Towards the end of last century Büchner,[128] a German physician, published a materialistic code of morality that made a considerable sensation. He wrote as follows on the question now before us: “What we term the moral sense arose from the social instincts and habits which, under pain of extinction, are developed in every society of men and animals. Morality depends on sociability, and varies with the peculiar conditions of each particular association. As man is essentially a social animal, and to be regarded, apart from society, merely as a wild beast, it is plain that the needs of the community must impose on him certain restrictions and directions that in time will pass into a settled code of morals.” Half a century later practically the same idea was repeated. Haeckel,[129] the well-known German naturalist, expressed it as follows in a volume that appeared a few years ago:— “Modern science shows that the feeling of duty does not rest on an illusory ‘categorical imperative,’ but on the solid ground of social instinct, as we find it in the case of all the social animals. It regards as the highest aim of all morality the re-establishment of a sound harmony between egoism and altruism, between self-love and the love of one’s neighbour.... If a man desire to have the advantage of living in an organised community he has to consult not only his own fortune but also that of the society and of the ‘neighbours’ who form the society. He must realise that its prosperity is his own prosperity, and that it cannot suffer without his own injury. This fundamental law of society is so simple and so inevitable that one cannot understand how it can be contradicted in theory or in practice; and yet that is done to-day and has been done for thousands of years.” The sexual and family instincts may be satisfied in many different ways, and this is also the case with the social instincts. Onanism and perverted passion may satisfy the sexual instinct; celibacy, artificial abortion and infanticide exist alongside the love of the wife and the parental cares. So also the social instinct of a criminal may be satisfied by his association with other criminals. It is well known that the most hardened criminals have their own codes, and they join faithfulness to their own companions to an atrocious attitude towards the rest of the world.