In the year 1890 I tried an experiment upon young flounders, with the object of discovering what would be the effect upon the lower sides of the fish if these sides were continually exposed to daylight. Under ordinary conditions the upper side of the fish is dark-coloured, the lower side white; and the upper side is exposed to light, while the lower side being usually in contact with the ground, and always turned away from the sky, is protected from the light. A connection between the difference of the two sides in relation to light and in coloration naturally suggests itself. If the difference in coloration is due to the difference in the exposure of the sides to light, when the lower side of the fish is kept exposed to the light it ought to become coloured.