49:2.16

It is quite impossible for you to envisage the environment which prevails during the early ages of some worlds. These unusual conditions make it necessary for the evolving animal life to remain in its marine nursery habitat for longer periods than on those planets which very early provide a hospitable land-and-atmosphere environment. Conversely, on some worlds of the superbreathers, when the planet is not too large, it is sometimes expedient to provide for a mortal type which can readily negotiate atmospheric passage. These air navigators sometimes intervene between the water and land groups, and they always live in a measure upon the ground, eventually evolving into land dwellers. But on some worlds, for ages they continue to fly even after they have become land-type beings.