192 Univcrsifjj of California Publications i)i Zoology. [Vol.7 



anlage is insufficient to carry on oxidation of tyrosin beyond the 

 yellow, or at farthest, the orange stage. (See Riddle, 1909.) 



It is not inconceivable that the character of diminished color- 

 production in the new habitat may have been induced many 

 years later than the date of importation, and have resulted from 

 the accumulated effects of some one or more environmental fac- 

 tors, not necessarily the same on all of the islands. But having 

 once arisen, this character would have been intensified by close 

 breeding, as others are known to be in cases of the breeding of 

 domestic animals. 



The exact nature of the colors and the manner of their pro- 

 duction in birds seem to remain practically unknown. 



Solution of the whole problem will doubtless rest upon a 

 searching analysis of the conditions surrounding the linnet in : 



all its habitats, together with experimentation upon living birds ; 



under control. But at the present time to assert emphatically f 



any particular factor or group of factors as the prime stimulus, ♦ 



does not seem justifiable. The problem is an attractive one for I 



investigation. 



In closing this paper, I wish to acknowledge generous criti- 

 cism from Professor H. B. Torrey, of the University of Cali- 

 fornia. 



