No one is less inclined than I am to place any limit upon the extension of human knowledge. Yet the extension of our scientific knowledge, in the widest sense of the term, is always subordinate to certain conditions. The most attentive examination, even of a human work, will never teach us anything of the _processes_ which have permitted its realisation. The cleverest watchmaker, if he has not followed studies perfectly foreign to his vocation, will know nothing of the origin of iron, of its transformation into steel, of the rolling and tempering of a main spring. The minutest study of that metallic ribbon which he knows so well, will tell him nothing of its origin, nothing of the process of its fabrication. To know more he must leave his shop and visit the furnaces, the forges, and the rolling mills. In the works of nature it is the same. With nature as well as with ourselves, the phenomena which _produce_ are very different from those which _preserve_, and from those displayed in the _object produced_. The most complete anatomical and physiological study of an animal or of a full-grown plant will certainly teach us nothing about the metamorphoses of the microscopic cell from which sprang the dog, the elephant, and man himself. Now hitherto we have only directed our attention to species already formed. We can therefore learn nothing more relative to their mode of production. But we know that the _unknown cause_ which has given birth to extinct and living species has been manifested at different times and intermittingly upon the surface of the globe. Nothing authorises us to suppose that it is exhausted. Although it appears to have generally acted at times which correspond to great geological movements, it is not impossible that it may be at work on some point of the earth even at this epoch of relatively profound rest. If this is the case, perhaps some happy chance will throw a little light upon the great mystery of organic origins. But until experience and observation have taught us something, all who wish to remain faithful to true science, will accept the existence and succession of species as a primordial fact. He will apply to all what Darwin applies to his single _prototype_; and, in order to explain what is still inexplicable, he will not sacrifice to hypotheses, however ingenious they may be, the exact and positive knowledge which has been won by nearly two centuries of work. CHAPTER XI. ORIGIN OF THE HUMAN SPECIES.—DIFFERENT HYPOTHESES. I. The preceding chapter might enable me to dispense with a discussion of the applications which have been made of Darwinism to the history of man. Nevertheless, apart from the curious points in the subject itself, some discussion of it will be necessary, for it will not be devoid of instruction.