218 
ZOOLOGY: 5. O. MAST 
dark gray to the human eye. On a gray background, however, even if 
it appears much lighter than the white in low light intensity, they be- 
come gray, on a black, black, etc. If however conditions are so arranged 
that the eyes receive no Hght direct from above, that is, receive only 
light reflected from the background, they become maximum white no 
matter what the shade or pattern of the background may be. This 
shows that the changes in shade and pattern and probably also color 
are in some way dependent upon the relation between the Hght received 
directly and that received by reflection from the background. 
In accord with this, if the light from above is abnormally strong the 
fish ought to become dark on a white bottom. Apparatus was devised 
by means of which such illumination could be produced but it was found 
that when the light from above was abnormally strong the fish did not 
respond at all. The significance of this negative result is still somewhat 
problematic. 
d. Comparison of the skin with the bottom: In specimens with the ven- 
tral eye removed it was possible to keep the anterior part, with the ex- 
ception of the eye, so thoroughly covered with sand that the fish could 
not see any of the skin. Under these conditions adaptation occurred 
just as accurately and rapidly as it did when the skin could be seen. 
This shows that the skin is not necessarily compared with the bottom 
by the fish in the process of adaptation. 
4, Selection of background. After flounders have been, for some time, 
on a given background they tend to return to this background when 
put onto another near it. That is, they tend to select the background 
on which they are least conspicuous. This was conclusively proved 
with reference to shade and fairly definitely with reference to color. 
The response is, however, of such a nature that, under normal conditions, 
it is probably of little value in conceaHng the animals ; but it does throw 
some light on the question of vision referred to later. 
5. Biological significance of simulation of the background. Some hold 
that the phenomenon of simulation is purely accidental and that it has 
no biological value: others maintain that it functions as a protection 
from enemies or in capturing prey; a few contend that it serves chiefly 
to regulate the temperature of the body; and some even hold that it 
functions in all of these ways and in still others. Unfortunately, how- 
ever, none of these ideas are supported by experimental evidence. In 
no case has it ever actually been demonstrated that the response in ques- 
tion has any value whatsoever. This statement includes the results 
obtained in work on the flounders, although the experiments outHned 
have as yet been only partially completed. 
