to reproduce, and so the species will gradually become extinct. Every
species normally increases, therefore, in some geometric ratio. Now,
this tendency to reproduce in some geometric ratio, which characterizes
all living organisms, means that any species, if left to itself, would
soon reach such numbers as to occupy the whole earth. Darwin showed, for
example, that though the elephant is the slowest breeding of all
animals, if every elephant lived its normal length of life (one hundred
years) and to every pair were born six offspring, then, at the end of
seven hundred years there would be nineteen million living elephants
descended from a single pair. This illustration shows the enormous
possibilities of any species reproducing in geometric ratio, as all
species in order to survive must do.

That this tendency to increase in some geometric ratio applies also to
man is evident from all of the facts which we know concerning human
populations. It is not infrequent for a people to double its numbers
every twenty-five years. If this were continued for any length of time,
it is evident that a single nation could soon populate the whole earth.

(2) _Heredity._ Heredity in organic evolution secures a continuity
of the species or racial type. By heredity is meant the resemblance
between parent and offspring. It is the law that like begets like.
Offspring born of a species belong to that species, and usually resemble
their parents more closely even than other members of the species.

It is evident that heredity is at work also in human society as well as
in the animal world. We do not expect that the children born of parents
of one race, for example, will belong to another race. Racial heredity
is one of the most significant facts of human society, and even family
heredity counts in its influence far more than some have supposed.