348 TROPICAL WILD LIFE IN BRITISH GUIANA 



FIG. 116. DIAGRAM OF WING DEVELOPMENT OF THE BLACK-NECKED TOUCAN 



The few passerine birds examined show two main meth- 

 ods of variation during growth, especially between the hand 

 and the forearm. In one, the hand first increases in propor- 

 tional length and then shortens to adult size, being offset in 

 the opposite direction to some extent by the forearm. In the 

 second, opposite conditions obtain; the hand of the nest- 

 ling always is shorter 1 than the adult and the forearm longer. 



Galeoscoptes carolinensis (Linn.) 



Arm Forearm Hand 



Embryo 26.5 mm. 23.8 mm. 20. mm. 



Newly hatched 25. " 20.7 " 23.5 " 



3 day old nestling 23.8 " 20.3 " 26.2 " 



6 " " 22. " 22. " 25.8 " 



Adult 20.2 " 26.6 " 23. 



A typical example of the first method may be found in 

 the catbird of North America (Galeoscoptes carolinensis). 

 There is a steady decrease in the length of the arm which 

 commences in the newly-hatched nestling. The forearm 

 increases at about the same rate, while the hand first in- 



1 The words "short" and "long" in the sense here used do not mean that the 

 hand grows shorter or longer in actual length, .but in proportional length only. 



