ABSTRACT
Birds, unlike the earth-bound mammals, have the advantage of living in three dimensions, an advantage that has allowed them to use a great variety of ecological habitats throughout the world, and to evolve no less than 8,600 living species. Among the mammals one-fifth of the species are bats, which enjoy the same advantage and of which there are some 900 kinds; though they are more limited in their distribution by the nature of their food, which is not universally available. When we go upstairs or climb a tree we cannot claim to use the third dimension in the same way – we merely taste it but do not have it. In the sea we can, with the whales, enjoy the third dimension, which is perhaps one of the reasons for the popularity of Scuba diving – our attempts at flight are not comparable, for even a glider pilot is merely a passenger in his machine; perhaps a Scafa, a self-contained aerial flying apparatus, will one day be invented.
