r of animals of any particular species remains practically
the same. There may be a few more one year, and a few less another,
but on the average, year by year, the number of toads, the number of
blacksnakes, the number of field mice, remains sensibly the same.
Sometimes the rise of man brings an end to the wild population, and so
in the past animals have dropped out of the race. Yet in the long run
and for a considerable time the number of any species is constant. But
each animal produces offspring in quantities sufficient to far more
than replace himself as he dies out. In other words, animals increase
not by addition but by multiplication. Too many are born for all of
them to live. What becomes of the great mass of them? The answer is
they die; most of them die young. Only a few fortunate individuals,
favored by being a little stronger, a little more cunning, a little
more attractively colored than their mates, survive to carry on the
race.

The skillful gardener, looking over his flowers, finds a plant of more
than ordinary beauty and thrift of growth. When it comes to maturity
he keeps its seeds separate from those of the rest and next year
plants them by themselves. As they come up he weeds out all unthrifty
plants, only allowing the strongest to come to maturity. As they break
into bloom he plucks away all whose flowers do not come up to the high
standard he has set for himself. After a while he has but a few plants
left, but these are the thriftiest and bear the most beautiful
flowers. Again he allows these to mature and selects the seed of the
very finest. Next year the process is repeated. After a few
generations, usually three if the man is skillful enough, he has a
definite strain of flowers that will thereafter come true. This is the
process of artificial selection as carried on by man.

Darwin saw that Nature is constantly carrying on a similar process.
She produces seeds enough on almost any plant to clothe the world in a
few years if all of them could fall into proper ground and thrive like
their parents. A friend of mine found a mullein stalk that bore more
than seven hundred seed pods and averaged more than nine hundred seeds
to the pod, a total of more than six hundred and thirty thousand
seeds. If each of these could find lodgment on a plot eighteen inches
square, produce a similar number of seeds and plant them all, the
result would be overwhelming. The fourth generation would cover land
and sea,