DEVELOPMENT OF WING 347 



seres, conditions appear to be different from the preceding- 

 cases. There seems to be a more or less regular variation 

 between all three segments of the wing. The black-necked 

 toucan {Pteroglossus aracari) presents the most perfect ex- 

 ample of this. 



Pteroglossus aracari (Linn.) 



Arm Forearm Hand 



6 day nestling 45.2 mm. .54.6 mm. 39.5 nun. 



21 "' " 44.3 " 57.5 " 38. 



Adult 44. " 60.5 " 35. " 



Each segment of the wing shows a steady relative varia- 

 tion through all stages of development to maturity. The 

 arm lessens gradually; the forearm lengthens, and the hand 

 decreases. Curiously enough the increase of the forearm, to 

 all intents and purposes, equals the total decrease of the arm 

 and hand. 



The comparative shortness of the hand in the adult is 

 worth commenting upon. If an embryo could be examined 

 it would probably show a very differently proportioned 

 hand than even in the six-day-old nestling. As it is, the 

 length of this member is so much greater and the forearm 

 so much shorter in the six-day-old bird that it is evident that 

 at some former period, when tlie world was younger, the 

 adult had a more evenly balanced wing. 



The present shortness may be due to a steady decreasing 

 need for this member as an agent of flight. The flight of the 

 toucan is comparatively wxak and one of the conmion sounds 

 of the jungle is the heavy whir of their wings as they labor 

 from tree to tree. Perhaps, when climatic conditions were 

 different, they found it necessary to seek further afield than 

 today for their food, which now may be found in almost 

 every tree top. Individually, they appear now to live in one 

 small section the year round and their total wino* exercise 

 consists of a few short flights from one tree to another dur- 

 ing the day. 



