[Illustration: EARLY ICE AGE, WHEN MAMMOTHS ROAMED THE EARTH AND MAN WAS ARISING] The very accurate adjustments that render our earth suitable for the production and long-continued development of organic life, culminating in man, may be well shown by another consideration. If our earth had been 9,600 miles instead of 8,000 miles in diameter--a very small increase in view of the immense range of planetary magnitudes from Mercury to Jupiter--with a slight proportionate increase in density, due to its greater force of gravitative compression, its mass would have been about double what it is now. This would probably have led to its having attracted and retained double the amount of gases, in which case the water produced would have been double what it is--perhaps even more, because hydrogen gas would not then escape into space as it does now. But the surface of the globe would have been only one-half greater than at present; so that, unless the ocean cavities were twice as deep as they actually are, the whole surface of the earth--except, perhaps, a few tops of submarine volcanoes--would have been covered several miles deep in water, and all terrestrial life would have been impossible. [Sidenote: The Deep Atmosphere of Venus] From the various considerations here set forth it appears clear to me that no other planet of the solar system makes any approach to the conditions essential for the development of a rich and varied organic life such as adorns our earth. One only--Venus--has a sufficient bulk and density to give it the needful atmosphere; but as it receives about twice as much solar heat as does the earth, it is probable that its very deep atmosphere may be mainly due to the fact that a large proportion of its water is held in a state of vapour, its seas and oceans being proportionately reduced in extent. Judging from what happens on the earth, this would probably lead to an excessive area of deserts, and thus be inimical to life. But this planet appears to possess one feature which renders it fundamentally unsuitable for organic life. [Sidenote: Why there is no Life on Venus]