346 OSCAR RIDDLE. 



pulp cavity (see figures). Others have pointed out that the dif- 

 ferentiation of the barbs occurs later, i. e., at a higher level than 

 that of the barbules. There must be added to that the further 

 fact that some of the cells which enter into the former arise later 

 (at a higher level) than those which form the latter. A few ob- 

 servations on feathers from various birds, together with Strong's 1 

 figures of Sterna, afford considerable reason for believing that 

 these statements on the region of growth in the chick and the 

 dove may have among birds a very wide application. 



The Rate of Feather Growth. — The rate of feather growth has 

 not, so far as I am aware, been extensively or accurately studied. 

 Cunningham 2 found that certain tail feathers of Japanese fowls 

 grow at the rate of 3.5 mm. per day. Ostrich plumes are said 

 to grow about one inch per week. In Plymouth Rocks my 

 measurements show a very similar rate of growth. The rate 

 varies greatly in different feather tracts of the bird ; for example, 

 in a Plymouth Rock it was, in the primaries, secondary coverts 

 and body coverts, 4, 2.25, 1.75 mm. daily respectively. In 

 general, the rate bears a rather definite relation to the ultimate 

 length of the feather ; and is less at the proximal than at the 

 distal end of the feather. This is shown in the figures (PI. XIII., 

 Figs. 17-19); these indicate also the presence of fault-bars which 

 are laid down at distances corresponding to the figures given above. 



The ring dove shows a still more rapid feather growth. Seven 

 mm. of growth in 24. hours has occasionally been recorded in the 

 rectrices of these birds. The average for these birds is : rectrices 

 5-6 mm., primaries 5-6 mm., upper tail coverts 4 mm., primary 

 coverts 4 mm. It will be recalled that this is also the order of 

 frequency for the appearance of the defective areas in the various 

 feather-tracts. This and kindred observations establish beyond 

 doubt that the frequency of appearance of obvious fault-bars in 

 feathers is directly related — one might almost say proportional 

 to the rate of growth. Table I. gives some figures on the rate of 

 growth in doves. 



The Effect of Starvation on the Rate of Growth. — Starving 

 conditions when brought to bear on growing feather-germs pro- 



1 " Development of Color," loc. cit. 



2 Cunningham, J. T., "Observations and Experiments on Japanese Long-tailed 

 Fowls," Proc. Zool. Soc. of London, 1903. 



