types of social evolution possible. While the same factors are at work in the higher phases of evolution which are at work in the lower phases, yet it is evident that the higher phases have new and distinct factors. Sociology, being especially concerned with social evolution, has a new and distinct factor at work which we may call association, coöperation, or combination, and this it is which gives sociology its distinct place in the list of general sciences. Factors In Organic Evolution.--As has already been said, the factors which are at work in organic evolution generally are also at work in social evolution. We need, therefore, to note these factors carefully and to see how they are at work in human society as well as in the animal world below man. While these factors are not all of the factors which are at work in social evolution, still they are the primitive factors, and are, therefore, of fundamental importance. Let us see what these factors are. (1) _The Multiplication of Organisms in Some Geometric Ratio through Reproduction._ It is a law of life that every species must increase so that the number of offspring exceeds the number of parents if the species is to survive. If the offspring only equal in number the parents, some of them will die before maturity is reached or will fail 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.