188 University of California Puhlicafions in Zoology. [Vol.7 



various factors have been eliminated by control, might, in con- 

 nection with ecological field studies, go far towards determining 

 whether or not the causes and processes are the same in the 

 two cases. 



The three color types occur more purely in the linnets of 

 California. In the Hawaiian series, taking into account the 

 effects of wear in the individual feathers, the two color types 

 present are each much more variable. It would seem that, as 

 Tower (1906, p. 285) found with certain potato beetles, sudden 

 change into a new environment is followed by an increase in the 

 range of variation. 



As to the nature of the variations in the linnets, whether or 

 not inheritable, there is nothing known. At any rate in the 

 development of the Hawaiian type of linnet, it would seem that 

 there could be no chance for the action of any sort of selection 

 (sexual, natural, or artificial). The characters do not, by any 

 stretch of the imagination, appear to be of adaptive value. Tower 

 found in the case of potato beetles, that the same kind of char- 

 acters may be somatic, acquired during the post germinal period, 

 or germinal, inherited. It is possible that in the Hawaiian linnet 

 the yellowness is heritable, even though the same feature arising 

 in caged birds be obviously somatic. 



It has been supposed that change from red to yellow in 

 caged birds is in some way caused by change in food, or by 

 general deterioration in bodily vigor, or perhaps due to a lack 

 of a normal amount of muscular activity. The latter would, 

 according to this idea, reduce the amount of the products of 

 metabolism. Some dermal pigments have been described as 

 modified excretory products. 



Yellow, orange and red pigments are said to ])e in certain 

 animals the same basally — manifestations respectively of lesser 

 or greater degree of oxidation of a chromogen in the presence 

 of an enzyme. (See Riddle, 1909, p. 320.) I have, however, been 

 unable to find out anything appropriate in regard to the chemieo- 

 physiolouical basis of these colors in birds. 



That food can be the prime cause of the color-modification 

 in the linnet is possible ; but the following facts do not give this 

 explanation more than a remote probability. The crimson linnet 



