The social condition of mankind is also profoundly affected by climatic and other external circumstances. The intense cold of the Arctic and Antarctic regions is fatal to anything approaching a developed form of civilisation. Intense heat, on the other hand, although not incompatible with a certain degree of progress, is unfavourable to its permanence;[12] the extinct societies of the tropics, such as Cambodia, Mexico and Peru, affording instances of the operation of this law. It is impossible for man to get beyond the nomad state in the vast deserts of Northern Africa; and the extreme moisture of the atmosphere in other portions of the same continent puts an effectual check on anything like social advance. In some parts of the world social development has been hindered by external circumstances of another character, such as the want of wood, the scarcity of animals, the absence of edible fruits. In fact, it is only within a comparatively temperate zone that human society has been able permanently to assume highly complex forms and to build itself up on an extensive scale. In this zone, climate, while favouring man up to a certain point, has at the same time compelled him to eat bread in the sweat of his brow. It has compelled him to enter into conflict with natural obstacles, the result of which has been to call forth his powers of industry, of energy, of self-reliance, and to sharpen his intellectual faculties generally. In addition to exercising and strengthening these personal attributes, the climatic influences of what has been called the zone of civilisation have brought man's social characteristics more fully and elaborately into play. The nature of these influences has forced him to cooperate more or less closely with his fellows; while each step in the path of cooperation has involved him in another of a more complex kind. The growth of social cooperation is not necessarily accompanied by a corresponding development of the moral sentiments; increased cooperation in some cases involving a distinct ethical loss. In many directions, however, highly organised societies tend to evolve loftier types of morality; and it is in harmony with the facts to say that the highest moral types are not to be found where nature does most or where it does least in the way of providing food and shelter for man. [12] Ratzel. _Voelkerkunde_, i. 20.