Now plants and animals have been studied for a much longer period than man, and from an exclusively scientific point of view, without any trace of the prejudice and party feeling which often interferes with the study of man. Without having penetrated very deeply into all the secrets of vegetable and animal life, science has acquired a certain number of fixed and indisputable results which constitute a foundation of positive knowledge, and a safe starting point. It is there that the anthropologist must seek the _known quantities_ of which he may stand in need. Whenever there is any doubt as to the nature or signification of a phenomenon observed in man, the corresponding phenomena must be examined in animals, and even in plants; they must be compared with what takes place in ourselves, and the results of this comparison accepted as they are exhibited. What is recognised as being true for other organised beings cannot but be true for man. This method is incontestably scientific. It is similar to that of modern physiologists, who, since they are unable to experiment upon man, experiment upon animals, and form their conclusions upon the former from the latter. But the physiologist devotes his attention to the _individual_ only, and, therefore, examines little more than those groups which in their organisation approach most nearly to the being whose history he wishes to explain. The anthropologist on the contrary studies the _species_. The questions with which he has to deal are much more general, so he is forced to direct his attention to plants as well as to animals. This method is accompanied by its criterion; it allows the control of the various, answers which are often made to one question. The means of estimation are simple and easily applied. In anthropology, every solution to be sound, that is to say, true, should refer man in everything which is not exclusively human to the general recognised laws for other organised and living beings. Every solution which makes or tends to make man an exception, by representing him as free from those laws which govern other organised and living beings, is unsound and false. Again, when we reason and form our conclusions in this manner, we remain faithful to the mathematical method. To be received as true, a solution of a given problem must agree with admitted axioms, with truths previously proved. Every hypothesis which leads to results at variance with these axioms or these truths, is, on that account alone, declared false. In anthropology, the axiom or the truth which serves as a criterion is the fundamental, physical, and physiological identity of man with other living beings, with animals, with mammalia. All hypotheses at variance with this truth should be rejected.