DEVELOPMENT OF WING 353 



another or both of the others change in an opposite direction ; 

 the variation of one eqnals the total variation of the others. 

 C. Throughout the development of the wing in all the 

 birds above discussed, when there is a change of length in one 

 segment at anj?^ particular period of growth, that change is 

 balanced in the same period by an opposite variation of one 

 or both of the other segments. 



THE DEVELOPMENT OF THE PINIOX 



To a great extent, the length of the pinion in many birds 

 is regulated by their habits. It is interesting to note the 

 various changes that take place in this member throughout 

 the period that elapses from the embryonic stages to ma- 

 turity. It is possible to trace in them some of the changes 

 that have taken place through many generations, due pos- 

 sibly to the changes of environment which occurred during 

 the later geological periods. 



Commencing with the hoatzin we have the development 

 of the liand, outlined as follows: 



OpisthocomiiH hoazin (Illiger) 



Carpus Digitus 



Embryo 43. mm. 23.5 mm. 



10 day nestling 34. " 32.3 " 



14 " " 34.2 " 32.1 " 



Adult 39. " 27.5 " 



In the embryo the carpus — containing the metacarpals 

 of the second digit — is very long, being nearly twice as great 

 as the digitus or forefinger. The excess of length is a relic 

 of past ages when birds were not so far removed from their 

 reptilian ancestors as they are at the present day. It is un- 

 doubtedly a remnant of the elongated metacarpals of the 

 lizard. 



In the nestling the great carpus of the embryo decreases 

 in length and the digit increases proportionally. This is at 



