sea-water, who were distressingly affected by conditions to which they had never been subjected. What would happen? The weaker would probably be killed by the change in the conditions, just as some fresh-water fishes and animalculæ would be killed now if plunged into sea-water. The stronger would, however, become acclimatised, and would in the course of successive generations struggle to adapt their bodies to the new conditions. Thus the living organisms in the earth's early sea contrived to cut themselves off from being bathed with the novel carbonaceous water. They cut themselves off from it in the course of generations by closing themselves off from it with skin or membrane. Many of them stirred up their cells to secrete lime and exude it so as to form for themselves a more or less impervious covering or shell. Finally, as they grew to like the mineral water less, they continually made fresh experiments to avoid it, and the more enterprising and adventurous got out of the ocean altogether, and migrated to the air or the land, perhaps by way of the shore sands and muds. This period, when the ocean seems to have passed its best stage for supporting all forms of life, appears to have been that which is geologically known as the Cambrian. After this period there was a wealth of ferns, of animals able to leave hard traces of themselves in the fossil records. Before this period there was no physiological need for either skin or shell. But once the skin and shell had been developed, primarily as a defence against the sea-water, their great advantages for purposes of the struggle for life among all forms of animals soon made themselves felt, and so they were retained. CHAPTER XI LIFE IN OTHER WORLDS If life was not brought to the earth from another planet, then life was created, or originated, on the earth. Some of the conditions which attended its birth have been considered, and they amount to this: that the temperature of the earth, the elements of the earth, the conditions of the earth's surface, oceans, and atmosphere were exactly those which favoured the origination, the continuance, and the development of living things. The earth, among all the heavenly bodies which we can examine at all closely, is probably the only one on which life, as we know it, would have much chance of survival.