Popular Science Monthly 



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 repre- 

 sented 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 mus- 

 cled, 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 ac- 

 complish flight, the in- 

 terest centering in birds, 

 on account of their con- 

 siderable weight, rather 

 than in bats and insects 



The ancestor-bird, faithfully re- 

 produced 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 fa- 

 cilitate the comparison, and the re- 

 semblance 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 con- 

 structive difficulty has been over- 

 come 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 



As the wings in- 

 creased in power 

 the rear wing 

 decreased in size 



*-' 



Gradually the 

 tail shortened 

 and the feath- 

 ers lengthened 



The fan-shaped 

 tail of the bird 

 of to-day is a 

 kind of a rudder 



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 mus- 

 cles were weak. Its struc- 

 ture 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. Mean- 

 while 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 Tet- 

 rapteryx was built up around a 

 tail-like appendage and was not 

 all feathers under muscular con- 

 trol like that of the modern bird. 

 Nature evidently found it impos- 

 sible to change the bony structure 

 in less than millions of 

 S||y 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 ma- 

 chine 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 mono- 

 plane principle and 

 ' strong enough to pull 

 the machine safely, in 

 most cases, through dis- 

 turbing eddies of the 

 atmosphere. 



This decided change in the 

 machine took the place of all the 

 transformations in bone dimen- 

 sions, balance and muscle 

 strength by which the Tetrap- 

 teryx became a bird after start- 

 ing out in the world with an ana- 

 tomical 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. 



