544 W. H. LONGLEY 



in the cour.se of their lives continually pass and repass over 

 environments of different colors, and adds that it is widely 

 found in fishes. 



For at least thirteen West Indian species it appears that 

 Thayer's and Poulton's hypothesis is fully confirmed. This 

 implies only that in nature the color changes of the animals 

 vary with their environment, with which they tend to assimilate 

 them. It has not been demonstrated that the power of adaptive 

 color change actually confers any advantage upon its possessor. 



The reason the truth above has remained obscure is not far 

 to seek. One studying fishes goes naturally, especially if time 

 be limited, first to those places where, upon the whole, species 

 are most abundant. This will be almost invariably some mixed 

 bottom covered in part by algae, gorgonians and living coral, 

 in part covered by bare sand, coral skeletons and stone. But 

 over such a substratum confusion prevails, for the color of a 

 given fish at a given station is not always the same, since it may 

 be modified by a number of factors. The color of the underlying 

 bottom, the size of the patch over which the fish is at the moment 

 and the rate at which it is moving across it, as well as the level 

 at which it is swimming, aje all effective in determining its 

 appearance. Above all the fish's recent experience confounds 

 the observer, for two individuals that have just come from envir- 

 onments of different character may temporarily show the color 

 phases induced there over a third type of bottom to which 

 neither is adjusted. 



Months of study of the problem under the conditions described 

 may leave one with the feeling that there is, upon the whole, 

 some evidence of adaptive color change manifested vaguely in 

 a few species. When this appeared to be the case with Iridio 

 bivittatus, a specimen was placed in each of two 5-litre aquaria, 

 which had been wrapped in black paper after the bottom of 

 one had been covered with white sand and that of the other with 

 a mixture of brown and green algae. A little manipulation then 

 showed, when the jars were set out in the sunhght, that either 

 fish might be made the darker or lighter within a few seconds 

 by making proper disposition of the pair, the difference in color 



