CAUSE OF PRODUCTION OF " DOWN." I 75 



3". Occasionally feathers may be found which have been able to 

 continue their growth proximal to the downy portion despite the 

 inhibitory influences of the lack of food ; such feathers have been 

 found to represent a type of structure intermediate to the downy 

 and pennaceous formations. 



4. The cause of the modification at the base of the down is to 

 be traced to an interruption, or at any rate to the inadequacy of 

 the nutritive processes of the bird. This interruption is doubt- 

 less partly accounted for by the change of source of food from 

 the embryonic to the adult life. 



5. The apparent absence of such growth-marks in the feathers 

 of the duck and other anserine birds remains unexplained. 



6. The rate of growth of the two ends of a ju venal, pennaceous 

 feather, and of the proximal ends of all pennaceous feathers 

 which bear a plumulaceous proximal portion, is much slower 

 than the rate of growth in the central pennaceous part of the 

 feather. 



7. It seems probable that all plumulaceous structures are pro- 

 duced at a relatively slow rate of growth ; and also probable that 

 during their growth they have not enjoyed optimum nutritive 

 conditions. 



8. The formation of the quill is probably the direct result of a 

 progressive diminution of an already lessened food-supply. 



