PLATE 13 



VIEWS OF SPECIMENS 11 AND 12 



12 a Against background of gray (same gray as in 4 i), immediately after trans- 

 fer from dark sand. 



11 /-12 b Showing conditions of illumination in the white and gray jars, used 

 in the experiment described on pp. 440-443. The bottom of the white jar appears 

 much darker than that of the gray jar. 



11 ^-12 c Showing condition of these two fishes (photographed together on gray 

 bottom), after former had been kept four days on the (lighted) gray bottom and 

 the latter four days on the (shaded) white bottom. 



11 h-l2 d Appearance of the same two fishes, four days after the reversal of the 

 above conditions (11 being in white jar, 12 in gray). (The reader must not be mis- 

 led by the altered positions of the two fishes in the later picture. The identity of 

 each specimen is revealed by its size, no. 12 being the larger). 



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