358 TROPICAL WILD LIFE IN BRITISH GUIANA 



of the outer primaries, which comes as a secondary result, 

 makes it necessary to have a lengthened and stronger digitus 

 to stand the new conditions. Until we can investigate the 

 development of a swift or hummingbird it will be impossible 

 to say how truly this parallelism has been carried out in the 

 younger stages of the martin. 



From the development of the few pinions described, it 

 is an easy matter to see that their growth in general follows 

 along certain lines. Whether the pinions of all other flying 

 birds follow the same rule has yet to be ascertained. In all 

 these the carpus has a tendency to grow in an opposite direc- 

 tion to the digitus. In some the variation is greater, but 

 wherever there is any variation at all between the two the 

 tendency is opposite, and usually the decrease of one equals 

 the increase of the other. 



In birds that show traces of clawed fingers in their nest- 

 ling or adult stages, and which are more ancient in type, 

 both the carpus and digitus have a large individual variation 

 in their rates of development. On the other hand altrical 

 birds — birds that are born helpless — have little or no varia- 

 tion of these portions except when the flight is highly 

 specialized. 



