THE COLORATION OF REEF FISHES 577 



In this Lilliputian world one has a demonstration of the effect 

 of color and pattern which supersedes all argument. Fishes 

 come from nowhere and vanish into nothingness, not because 

 they have gone beyond the range of vision, but have wheeled at 

 right angles. The' secret involved in their disappearance is 

 largely this, that, as they turn, foreshortening reduces, even to 

 zero, tell-tale lateral markings upon which the eye had been 

 fixed. Blue-gray bodies, or blue-gray bands upon bodies of 

 other colors, are utterly resolved into the blue-gray haze which 

 surrounds them, and one is wholly unable to perceive the outlines 

 of objects easily within visible distance, as the distinctness of 

 certain markings testifies. 



Figure 8 illustrates the effect of differential visibility of two 

 colors combined in simple fashion. It shows, in other words, 

 the mode of operation of Thayer's 'ruptive pattern,' when it 

 appears upon a countershaded body. It is so efficient in the 

 case of Abudefduf, that when the fish is above the level of the 

 observer's eye, its silhouette becomes indistinct, or is inter- 

 rupted, at a distance of 4 or 5 yards, through the assimilation of 

 some of its color elements with the watery background against 

 which it appears. If it were wholly of the gray color, suitably 

 countershaded, it would be almost invisible at that distance, 

 except as its eye might reveal its position. Some simply colored 

 species of the upper levels, when appropriately viewed, are, in- 

 deed, the veriest ghosts of fishes. Tylosurus is a notable ex- 

 ample, and Sphyraena, though a large fish, has defied the camera 

 times without number, largely on account of its lack of contrast 

 with its background. 



At this point it may appear to the reader, that the presence 

 of the dark bars of Abudefduf, which prove so clearly that its 

 partial invisibility is attained well within the limit of vision, is 

 inconsistent with the argument that is being developed. For if 

 natural selection guides the evolution of such characters, why, 

 it may be asked, should the fish retain these marks, if its visi- 

 bility might be farther diminished by their suppression. This 

 fair question may be answered most readily by calling attention 



