Without troubling ourselves with enquiring into the whole system of the genus _homo_, we must at first examine these well-characterised centres of population which are entirely distinct as regards aspect and physiognomy. We must mark these centres with care, paying attention to all the physical, moral, and philological varieties which we are able to notice. M. Flourens has given some excellent principles for the study of animal species; he wishes simply to apply them to the study of human species; and from this connection, which nobody can contradict as a means of investigation, there arises a farther proof of the rank which we must give to man in the organic series. “We must observe the living animal,” said M. Flourens; “we must observe him for a long time, and also both sexes and all ages. We must study his nature, his instincts, and his intellect. Each of these things has its own characteristic in each animal, and it is by the whole of these characteristics put together that species is defined.” It is impossible to trace in a better manner the anthropologists’ task. When we have well studied a homogeneous centre of population under all its aspects, when we have rendered an account of its physiological, psychological, and philological characteristics, we may stop; and without prejudging anything concerning the area of this race, may then pass on to another centre, which we shall notice in the same way, without troubling ourselves with intermediary varieties, which will always be in a greater or lesser number wherever we do not happen to meet with a physical barrier, like the sea or a chain of mountains, which may separate the two centres which are to be observed. Then we shall, doubtless, have numberless shades and transitions; but these are merely the phenomena of hybridity, entirely secondary, and which ought not at all to influence our essays on anthropological classification. At a later period, when we know more, we shall be able to review all these intermediary varieties, when we understand their conditions of existence better. In this manner we must take care at the beginning to study certain countries, places of travel, and meeting, to which all the neighbouring races have given some portion of their blood. Such are most European countries, and such always was the Valley of the Nile and the Blue Nile. The streets of Cairo are not only a picturesque spectacle; from thence did Étienne Geoffroy borrow his grand views about the position of the genus _homo_ in nature; the man of science profits here as much in his search after truth as the artist in his search after the beautiful.