DEVELOPMENT OF JACANA 299 



period, of much greater size than it is today. Thus we can 

 imagine an ancestral bird with clawed thumb and finger that 

 nested in trees ; probably with a stronger flight, and certainly 

 with a better balanced and longer tail than at present. 



DIASTATAXY 



A crowding and reduction of the fourth secondary oc- 

 curs in the young chick, which in some way may be due to 

 the diastataxy of the wing. There are ten secondaries and 

 eleven coverts in the embryo, the extra covert being placed 

 between the fourth and fifth. It is small and raised slightly 

 out of line from the others, there also being a slight shifting 

 out of place of the one above. Here, however, the shifting 

 appears to cease and the coverts above remain in their regu- 

 lar positions. 



At hatching time or a little later, the extra covert falls 

 directly into line and now regularly becomes the fifth, while 

 the original fifth becomes the sixth, and so on. There is, 

 however, no fifth secondary to which it may become a covert. 

 All the down secondaries are in line. (Fig. 98.) 



Now comes a curious phase in the growth of the secon- 

 daries themselves. As they commence to grow rapidly, the 

 fourth is left far behind as a mere little bud, crowded and 

 pushed up out of line as was once the extra covert. After 

 a period it manages to regain the line and, at first very slowly, 

 to lengthen. Later, however, when the secondaries are near- 

 ly grown it more than recovers the strength it once lost and, 

 pushing quickly ahead, overtakes the rest before they are 

 fully matured. (Fig. 98.) 



It is hard to account for this condition, though the dias- 

 tataxy of the wing may have something to do with it. It 

 is possible that some particular stress exerted by the move- 

 ment of the changing coverts, may have caused it to be drawn 

 up, though why the down sheath should be in line and not 

 the main sheath, can be answered only by a more thorough 



