i)i:\i;i.()i'.Mi: NT oi' wise, nr/.i 



another oi- l)()tli of tlic otlicrs chaiinc in an ()p|)()sitc' direction: 

 the variation of one ecjnals the total variation of the others. 

 C. 'rhi-()n_i>hout the development of the win^' in all the 

 hirds above discussed, when there is a chan«>e of length in one 

 segment at any particular ])eriod of growth, that change is 

 balanced in the same period })y an opposite variation of one 

 or both of the other segments. 



THE DEVELOPMENT OK THE PIXIOX 



To a great extent, the length of the pinion in many l)irds 

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

 various changes that take place in this member throughout 

 the period that ehi])ses from the embryonic stages to ma- 

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

 that ha\e taken ])lace through many generations, due pos- 

 sibly to the changes of environment which occurred during 

 the later geological periods. 



Commencing wnth the hoatzin we have the development 

 of the hand, outlined as follows: 



OpIsfJt oco in II s h oazin ( I lliger ) 



Carpus Digitus 



Embryo 43. mm. 23.5 mm. 



10 day nestling 34. " 32.3 



C( 



14 " " 34.2 " 32.1 



27.5 



Adult 39. " 2" - " 



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 



