New Zealand have retained primitive features for millions of
years; so the aboriginal Australian and the Fuegian have remained
stagnant, in their isolation, for a hundred thousand years or
more; so the Chinaman, in his geographical isolation, has
remained unchanged for two thousand years. There is no more a
"conservative instinct" in Chinese than there is a "progressive
instinct" in Europeans. The difference is one of history and
geography, as we shall see.
To make this important principle still clearer, let us imagine
some primitive philosopher observing the advance of the tide over
a level beach. He must discover two things: why the water comes
onward at all, and why it advances along those particular
channels. We shall see later how men of science explain or
interpret the mechanism in a living thing which enables it to
advance, when it does advance. For the present it is enough to
say that new-born animals and plants are always tending to differ
somewhat from their parents, and we now know, by experiment, that
when some exceptional influence is brought to bear on the parent,
the young may differ considerably from her. But, if the parents
were already in harmony with their environment, these variations
on the part of the young are of no consequence. Let the
environment alter, however, and some of these variations may
chance to make the young better fitted than the parent was. The
young which happen to have the useful variation will have an
advantage over their brothers or sisters, and be more likely to
survive and breed the next generation. If the change in the
environment (in the food or climate, for instance) is prolonged
and increased for hundreds of thousands of years, we shall expect
to find a corresponding change in the animals and plants.
We shall find such changes occurring throughout the story of the
earth. At one important point in the story we shall find so grave
a revolution in the face of nature that twenty-nine out of every
thirty species of animals and plants on the earth are
annihilated. Less destructive and extreme changes have been
taking place during nearly the whole of the period we have to
cover, entailing a more gradual alteration of the structure of
animals and plants; but we shall repeatedly find them culminating
in very great changes of climate, or of the distribution of land
and water, which have subjected the living population of the
earth to the most searching tests and promoted every variation
toward a more effective organisation
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