306 TROPICAL WILD LIFE IN BRITISH GUIANA 



HJya : : : : eifcsa : 



FIG .101. DIAGRAM OF ONTOGENETIC VARIATIONS OF THE WING OF JACANA 



ments of the wing during the development of the chick. The 

 wing of the mature bird is this time used as a constant to 

 which those of immature stages are proportionally raised, so 

 that in every case their wings equal those of the adult in total 

 length. Thus if the embryonic wing equalled the parent in 

 actual size, the relative proportions of its segments would 

 be as indicated in the curve. 



The curve shows the directly opposite growth of the 

 arm and pinion throughout the whole developing period. 

 The forearm remains constant and takes a course midway 

 between the two. That the growth of the arm and pinion 

 should directly change about in the short period of time 

 between the first quarter and the half-grown stages must be 

 fraught with some meaning of which at present we have only 

 gleams of understanding. The lengthening of the pinion or 

 hand takes place during the early days of the chick, when 

 there are still traces of ancient climbing habits and possibly 

 it may be numbered as another disappearing arboreal char- 

 acter. The decrease of the arm may also be placed as a de- 

 clining character, for it finds an exact parallel in the young 

 hoatzin, that living echo of the past. The great decrease of 

 the hand in the half grown fledgling, when the wing begins 

 to prepare for flight, and the corresponding increase of the 

 arm further parallel the hoatzin and make one feel sure 

 that at one time this bird spent much of its time in trees or 

 at least reared its clawed young there. But this was long 

 ages ago, before the wing became a practically useless ap- 

 pendage to the young nestling. 



