li^H 



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 betAveen the hand 

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

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

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

 second, opposite conditions o])tain ; the hand of the nest- 

 liuii' always is shorter ' than the adult and the forearm longer. 



Galeoscoptes carolinciisis (Linn.) 



Arm Forearm Hand 



Embryo 2f)..5 mm. 23.8 mm. 20. mm. 



Newly hatched 2.5. '' 20.7 " 23.,5 " 



3 day old nestling 23.8 " 20.3 " 26.2 " 



(} " " " 22. " 22. " 2.5.8 " 



Adult 20.2 " 26.6 " 23. 



A ty])ical example of the first method may be found in 

 the catl)ird of North America {Galeoscoptes Caroline nsis) . 

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

 commences in the newly-hatched nestling. Tlie forearm 

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



' 'Ilic words "slioi-t" and "long" in tlir sense liere used do not mean that the 

 hand grows shorter or longer in actual Itngtli, l)ut in proj)()rtional length only. 



