362 OSCAR RIDDLE. 



11. A daily blood pressure rhythm with a minimum pressure 

 occurring doubtless between 1:00 A. M. and 5:00 A. M. is pres- 

 ent in birds. 



12. The reduced nutrition brought about daily by this mini- 

 mum blood pressure ; the disadvantageous position, in relation 

 to the blood, of the pigment and barbule elements of the feather ; 

 together with the very rapid rate at which feathers grow, furnish 

 the complex of conditions which bring unfailingly into existence 

 a fault-bar, and to a more or less appreciable extent a light funda- 

 mental bar, at perfectly regular intervals in the entire length of 

 every feather formation. 



13. The melanin pigment of the feathers of birds shows, under 

 favorable conditions, quantitative variations of the pigment pro- 

 duced in response to changes in the available food- supply. This 

 is an additional evidence that this pigment is not a derivative of 

 haemoglobin, but of the serum or cell proteids. 



14. The light and dark fundamental bars are universal in 

 feathers (Whitman) and have been shown by the writer to repre- 

 sent different conditions and periods of growth ; the dark funda- 

 mental bars (of pigmented feathers) being expressions of growth 

 and pigment production under the most favorable conditions ; 

 these conditions obtain during the day and the first part of the 

 night ; on the other hand, the light fundamental bar is brought 

 into existence, as stated above, by fault-bar producing conditions, 

 i. e., by the low nutritive conditions which obtain during the later 

 hours of the night. 



15. These results furnish a description in the terms of phys- 

 iology, of the mechanism of the "inheritance" of certain funda- 

 mental color-characters of all birds. 



16. The fundamental bars furnish the starting point for all 



evolutionary studies on the color-characters of birds. 



The Hull Zoological Laboratory, 

 University of Chicago. 



