238 PRIMARY FACTORS OF ORGANIC EVOLUTION. 



d. The Effect of Light on the Colors of Flatfishes. 



It is well known that the side of the body which is 

 uppermost in the normal position in the flatfishes 

 (Pleuronectidse) is colored generally with dark tints, 

 and frequently with a distinct pattern, while the lower 

 side is white. This is due to the absence from the 

 lower side of the chromatophorae or pigment-contain- 

 ing cells, which are abundant on the upper side. The 

 young fish has chromatophorae on both sides as it has 

 its eyes also in the normal position, but as the fish 

 turns the left side upwards and the right eye gradually 

 rotates to the left side, the chromatophorae disappear 

 from the right side, which thus becomes colorless. 



Prof. J. T. Cunningham 1 experimented with young 

 flounders taken at the beginning or middle of their 

 metamorphosis, by placing a mirror below the aqua- 

 rium in which they were kept, at an angle of 45, and 

 cutting off the light from above by an opaque cover. 

 In the great majority of the specimens treated in this 

 way, after several months, although no effect was pro- 

 duced upon the eyes, more or less of the skin of the 

 lower side was pigmented. He thus showed that the 

 absence of pigment on that side in the normal fish is 

 due to its position in shadow, where little light can 

 reach it. 



e. The Effect of Feeding on Color in Birds. 



Mr. F. E. Beddard cites the following remarkable 

 example of the direct effect of internal physical causes 

 in producing change of coloration. 2 



1. Zoologischer Anzeiger, 1891. 



2 Animal Coloration, an Account of the Principal Facts and Theories Relat- 

 ing to the Colors and Markings of Animals. By Frank E. Beddard, M. A. 

 Cxon., F. R. S. E. With Four Colored Plates; and Wood-Cuts in the Text. 

 London . Swan Sonnenschein & Co. New York : Macmillan & Co. 1892. 



