Volant Adaptation in Vertebrates

Abstract

1. Volant evolution has occurred seventeen times among vertebrates, ten of which are merely adaptations for more or less prolonged soaring leaps, while in seven instances in all probability true flight has been developed. 2. Soaring implies, with but one exception, the development of a fold of skin along the creature's flanks supported in one instance by the extension of the ribs beyond the body wall, but generally stretched between the fore and hind limbs. This fold is often supplemented by others in front of the fore limbs and between the hind limbs sometimes involving the tail. 3. True flight always implies a more or less profound modification of the fore limbs which become, as a consequence, unsuited to ordinary progression. True flight has been developed once in each of the classes of strictly air-breathing vertebrates, and probably at least four times among fishes. 4. With the exception of the fishes, soaring implies also present or ancestral arboreal adaptation and this may apply as well to the true fliers. It is certainly true of the bats, possibly true of the birds, but of the pterodactyls one cannot be certain. 5. Besides the primary modifications which constitute the machinery of flight, other portions of the body, especially the nervous system, the sense and the nutritive organs, may exhibit secondary volant characteristics. These, as with the primary modifications, are in direct proportion to the powers of flight.