Because men have always been interested in the study of themselves, the subject of racial evolution is literally enormous, and the attempt to give anything like a complete description of what is known would obviously be futile.  But it is possible to obtain a clear conception of certain of the fundamental principles that fall into line with the other parts of the doctrine of organic evolution with which we have now become acquainted.  The main questions, therefore, may be stated in simple terms.  The first deals with the evidences as to the reality of evolution during the historical and prehistoric development of the various types of man from earlier common ancestors; the second asks whether the lines of racial evolution are further continuations of the line leading from ape-like ancestors to the human species as a type.  In order to give the proper perspective, it will be well to state at the present juncture, first, that the various kinds of men do not vary from each other in a chance manner so as to show all possible types and varieties, but that they fall into natural groups or families distinguished by certain common characteristics, just as do all other kinds of species of animals; in the second place, it appears that some of the differences between the races denoted higher on structural accounts and the lowest forms of man are of the same nature as those observed in the review of the various species of primates from the lemurs to man.

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It is best to look at the whole question in a very simple and common-sense way before undertaking an extended examination of the details of human diversity.  The most casual survey of the peoples that we know best because of our own individual nearness to them enables us to realize that the races now upon the earth have not existed forever and ever, or even for the age of 6000 years as contended by Archbishop Ussher.  They have all come into existence as such, and they differ from their known antecedents; so that at the very outset common-sense leads us to accept evolution as true, if we admit that human races have changed during the course of recent centuries.  We know, for example, that the so-called Mexicans of to-day are a people produced by a fusion of Spanish conquerors and Indian aborigines the Mexican is neither Spaniard nor Indian, though he may resemble both in certain respects; he is a product of natural evolution, accomplished in this case by an amalgamation of two contrasted types.  When we speak of the American people, we must realize that it too has come into existence as such, and even, indeed, that it is in the actual process of evolution at the present time.  The various foreign elements that have been added during the last few decades by the hundreds of thousands are becoming merged with the people who preceded them, just as the Dutch and the French and the English coalesced during the days of early settlement to form the young American nation.