bats it has developed into a sort of webbed wing. But if any of the early birds happened to possess an altered hair-like or scale-like covering—the relic, perhaps, of some common reptilio-mammalian ancestor—which afforded them any extra grip upon the air through which they fell rather than floated, then those individuals would thereby gain an extra chance of catching prey or escaping enemies, and therefore of survival in the constant rivalry of species with species. The more perfect these organs became, the more closely adapted to the function of flight, the greater the advantage the bird would derive from their possession, and therefore the better the chance of survival which it would obtain. Thus, apparently, the most aërial birds have the largest and strongest quills, and the most quill-like plumes, while the running and diving birds have either never developed these adjuncts in their highest form, or else have lost them by disuse.
Let me take down one of the peacock's feathers, which stands on the mantelpiece in this Vallauris vase, and closely examine its structure. It consists of a long central shaft, horny and tubular at the lower end, and filled above with a soft, white, spongy matter; while a number of little barbed branches are given off on either side, curiously interlaced by means of tiny hooked filaments, whose myriad threads are far too numerous for the most industrious critic to count up. Everybody knows that this tubular structure combines in the highest degree the mechanical requisites of lightness and strength; and everybody has read that it is employed with the self-same object by human engineers, in such constructions as the great bridges which span the Menai Straits or the St. Lawrence at Montreal. Evidently this peacock's feather, though now converted to a purely ornamental function, was originally developed for the purpose of flight. If I doubt it for a moment, I need only look at the quill-pen in my desk over yonder. That flat blade, close-textured and strongly woven, clearly belongs to a flying organ; and this beautiful mass of green and golden waving plumelets is evidently modeled on the self-same plan. It is useless, or next to useless, now, for flight; but it still bears clear traces of its original function in the structure and arrangement of its shaft and barbs.
Next, let me look at the little downy feather I have abstracted from the Indian cushion. This is not a flying organ, nor did its representative on any early ancestor ever fulfill a similar office. Light, warm, soft, fluffy, its whole object is decidedly that of clothing against chilly weather, and protection against thorns or other rough bodies. Yet when I examine it closely, I see that the same general ground-plan still runs through it, as that which ran through the goose-quill and the peacock's tail-covert. "How comes this?" I ask myself; "here we have a small, delicate, almost fleshy shaft, instead of the horny quill; and a feeble set of downy barbs instead of the strong, well-woven blade: yet the main features remain unaltered, though the function is entirely different. How can I account for this resemblance?"
