354 TROPICAL WILD LIFE IN BRITISH GUIANA 



FIG. 118. DIAGRAM OF HAND DEVELOPMENT OF THE HOATZIN 



the period when the claws of the hoatzin are at their greatest 

 (levelopnient. At maturity the eoiulitioiis again change; the 

 carpus elongates and the digitus shortens, but the difference 

 is not so great as it w as in the embryo. 



Much the same condition exists in the young chick 

 trumpeter. The carpus, in the six-day chick, is almost twice 

 the digitus in length, but at matui-ity only exceeds it by a 

 few millimeters. This increase of the digitus, however, does 

 not, as in the hoatzin, equal the decrease of the carpus, though 

 the delayed growth of one and the slight elongation of the 

 other is significant. 



Psojjhia crepitans (Linn.) 



Carpus Digitus 



r> day chick 37.o mm. 22.8 mm. 



Aduit 27. " 23.5 " 



A better maintained balance in the hand^as far as as- 

 certained — is found to a more or less extent in passerine birds 

 and others that do not have better than an average flight. 

 The variation of each portion in the individual is diamet- 

 rically opposite. This is especially true in nestlings, while 

 in the embryo the variations are as a rule greater and oppo- 

 site. Such is the case with birds like the kiskadee, yellow- 



