EXTERNAL FEATURES 39 



they are precipitated as granules or held in suspension in 

 saturating oil and fat. Now, it is plain that vicissitudes or 

 changes in the nutritive supply or in the environment may 

 influence the pigmentation of a feather ; and the result is 

 technically called an individual modification. Something 

 peculiar in the food may result in something peculiar in the 

 pigmentation ; thus cayenne pepper affects the colouring 

 of canaries. Or some injury to the young feather may 

 result in the non-deposition of pigment, as we see in the 

 individual white feathers that may occur on a blackbird or 

 crow. 



The development of a normal hereditary character 

 (nature) requires appropriate normal conditions (nurture) ; 

 and the colouring of a bird may depart from what is usual 

 because of something peculiar in the conditions of life. A 

 change in nurture (whether environmental, nutritional, or 

 functional) brings about an internal change that alters the 

 expression that the development of a hereditary character 

 finds. 



Fault-bars. — It has been shown by O. Riddle (1908) 

 that what are called " fault-bars," weak areas interrupting 

 the fundamental barring, are due to malnutrition. They 

 may be produced artificially by giving the birds amyl-nitrite 

 which reduces the blood-pressure, or by altering the food 

 disadvantageously. They are usually laid down at night 

 when the blood-pressure is normally low. The dark 

 (melanin) pigment is probably derived from proteins in the 

 blood or in the formative cells of the feather, and the 

 quantity available will naturally vary with the food supply. 

 " The reduced nutrition, brought about daily by the 

 minimum 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 fundamental bar, at 

 perfectly regular intervals in the entire length of every 

 feather formation." It is evident that interpretations of 



