230 METAMORPHOSIS AND 



dactyls also the sternum is produced into a carina 

 along the median line. The reduction of the digits 

 of the wing in birds to three, with the bones firmly 

 united together, would follow from their use in flight 

 and their disuse as digits, and it would seem, from 

 the fact that the flight-feathers must have been 

 always on the posterior edge of the wing, and that 

 the ulna is larger than the radius, that the three 

 digits which have persisted are the 3rd, 4th, and 5th, 

 and not the 1st, 2nd, and 3rd as usually taught. A 

 comparison of the hind-limbs of birds with those 

 of bats and pterodactyls suggests strongly that the 

 patagium flyers have arisen from arboreal or climb- 

 ing animals, while the birds arose from terrestrial 

 forms which acquired the bipedal habit, as certain 

 reptiles have. An arboreal animal would necessarily 

 use all four limbs, as climbing animals actually 

 do. The wings of birds, on the other hand, 

 would have arisen from the endeavour to increase 

 speed by movements of the fore-limbs. The 

 perching birds would therefore have arisen by 

 later adaptations after the power of flight had been 

 evolved. 



Complete recapitulation does not occur in the 

 development of the digits of the wing. Only a 

 rudiment of a fourth digit has been found in the 

 embryonic wing, not, as might be expected, rudiments 

 of five digits of which two disappear. The meta- 

 carpals are free, not united as in the adult, and there 

 are separate distal carpals, which in the adult are 

 united with the metacarpals. In other respects the 

 modifications of wings and sternum are so obviously 

 adaptive that it is difficult to beheve that the 

 reduction of digits was not due to disuse. This is 



