Whether this very remarkable theory of the origin of our moon is the true one, and if so, whether the explanation it seems to afford of the great oceanic basins is correct, I am not mathematician enough to judge. The tidal theory of the origin of the moon, as worked out mathematically by Professor G.H. Darwin, has been supported by Sir Robert Ball and accepted by many other astronomers; while the researches of the Rev. Osmond Fisher into the _Physics of the Earth's Crust_, together with his mathematical abilities and his practical work as a geologist, entitle his opinion on the question of the mode of origin of the ocean basins to the highest respect. And, as we have seen, the existence of these vast and deep ocean basins, produced by the agency of a series of events so remarkable as to be quite unique in the solar system, played an important part in rendering the earth fit for the development of the higher forms of animal life, while without them it seems not improbable that the conditions would have been such as to render any varied forms of terrestrial life hardly possible. FOOTNOTES: [16] For a fuller account of this Arctic fauna and flora see the works of Sir C. Lyell, Sir A. Geikie, and other geologists. A full summary of it is also given in the author's _Island Life_. [17] Professor G.H. Darwin states that it is nearly certain that no other satellite nor any of the planets originated in the same way as the moon. CHAPTER XIII THE EARTH IN RELATION TO LIFE: ATMOSPHERIC CONDITIONS We have seen in our tenth chapter that the physical basis of life--protoplasm--consists of the four elements, oxygen, nitrogen, hydrogen, and carbon, and that both plants and animals depend largely upon the free oxygen in the air to carry on their vital processes; while the carbonic acid and ammonia in the atmosphere seem to be absolutely essential to plants. Whether life could have arisen and have been highly developed with an atmosphere composed of different elements from ours it is, of course, impossible to say; but there are certain physical conditions which seem absolutely essential whatever may be the elements which compose it. The first of these essentials is an atmosphere which shall be of such density at the surface of the planet, and of so great a bulk, as to be not too rare to fulfil its various functions at all altitudes where there is a considerable area of land. What determines the total quantity of gaseous matter on the surface of a planet will be, mainly, its mass, together with the average temperature of its surface.