As we advance to the colder regions of the earth man becomes a flesh-eater, and his carnivorous appetite increases as the external temperature diminishes. Eventually we reach the coldest zones, and the human being there devours enormous quantities of fat to supply the necessities of his condition. It must necessarily follow, that the inhabitants of the tropics do not produce so much carbonic acid as those who dwell in colder regions. In the first place, their habits of life are different, and they are not under the necessity of maintaining animal heat by the use of artificial combustion, as are the people of colder climes. The vegetation of the regions of the tropics is much more luxuriant than that of the temperate and arctic zones. Hence an additional supply of carbonic acid is required between the torrid zones, and a less quantity is produced by its animals. These cases are all met by the great aërial movements. A current of warmed air, rich in oxygen, moves from the equator towards the poles, whilst the cooler air, charged with the excess of carbonic acid, sets in a constant stream towards the equator. By this means the most perfect equalization of the atmospheric conditions is preserved. The carbonic acid poured out from the thousand mouths of our fiery furnaces,--produced during the laborious toil of the hard-working artizan,--and exhaled from every populous town of this our island home,--is borne away by this our aërial currents to find its place in the pines of the Pacific Islands, the spice-trees of the Eastern Archipelago, and the cinchonas of Southern America. The plants of the valley of the Caucasus, and those which flourish amongst the Himalayas, equally with the less luxuriant vegetation of our temperate climes, are directly dependent upon man and the lower animals for their supply of food.