If we select another group, we find that the birds present a sharply defined class with very definite characters. The definition of the group runs as follows: “Birds are characterized by the presence of feathers, their fore-limbs are used for flight, the breast-bone is large and serves for the attachment of the muscles that move the wings; outgrowths from the lungs extend throughout the body and even into the bones and serve as air sacs which make the body more buoyant. Only one aortic arch is present, the right, and the right ovary and oviduct are not developed. The eyes are large and well developed. Teeth are absent. We have here a series of strongly marked characteristics such as distinguish hardly any other class. Moreover, the organization of existing birds is, in its essential features, singularly uniform; the entire class presenting less diversity of structure than many orders of Fishes, Amphibians, and Reptiles.”[1] The feathers are the most unique features of birds, and are not found in any other group of the animal kingdom; moreover the plan on which they are formed is essentially the same throughout the group, yet in no two species are the feathers identical, but differ not only in form and proportions, but even in the character of the barbs and hooks for holding the vane together. The modification of the fore-limbs for flight is another characteristic feature; yet in some birds, as the ostrich and kiwi, although the wing has the same general plan as in other birds, it is not used for flight. In the latter it is so small that it does not project beyond the feathers, and in some birds, as in the penguins, the wings are used only as organs for swimming. Footnote 1: Parker and Haswell: “Text Book of Zoology.” In spite of these differences we have no difficulty in recognizing throughout the group of birds a similarity of plan or structure, modified though it be in a thousand different ways.