In general the physical and mental form of conscious beings is an expression of the character of the planet on which they live. On certain very large and aqueous planets, for instance, we found that civilization had been achieved by marine organisms. On these huge globes no land-dwellers as large as a man could possibly thrive, for gravitation would have nailed them to the ground. But in the water there was no such limitation to bulk. One peculiarity of these big worlds was that, owing to the crushing action of gravitation, there were seldom any great elevations and depressions in their surface. Thus they were usually covered by a shallow ocean, broken here and there by archipelagos of small, low islands.