232 CALIFORNIA ACADEMY OF SCIENCES. 



erally grows paler from the breast towards the tail but 

 the abdomen is more protected from the direct influence 

 of reflected light than the breast, because, especially 

 during the middle of the day when the influence would 

 be strongest, it would be protected by the bird's shadow. 

 This is supposing that the bird is habitually upon the 

 ground, but if it were habitually among the trees or 

 bushes it would be protected by them. 



I would accordingly suggest the very opposite view of 

 the white color of the under parts of animals. We have 

 seen that color is a normal product of growth, which 

 would at first be thrown out upon the integument in a 

 perfectly indiscriminate manner. The influence of 

 sunlight would, in the course of time, tend to attract 

 the greater amount of pigment to that part of the body 

 most exposed to the light. Thus there are many in- 

 stances where two pigments have apparently combined 

 upon the back, and perhaps the upper part of the breast, 

 while only one, and that one not very intense, remained 

 upon the belly, often fading into pure white on the 

 under tail coverts. This state of affairs may be fre- 

 quently found among the warblers, where the back is a 

 dark olive green, and the breast a light yellow. Plates 

 XVIII and XIX of the colors of the genus Dendroica well 

 illustrate this point, for it will be noticed how uniformly 

 the abdomen and under tail coverts are pale colored and 

 unmarked. According to this view there would be a 

 general tendency for the bulk of pigment to be directed 

 to that part of the body which received the most sun- 

 light, but if it happened that some advantage, either 

 from recognition, protection, or sexual adornment, was 

 to be gained by retaining patches of color upon the 

 breast, or by having the breast uniformly colored some 

 dark shade, natural selection would counteract the influ- 

 ence of sunlight. 



