158 Degeneration in the Ostrich 



time, and to afford details as to the manner in which the process takes 

 place. In the embryo, however, many earlier stages are represented 

 which indicate just as truly the manner of degeneration, even though 

 the process is confined to the egg. 



In all modern birds the third digit ^ is greatly reduced in size 

 compared with the second and first, and is buried in the muscles of 

 the wing. The first is also small, but is free and bears its own feathers, 

 representing the ala spuria or bastard wing, while the second is axial 

 and constitutes by far the greater part of the fore-wing. In the oldest 

 known fossil bird, Archaeopteryx, all three digits were free and clawed, 

 and perhaps bore feathers. In the adult ostrich the first and second 

 are always free and clawed, while the third is unclawed and usually 

 hidden; sometimes however it projects quite freely, and appears as a 

 distinct third finger (PI. V, fig. 2). 



In the embryo the third digit is clearly seen at about the tenth day 

 of incubation, and is then of equal size with the second ; but from this 

 stage onwards the latter greatly outstrips the former (PI. VI, fig. 5), and 

 within a few days of hatching the third shows only as a small, triangular 

 projection from the post-axial border of the wing. After hatching it 

 generally disappears and no trace is seen at the surface, but in some 

 instances it persists, and may be altogether free from the rest of the 

 wing — an approach to the ancestral free third digit (PI. V, fig. 2). All 

 intermediate stages between the extremes occur. Some writers assert 

 that a claw appears on the third digit (Parker and Haswell, Text-Book of 

 Zoology, 1910, Vol. xi. p. 421), but this has never been found in the 

 hundreds of birds coming under the writer's observation, although 

 looked for specially. As the ungual phalanx, which on the reptilian 

 phalangeal sequence would be the fourth, has wholly disappeared the 

 occurrence of the claw becomes most improbable. Judging from 

 the retrogressive stages in the ostrich, the order of disappearance 

 of the constituent parts of clawed digits is as follows : the claw first, 



1 In this and what follows the more usual method is adopted of regarding the bastard 

 wing of birds as representing the first digit of the pentadactyle fore-limb and the other 

 two as the second and third, following in this the views of Gegenbaur, Parker and others, 

 as contrasted with those of Owen and Leighton, who consider the bird's digits as the 

 second, third and fourth of the pentadactyle series. The conditions in the ostrich would 

 appear to be conclusive of the former view, for the first digit bears two phalanges, and as 

 the distal one is an ungual phalanx, no phalangeal reduction can have taken place. The 

 second digit has the normal succession of three phalanges and the last bears the claw. 

 Any loss of phalanges is from the distal to the proximal, and if a distal phalanx were lost 

 the claw would disappear with it, as a more proximal phalanx would not become the 

 ungual. 



