Thus if we start with the existing state of things, we find a great variety of human races actually prevailing, located in different parts of the world, and of fundamental types so dissimilar as to constitute what in animal zoology would often be called separate species,[16] and yet fertile among themselves, and so similar in many physical and mental characters as to infer an origin from common ancestors. And we can infer from history that this was so to a great extent 6000 years ago, and that the length of time has been insufficient to produce any marked changes, either in physical or linguistic types of the different fundamental races. [16] Topinard, one of the latest and best authorities, says in his book on Anthropology: "We have seen the marked difference between woolly and straight hair, between the prognathous and the orthognathous, the jet black of the Yoloff and the pale complexion of the Scandinavian, between the ultra-dolichocephalic Esquimaux or New Caledonian, and the ultra-brachycephalic Mongolian. But the line of separation between the European and the Bosjesman, as regards these two characters, is, in a morphological point of view, still wider, as much so as between each of the anthropoid apes, or between the dog and the wolf, the goat and the sheep." Was this always so, and what inference can be drawn as to the much-disputed question between monogeny and polygeny, that is, between the theory of descent from a single pair in a single locality, and that of descent from several pairs, developed in different localities by parallel, but not strictly identical, lines of evolution? This is a question which cannot be decided off-hand by _à priori_ considerations. No doubt Darwinism points to the evolution of all life from primitive forms, and ultimately, perhaps, from the single simplest form of life in the cell or protoplasm. But this does not necessarily imply that the more highly specialized, and what may be called the secondary forms of life, have all originated from single secondary centres, at one time and in one locality. On the contrary, we have the authority of Darwin himself for saying that this is not a necessary consequence of his theory. In a letter to Bentham he says--"I dispute whether a new race or species is necessarily or even generally descended from a single or pair of parents. The whole body of individuals, I believe, became altered together--like our race-horses, and like all domestic breeds which are changed through unconscious selection by man." The problem is, therefore, an open one, and can only be solved (or rather attacked, for in the present state of our knowledge a complete solution is probably impossible) by a careful induction from ascertained facts, ascending step by step from the present to the past, from the known to the unknown.