The wings of birds, when they are developed so as to minister to flight, are all made upon one plan; but as examples of the variation which the organs contributing to make the fore limb manifest, I may instance the short swimming limb of the Penguin, the practically useless rudiment of a wing found in the Ostrich or Kiwi, and the fully developed wing of the Pigeon. The wings of birds obtain an extensive surface to support the animal by muscular movements of three modifications of structure. First, the bones of the fore limb are so shaped that they cannot, in existing birds, be applied to the ground for support and be used like the limbs of quadrupeds, and are therefore folded up at the sides of the body, and carried in an unused or useless state so long as the animal hops on the ground or walks, balancing its weight on the hind legs. Secondly, there are two small folds of skin, less conspicuous than those on the arms of Geckos; one is between the wrist bones and the shoulder, and the smaller hinder membrane is between the upper arm and the body. These membranous expansions are insignificant, and would in themselves be inadequate to support the body or materially assist its movements. Thirdly, the bird develops appendages to the skin which are familiarly known as feathers, and the large feathers which make the wing are attached to the skin covering the lower arm bone named the ulna, and the other bones which represent the wrist and hand. The area and form of the bird's wing are due to individual appendages to the skin, which are unknown in any other group of animals. Between the extended wing of the Albatross, measuring eleven feet in spread, and the condition in the Kiwi of New Zealand, in which the wing is vanishing, there is every possible variation in size and form. As a rule, the larger the animal the smaller is the wing area. The problem of the origin of the bird's wing is not to be explained by study of existing animals; for the rowing organ of the Penguin, which in itself would never suggest flight, becomes an organ of flight in other birds by the growth upon it of suitable feathers. Anyone who has seen the birds named Divers feeding under water, swimming rapidly with their wings, might never suspect that they were also organs of aerial flight. The Ostrich is even more interesting, for it has not developed flight, and still retains at the extremities of two of the digits the slender claws of a limb which was originally no wing at all, but the support of a four-footed animal (Fig. 46, p. 130). FLYING MAMMALS