354 TROPICAL WILD LIFE IN BRITISH GUIANA 



FIG. 118. DIAGRAM OF HAND DEVELOPMENT OF THE HOATZIN 



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

 develo])nient. At maturity the conditions again change; the 

 carpus elongates and the digitus shortens, l)ut the difference 

 is not so great as it was in the embryo. 



^luch the same condition exists in the young chick 

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



Digitus 

 22.8 mm. 

 23..5 " 



-as far as as- 



Psophia crepitans (T.inn.) 

 Carpus 



6 day chick 37. .5 mm. 



Adult 27. 



A better maintained balance in the hand- 

 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 o])])()site. This is especially true in nestlings, while 

 in the embryo the variations are as a rule greater and 0])po- 

 site. Sucli is the case with l)irds like the kiskadee, yellow- 



