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.
Pages:
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115