I^O OSCAR RIDDLE. 



say, many of the necessary cells never arise at all and. those 

 which are to be found in the barbule region never attain the 

 normal size of such cells. Davies recognized, to a certain extent 

 at least, a deficiency of growth of particular parts of this region. 

 He states, " Gewohnlich verschwinden (in the quill) die Leisten 

 nicht vollkommen, obgleich sie eine bedeutende verminderung 

 ihre Grosse erfahren " (p. 581). On the other hand, one kind of 

 differentiation, i. e., cornification or development of keratin pro- 

 ceeds without interruption in all of these cases. 



Jones further states that apparently this " lack of differentia- 

 tion ... is induced by a reduced blood supply to this part of 

 the feather, etc." He is here speaking of the conditions in the 

 "quill "-formations only and does not apply this statement to the 

 other form of down modification which he considers (p. 11) the 

 typical one. He does not furnish any evidence for the statement 

 just quoted, and does not refer to the direct and conclusive evi- 

 dence which my paper ('07) supplies — and I may add that this 

 is still the only direct evidence we have — that a reduced blood 

 supply tends to produce just such feather modifications as are 

 represented in the quill of the down. 



Jones' observation that there appears to be no growth of the 

 down in altricial birds during " this critical period " is important 

 and suggestive as supporting the view that wherever we find the 

 " down " we can assert that it signifies defective nutritive condi- 

 tions in the bird at the time that part of the feather was grown. 

 It is, however, doubtful whether Jones had anything similar to 

 this in mind, for it will be seen that his last word on this subject 

 is that "in the robin (the only specific case cited) the so-called 

 ' quill ' was formed at the proximal end of the down by the rapid 

 drying of the imperfectly formed barb-vane ridges." 



Finally, it should be noted that Jones states that in the case 

 of the young robins the " quill " was formed during the fourth 

 to the eighth days after hatching. It seems to me extremely 

 probable that the first four days after hatching were even more 

 important in producing the modification than the four succeeding 

 ones. 



