For a species of one of the inferior animals to have attained the level of man, it was necessary that, following an universal law, it must have undergone variations both corporally and mentally, during a long succession of generations; the primary causes of these variations is not clearly understood, but it has been proved that the conditions of life or environment to which the living beings submitted were potent agents in the renewal of phenomena. Like all other creatures man increased out of all proportion to his means of subsistence, and thus began the struggle for existence, when those who were best equipped for the fight survived in the greatest numbers, and left the greatest number of robust descendants. Man acquired the capability of expressing his wants by means of language, at first, perhaps, little different from that of the inferior animals, but the continued use of language reacting on the brain furnished a means for the further development of those mental faculties which of themselves constitute a real distinction between man and beast. This difference, however, does not become pronounced until a certain period of man’s existence, as during the earliest stages the intelligence of the newly created human beings does not differ from that of other mammifers. It begins to dawn a little later, then gradually increases, and at last becomes most strongly marked, even if a comparison be made between the intelligence of a highly developed monkey and that of the lowest savage, who has failed perhaps to find words with which to express the most elementary emotions. But men are not all on the same level; without speaking of the vast difference that exists between the faculties of a Papuan and those which we know to have been possessed by a Newton or a Kant, we notice a very sensible difference between the mental powers of two individuals of the same race; but we always find these extremes are connected by shades of difference which gradually melt imperceptibly the one into the other. Darwin arrives at the conclusion that the distinctions to be drawn between the intellect of man and the intelligence of animals is one of degree rather than of kind.