ironment that we are going to survey the panorama of the advance of
terrestrial life.

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
survi