Page:Evolution of Life (Henry Cadwalader Chapman, 1873).djvu/188

138 That which, under the ordinary laws of vegetation, would have developed as a leafy branch, here develops as a flower; its several organs appearing under forms some of them slightly and others extremely different in aspect and in office from the foliage. But they all have a common nature and a common origin, or, in other words, are homologous parts. When, therefore, the floral organs are called modified or metamorphosed leaves, it is not to be supposed that a petal has ever actually been a green leaf, and has subsequently assumed a more delicate texture and hue, or that stamens and pistils have previously existed in the state of foliage, but only that what is fundamentally one and the same organ develops, in the progressive evolution of the plant, under each or any of these various forms." The visceral arches of the Vertebrata are among the many illustrations of this idea offered by the animal kingdom. The visceral arches (Figs. 178 to 181, c) consist of thickenings or papillai situated behind the primitive eye, and below the primitive ear. They are present in the early stages of all Vertebrata, and are much modified in the course of development. The branchial arches supporting the gills in Fishes represent best their primitive condition, while in remaining Vertebrata they are used partly in the formation of the lower jaws, partly in the formation of the organs of hearing.

The subject of Embryology is as intimately related to Geology as to that of Anatomy, for the changes through which plants and animals pass in the course of development are essentially the same changes through which life in general has passed from its first appearance to the present time, for not only are the transitory stages of the higher animals permanently represented by the lower, but they are also permanently represented by the fossils. In other words, the development of the most complex plant, and of the most highly-organized animal, is an epitomized