ACQUIRED CHARACTERS 313 



the Amazon valley, afford clear evidence of this theme — the 

 transmissibility of acquired characters. 



In this shoulder girdle (opposite) the keel of the breast-bone 

 has become reduced to a small triangular plate at the extreme 

 hinder end of the sternal plate, and is further peculiar in 

 that its free edge is unusually broad, and during life is covered 

 by a callous pad, such as is met with on the breast of the 

 Ostrich. The furcula, or merry-thought, has been transformed 

 so that the median spur known as the " hypocleideum '' is 

 drawn out into a long bony style fused with the sternal plate. 

 Finally, the coracoids have fused with the sternal plate at one 

 end, and with the free ends of the furcula at the other. 



These peculiarities are, he contends, the result of the pressure 

 exerted by the enormous crop, which is unique in character, 

 since it has developed thick muscular walls and a horny internal 

 layer whereby it has been enabled to assume the functions of 

 the gizzard, which organ has, in consequence, degenerated till 

 it is now no larger than, say, a large acorn. 



The series of anatomical changes here outlined are unques- 

 tionably striking. 



The broadened free edge of the sternal keel and its callous 

 pad are set down by Dr. Gadow as the direct effect of the re- 

 sponse to external stimulus, since this pad is used as a support 

 to the body when the bird is at rest perched on a bough. 

 Similarly, the peculiar modifications of the shoulder girdle are 

 attributed to the accumulated effects of the pressure of the great 

 muscular crop on the furcula and end of the sternal keel. But 

 no attempt is made to account for the origin of the supposed 

 cause of this modification of the skeleton — the change in the 

 function and consequent increased size and weight of the crop. 



The interpretation of this case, it would seem, is rather one 

 of correlated variation. The change in the direction of the 

 transformation of the crop, we may suppose, was the begin- 

 ning of this story, which ran on to its final conclusion only 

 because accommodation could be found for the increased bulk 

 by the gradual retreat of the keel of the breast-bone, no longer 

 all-important as a support for the flight muscles since flight 

 itself has become almost dispensed with. 



And this interpretation gathers increased probability since 

 what may well have been the initial stages in the transforma- 



