226 



ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



the growth of organs. There are some other birds, I would remark, 

 wherein also we find a hypertrophied growth of the windpipe. But 

 the course taken has been very different. In the painted snipe 

 {Rhynchea) and the purple manucode {Phonygama) — one of the 

 birds of paradise — the windpipe has come to form long coils or 

 loops between the breast muscles and the skin, while in the spoonbill 

 it forms a figure-8 loop immediately under the lungs. But here, 

 meeting with no resistance, no secondary modification of any other 

 organ was needed. 



Let me cite yet one more case — and I could find a hundred. This 

 is furnished by the hoatzin. It feeds largely on the fruit and leaves 

 of the thorn-tree {Drepanocarpus lunatus) and of a species of 

 Echites. One or other of these has probably some astringent prop- 

 erties, that brought about a thickening of the walls of the crop, which 

 gradually assumed the form and functions of the gizzard, now re- 

 duced in size and superseded. As a consequence, the pressure of this 

 gizzard-crop against the anterior border of the keel of the sternum 

 gradually forced it back, a process accelerated by the fact that the 

 breast muscles had already become reduced from lack of use, as in 

 our waterhen. The crop developed, in short, at the expense of the 

 keel. As a result all that is now left of the keel is a small, triangular 

 projection at the extreme hinder end of the sternum. Furthermore, 

 this pressure of the crop has forced the furcula upward, so that it 

 has come to lie between, and parallel with, the coracoids, while its 

 conjoined extremities have fused with the sternal plate. 



Here, surely, we have unmistakable evidence of the effects of per- 

 sistent stimuli, such as mold every organ of the body according to 

 the intensities and nature of its use. 



It seems impossible to attribute these instances of "reciprocity" in 

 development to "natural selection", nor can they be set down to the 

 action of the "environment." In the case of the windpipes we have 

 two totally different organs involved, one belonging to the respira- 

 tory, the other to the skeletal systems, producing, as I have shown, 

 reciprocal changes. But whether this excessive lengthening of the 

 windpipe is due, in the first place, to hypertrophy, or whether it has 

 been brought about by some unsuspected and undiscovered stram in 

 the production of the voice, is a matter for further investigation. 

 In the hoatzin, then, we have a very singular relationship set up be- 

 tween the alimentary canal on the one hand, and the skeleton on the 

 other. They are precious sign posts in our search for the causbs 

 governing the transformations of living bodies. 



No less important are cases — and they are legion — of "degrada- 

 tion" or "retrogression", sometimes strangely associated with the 



