146 Zoologica: N. Y. Zoological Society. [I; 5 



This has been hinted at by Dr. Stejneger* but unfortunately 

 for his illustration, he has used a feather exhibiting an ordinary 

 fault-bar, caused, as we know, by some sudden and local disturb- 

 ance in nutrition, either of the bird as a whole or of a particular 

 feather follicle. Similar faults may result from so slight a cause 

 as the difference in blood pressure during the sleeping and wak- 

 ing hours. This results in a transverse faulting across the 

 entire vane — affecting the tips of the proximal barbs and the 

 centers or bases of those more distal ones which are in process 

 of development at the period in question. 



The weakness of the vane of the Motmot's tail is far more 

 fundamental, in reality an organic inheritance ; the faults or de- 

 generation affecting only the bases of the barbs within a definite 

 limited area — and the faulting thus being longitudinal. Of con- 

 siderable interest is Stejneger's illustration of a specimen in the 

 collection of the United States National Museum, where a slight 

 denudation — symmetrical in the two feathers — exists on a pair 

 of rectrices which have only just escaped from the enveloping 

 sheath. This would seem to indicate that physiological denuda- 

 tion has already begun in nature and that the bird's part is be- 

 coming a subordinate one. 



Under a magnification of twenty-five diameters, the distinc- 

 tion between the normal and the weakened portions of the vane 

 is seen to consist in the dwarfing or absence of the barbules, and 

 of a very slightly reduced diameter of the basal part of the barb 

 itself. The degeneration of the barbules has been greater on the 

 inner side next to the shaft, where in many cases the barbules 

 have totally disappeared. 



Under the lens, the clean mechanical work of the purely 

 physiological action as compared with the clumsier trimming of 

 the rough beak is plainly visible ; the shaft being cleaner and the 

 barbule stumps more regular in the former case. 



The physiologically denuded tail feathers reached their full 

 growth within a period of six weeks, and on October 1st, 1909, 

 were in their turn plucked out for examination and reference. 

 The bird had of course by this time completely finished its moult 

 and was in perfect condition. 



Part V. 



The final stage of this experiment was the observation of 

 the growing tail feathers, succeeding those plucked out October 



*Riverside Natural Histon', Vol. iv, p. 399. 



