348 



TROPICAL WILD LIFE IN BIirriSH (lUIANA 



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



The few passerine birds examined sliow two main meth- 

 ods of variation (hn-int^- growth, espeeially between the hand 

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

 tional leng'tli and tlien shortens to a(hdt size, l)eing offset in 

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

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

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



Galeoscoptcs Cdroliiicfisis (I^inn.) 



Arm Forearm Hand 



Embryo 2(1.5 mm. 23.8 mm. 20. mm. 



Newly hatched 2,5. " 20.7 " 23. .5 " 



3 day old nestling 23.8 " 20.3 " 26.2 " 



6 " " " 22. " 22. " 25.8 " 



Adult 20.2 " 26.6 " 23. 



A typical example of the first method may be found in 

 the catl)ird of North America {Galco.scojjfc.s caroJiucusis) . 

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

 commences in the newly-hatched nesth'ng. The forearm 

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



* The words "sliort" Jiiul "loiia" in the sense here used do not mean that tlie 

 hand grows shorter or loniicr in actual length, hut in ])r(>])ortional length only. 



