(3) _Variation._ This factor in organic evolution means that no two
individuals, even though born of the same parents, are exactly like each
other. Neither are they of a type exactly between their two parents, as
theoretically they should be, since inheritance is equal from both
parents. Every new individual born in the organic world, while it
resembles its parents and belongs to its species or race, varies within
certain limits. This variation so runs through organic nature that we
are told that there are no two leaves on a single tree exactly alike.
The result of this variation, the causes of which are not yet well
understood, is that some individuals vary in favorable directions,
others in unfavorable directions. Some are born strong, some weak; some
inferior, some superior.

It is evident that variation characterizes the human species quite as
much as other species, and indeed the limits of variation are wider,
probably, in the human species than in any other species. Man is the
most variable of all animals, and human individuality and personality
owe not a little of their distinctiveness to this fact.

(4) _The Struggle for Existence._ Individuals in all species, as we
have seen, are born in larger numbers than is necessary. The result is
that a competition is entered into between species and individuals
within the species for place and for existence. This competition or
struggle results in the dying out of the inferior, that is, of those who
are not adapted to their environment. The gradual dying out of the
inferior or unadapted through competition results in the survival of the
superior or better adapted, and ultimately in the survival of the
fittest or those most adapted. Thus the type is raised, and we have
evolution through natural selection, that is, through the elimination of