If, on the contrary, these groups can be traced to one common primitive stock, if there is _but one single species_ of man, the differences, sometimes so striking, which separate the groups, constitute a problem similar to that of our animal and vegetable races. Further, man is found in all parts of the globe, and we must account for this dispersion; we must explain how the same species has been able to accommodate itself to such opposite conditions of existence as those to which the inhabitants of the pole and the equator are subject. And lastly, the _simple affinity_ of naturalists is changed into _consanguinity_; and the problems of _filiation_ are added to those of _variation_, _migration_, and _acclimatisation_. It is clear that, independently of every religious, philosophical, or social consideration, this science will differ entirely in character as we consider it from a polygenistic point of view, or according to theories of monogenism. III. If the former of these doctrines claims such a large number of adherents, the reason may for the most part be found in the causes mentioned above. But its seductive simplicity and the facility which it seems to lend to the interpretation of facts also stand for a great deal. Unfortunately these advantages are only apparent. Polygenism conceals or denies difficulties; it does not suppress them. They are suddenly revealed, like submarine rocks, to anyone who tries, however little, to go to the root of the matter. The case is the same with this doctrine as with the _Systems_ of classification formerly employed in botany and zoology which rested upon a small number of arbitrary data. They were undoubtedly very convenient, but possessed the serious fault of being conducive to most erroneous opinions from a destruction of true relations and an imposition of false connections. Monogenism acts in the same manner as the _Natural Method_. The zoologist and the botanist are by this method brought face to face with each problem which is put before them under every aspect. It often displays the insufficiency of actual knowledge, but it is the only means of destroying illusions, and of preventing a belief in false explanations. It is the same with Monogenism. It also brings the anthropologist face to face with reality, forces him to investigate every question, shows him the whole extent of each, and often compels him to confess his inability to solve them. But by this very means it protects him against error, provoking him to fresh investigation, and from time to time rewards him with some great progress which remains an acquisition for ever. I shall return to these considerations, the truth of which will be better understood when the principal general questions of anthropology have been reviewed. Henceforward I shall attempt to justify as briefly as possible the preceding criticisms and eulogies. CHAPTER III. SPECIES AND RACE IN THE NATURAL SCIENCES.