576 W. H. LONGLEY 



of coloration is protective, the other is not." Yet it appears that 

 the ranges of the two species differ both in horizontal and vertical 

 extent. 



The Iridio occurs on the turtle-grass flats, to whose color it is 

 able to adjust itself, but the Thalassoma does not range over the 

 larger areas of this character. Not one has been taken among 

 scores of Iridioes caught with the seine on grassy bottoms, nor 

 do they come about baits 'set out in such places. In other 

 respects the horizontal ranges of the two specise seem to coincide. 



It was a long time before difference in their vertical distribution 

 was noticed which was sufficient to suggest an appeal to experi- 

 ment, but when this was undertaken it was demonstrated with 

 ease that the Thalassoma rises readily to higher levels than the 

 Iridio. If food be placed on coral heads of different height, 

 Thalassoma will arrive first at the higher stations and may be 

 the only visitor, although Iridioes swim about at lower levels. 

 Again, if in a tank filled with water to a depth of 15 inches a 

 vertical partition a foot high be erected, and specimens of the 

 two species be placed on one side, a Thalassoma will swim over 

 the obstruction first, and may recross twenty times, before the 

 first Iridio finds its way over. Each experiment, therefore, 

 clearly confirms the suggestion that led to their performance. 



But though there is correlation between blue color and a fish's 

 habit of swimming at intermediate levels in water of moderate 

 depth, its meaning may not be apparent, for Wallace ('77) writes, 

 and the explanation is approved by Poulton ('90), that "look- 

 ind down on the dark back of a fish it is almost in\dsible, while, 

 to an enemy looking up from below, the light under surface 

 would be equally invisible against the light of the clouds and 

 sky." However, at a depth of 15 to 20 feet in clearest water, 

 under a tropical sky, one does not see the fishes above against 

 a mere light or whitish background. The sky seems to have 

 fallen, for one stands beneath a dome of blue, paling toward the 

 zenith and deepening toward the horizon. Little is visible at 

 20 feet. The 'atmosphere' of the painter is thick; it is apparent 

 almost at arm's length, and softens the outlines of all but the 

 nearest objects. 



