Some of these variations are indifferent, some
deteriorations, some improvements, that is, they are such as enable the
plant or animal to exercise its functions to greater advantage.
Third, the law of Over Production. All plants and animals tend to
increase in a geometrical ratio; and therefore tend to overrun
enormously the means of support. If all the seeds of a plant, all the
spawn of a fish, were to arrive at maturity, in a very short time the
world could not contain them. Hence of necessity arises a struggle for
life. Only a few of the myriads born can possibly live.
Fourth, here comes in the law of Natural Selection, or the Survival of
the Fittest. That is, if any individual of a given species of plant or
animal happens to have a slight deviation from the normal type,
favorable to its success in the struggle for life, it will survive. This
variation, by the law of heredity, will be transmitted to its offspring,
and by them again to theirs. Soon these favored ones gain the
ascendency, and the less favored perish; and the modification becomes
established in the species. After a time another and another of such
favorable variations occur, with like results. Thus very gradually,
great changes of structure are introduced, and not only species, but
genera, families, and orders in the vegetable and animal world, are
produced. Mr. Darwin says he can set no limit to the changes of
structure, habits, instincts, and intelligence, which these simple laws
in the course of millions or milliards of centuries may bring into
existence.
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