162 Degeneration in the Ostrich 



those of the leg. We have in the ostrich a marked retention of archaic 

 characteristics in certain directions along with a high degree of speciali- 

 zation in others. Structurally the digits of the wing are among the 

 least degenerate of all living birds, while those of the foot are the most 

 reduced. 



Fourth Outer Toe and Claw, 



The small, outer toe of the ostrich is the fourth of the pentadactyle 

 foot, the first,' second and fifth having already disappeared, though 

 Dr Broom has shown there are traces of all five metatarsals in the 

 embryo chick of ten and eleven days' incubation. At about two weeks' 

 incubation the second toe shows as a very small projection, reminiscent 

 of the three-toed ancestor (PI. VI, fig. 5), and in the adult the distal end of 

 its metatarsus remains as a small knob on the tarso-metatarsus, its 

 proximal end being fully developed. Following upon the losses which 

 have already taken place in the foot, the next toe in the sequence to 

 disappear will be the fourth, and in the end the middle third will be 

 the only one remaining, as in the case of the modern horse. Apart 

 from the great diiference in size- between the fourth and third, other 

 facts with regard to the claw and the scales on the upper surface give 

 every reason for thinking that the former is in a retrogressive phase. 

 Though small in size the normal sauropsidan number of phalanges — 

 five — is still retained, the ungual phalanx naturally persisting so long 

 as any vestige of the claw remains. 



The scutellation on the upper surface of the fourth toe is short and 

 continuous, only seven to ten scales being usually present, with little 

 variation among the members of the race (Fig. 7). The scales cover 

 only a small distal part of the digit, while comparison with other birds 

 would lead us to expect they would extend the whole length, in con- 

 tinuity with those along the tarsus. It may be presumed that the 

 factors for the missing scales have wholly disappeared from the germ 

 plasm, for no evidence of them is ever forthcoming, even in the 

 embryo. Along with the retrogressive claw, their absence is held to 

 be so much evidence in support of the degeneration of the toe. As 

 no differences of any moment however have been observed in the 

 number, among either the northern or southern birds, it is probable 

 they are not at present in a degenerative phase, in marked contrast 

 with the claw. Evidence to be presented later, in connection with the 

 scutellation over the third toe, proves that the scales do not drop out 

 singly in an ordinal succession like the coverts and remiges, but in 



