We must return to the subdivision proposed by Charles Bonaparte,--a _family_. Man constitutes a simple family in the Order of _Quadrumana_, distinguished by characteristics precisely equal in importance to those which make a difference between other similar groups in the class of mammalia, that which even comes to the assistance of the adversaries of the human kingdom, and the partisans of the zoological system. For want of positive characteristics taken from the extremities, which could never,--in the eyes of true naturalists, as we have just said,--favour a serious distinction between man and other quadrumana, a characteristic in dentition has been discovered, remarkable for its constancy even in the most degraded and animal-like races, and which, first and foremost, distinguishes man from the group which immediately follows him in the zoological series. This characteristic, upon which Professor Owen has, in many places, insisted,--like the two Cuviers,--but with an entirely new vigour,[106] is the contiguity of the teeth and the _continuity_ of their crowns, not one of which ever extends beyond the level of the others.[107] Thus it is for man, like the rest of the mammalia; it is the dental system which gives us the best characteristic. A new proof that the study of mankind and that of animals ought to be conducted in one and the same manner; a proof, indeed, that these two studies are two parallel branches, intimately united, of one and the same science. CHAPTER IV. ANATOMICAL, PHYSIOLOGICAL, AND PATHOLOGICAL VARIETIES. We have endeavoured to prove in the preceding pages the specific unity of the biological phenomena in each Order, which are to be found among the superior animals and man. This unity has led us necessarily to another, that of _method_; and we have just seen that man forms simply a family, that is to say, a very secondary division in the zoological series. But we have only taken the first step of the path in which we have to travel. The genus _homo_ shows many varieties, and many dissimilarities. We must try to estimate their value, and to find out what the divisions to be established between what we commonly call _races_ of men may be worth. Now, the only rule to be followed here is naturally that which is applied by all zoologists to the other individuals composing the animal series. The only way to arrive at such a goal will be, first of all, the study of the physical differences, the necessary basis of a rational classification. Thus we shall, at least, have important, and what is more, comparable results.