The way an animal gets around in the sea and or in the air depends, fundamentally, on the density and viscosity of its milieu. Birds use the low density and viscosity of the atmosphere to fly or glide, with the range of their movement being limited only by their endurance and the height to which they can fly. This is similar to the fishes and similar marine creatures who can also move three-dimensionally within their aqueous milieu though unlike the terrestrial creatures which live on the, essentially, two dimensional surface of the earth. The latter have evolved locomotive mechanisms which are suited to movement dominated by gravity. In the case of humans, for example, a two-legged mode of locomotion suited to this situation has evolved. To a certain extent, birds may seem to be independent of gravity as they fly around in the air. However, unlike marine creatures in their aqueous milieu, if they stop flying they immediately crash to the ground – unless, of course, they can glide for a little while.