THE WONDER OF LIFE 537 



the ripple-marks of internal tides (periodic or rhythmical 

 changes in metabolism and growth), we may refer to the 

 suggestive observations of Eiddle (1908) on fault-bars 

 in feathers. Fault-bars are weak areas interrupting the 

 fundaanental barring of the feather, and they appear to be 

 due to malnutrition or to defective nutrition. They may 

 be produced by feeding the birds with Sudan III, by 

 unwholesome conditions, or by using amyl nitrite to reduce 

 blood-pressure. They are usually laid down at night, 

 when the blood-pressure is normally lower than during the 

 day. The structurally weakened areas tend to be less 

 pigmented, and it has been shown that the production 

 of the dark (melanin) pigment in feathers may show 

 quantitative fluctuations corresponding to changes in the 

 available 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.' 



This is of very great importance, for we are here be- 

 ginning to see how an alternation in the rhythm of internal 

 processes may have far-reaching external results — which 

 afford much more than raw material for Selection to 

 work on. 



Physiologically useful Pigments. — ^Having recognized 

 that pigments occur in an organism as waste-products, 

 reserve-products, or by-products, and that there need not 



