§ 294. Dermal structures of another class are developed mainly, if not wholly, by the actions of external causes on species rather than on individuals. These are the various kinds of clothing--hairs, feathers, quills, scales, scutes. Though it is no longer thought as at one time that all these various tegumentary structures are homologous with one another, yet it is unquestionable that sundry of the more conspicuous ones are. Those which are extremely unlike may be seen linked together by a long series of graduated forms. A retrograde metamorphosis from feathers to appendages that are almost scale-like, is well seen in the coat of the Penguin. There is manifest a transition from the bird-like covering to the fish-like covering--a transition so gradual that no place can be found where an appreciable break occurs; and if the scale-like appendages are not truly scales yet they exemplify an extreme metamorphosis. Less striking, perhaps, but scarcely less significant, are the modifications through which we pass from feathers to hairs, on the surfaces of the Ostrich and the Cassowary. The skin of the Porcupine shows us hairs and quills united by a series of intermediate structures, differing from one another inappreciably. Even more remarkable are certain other alliances of dermal structures. “It may be taken as certain, I think,” says Prof. Huxley, “that the scales, plates, and spines of all fishes are homologous organs; nor as less so that the tegumentary spines of the Plagiostomes are homologous with their teeth, and thence with the teeth of all vertebrata.” Further details concerning these tegumentary structures are not needful for present purposes, and are indeed but indirectly relevant to the subject of physiological development. Here they are of interest to us only by involving the general question--What physical influences have brought them into existence? Still with a view to definite presentation of the problem, it will be well to contemplate the mode of development common to the most familiar of them.