A Bird with Four Wings
When Nature decided to evolve a bird out of a reptile she molded a four-winged flyer curiously like the first flying machine
BIRDS came later than fishes and reptiles in the evolution of life. But what manner of creature was it that linked fish with bird? What was the first bird that ever flew? Fossil remains and imprints have so far given only scant information as to how the feathered descendants of the fish or the reptile gradually came into possession of the power of flight. Intermediate links in the development have left but few and faint traces, and this is due, at least in part, to the extreme remoteness of the transition period during which birds became birds from whatever they were before. The change was so radical that it required millions of years by Nature’s slow methods.
Fortunately a single natural document has come down through the ages which goes far toward explaining the mechanics that made the elongated swimming fish or crawling reptile fit for sustaining, propelling and balancing itself in the air, and this document, as it happens, also establishes a most curious and interesting parallel between Nature’s experiments in flying and those fundamental experiments by Professor Langley of the Smithsonian Institution in Washington to which the inauguration of the airplane era in flight is due, more than to any other one cause. The document referred to is a true document; for it was found printed by natural forces on the rocks of the Solinghofen quarry in Germany. Here, in a formation not less than seven million years old, one of the ancestors of the
modern bird, a strange feathered reptile, had been imprisoned, caught unawares perhaps in one of the every-day upheavals of that formative age when mountain ranges, continents and oceans were still in the making. Its skeleton, its outlines and its feathers are here preserved, stamped in unmistakable distinctness in stone, which is now hard but which must have been plastic as clay when the fluttering creature was seized in deadly embrace. More than twice as old as any of the prehistoric monsters reconstructed from their bones in our museums of natural history, the fossil imprint of the Tetrapteryx, as this creature has been named (meaning “four-winger”), represents an indisputable and descriptive record of perhaps the earliest feathered flyer.
The Tetrapteryx record was discovered fifty-five years ago, but science has only recently undertaken to interpret it mechanically. William C. Beebe, while curator of birds at the New York Zoological Park, demonstrated that several species of modern birds, and especially the white-winged dove, show very marked traces of just such wings on the legs, called pelvic wings, as the Tetrapteryx record reveals. On the very young dove, at the time when its body is still bare but for the sprouting flight feathers of wings and tail, twelve flight feathers and six coverts begin to grow from the outer and upper edge of the leg, extending in two rows from the knee almost to the base of the tail. While the growth of these tell-tale feathers is soon arrested and is covered up
in the surrounding plumage, so that the grown bird shows only traces, the fact that the young of the species pass rapidly through the same evolution that is represented in the succession of innumerable generations of their ancestry, almost clinches the conclusion that birds are descended from a type equipped with wings on all four limbs, as the Tetrapteryx, and that Nature has learned gradually to replace four small and imperfect wings, weakly muscled, by two larger and stronger wings under perfect control.
Frederic A. Lucas, Director of the American Museum of Natural History, called attention last year in the American Museum Journal to the great force of the evidence which has thus been collected to prove how Nature learned to accomplish flight, the interest centering in birds, on account of their considerable weight, rather than in bats and insects.
The ancestor-bird, faithfully reproduced from the record, and the ancestor-airplane are presented in illustration herewith, side by side.
The dimensions of the bird have been relatively exaggerated to facilitate the comparison, and the resemblance in structure is striking.
Langley’s "aerodrome” repeatedly flew over the Potomac in 1895, sustaining its own weight in the air for more than one minute at a time by the action of its two pairs of planes or wings and two rotary propellers, of five to six feetdiameter, driven from a diminutive steam engine developing one to one and one-half horsepower. The necessity for placing the power equipment and the propellers amidships called for an elongated body for the machine as a whole, so that the weight might be evenly supported by planes at the rear as well as in front. This constructive difficulty has been overcome in modern airplanes, but it was decisive for Langley’s machine, with its small power, in the same degree and almost for the same reasons as for the
original, four-winged bird. The latter came from a race whose fore and hind limbs were spaced well apart, whose legs were relatively heavy and whose arm muscles were weak. Its structure had to be modified by hereditary influences before it would balance at all in the air, hung from the arm sockets alone, as birds do. Meanwhile Nature did practically as Langley did. She adopted the compromise solution of upholding the rear weight by large feathery extensions from the legs and tail, and it may be noticed that the fantastic feather tail of the Tetrapteryx was built up around a tail-like appendage and was not all feathers under muscular control like that of the modern bird. Nature evidently found it impossible to change the bony structure in less than millions of years, working from the basis of a reptile with only growth and heredity as the tools at command, but she could make feathers grow in the place of horny scales, which are made of almost the same material, by a comparatively brief evolution.
To transform Langley’s machine into the modern airplane was a task much simplified by the advent of the compact and powerful gasoline engine, small enough to be moved forward in line with the support from planes arranged on the biplane or monoplane principle and strong enough to pull the machine safely, in most cases, through disturbing eddies of the atmosphere.
This decided change in the machine took the place of all the transformations in bone dimensions, balance and muscle strength by which the Tetrapteryx became a bird after starting out in the world with an anatomical construction very much like an animated parachute or gliding machine. But the mechanical flyer is still an infant compared with Nature’s eon-old product.