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. (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 the unfit.