COLORATION OF FISHES LONGLEY. 483 



Each individual which one sees commonly at such a place, if it is 

 not frankly a stray specimen, is adjusted to some one or more of the 

 various factors which make up its complex environment of the mo- 

 ment. To others it is more or less unadapted, and its lack of adapta- 

 tion in so far as color is concerned may make it appear conspicuous. 

 It is impossible to consider its conspicuousness functional, however, 

 something which nature has elaborated through selection on account 

 of its revealing quality. It is not even possible to look upon it as 

 something of no significance, upon which nature has placed no 

 check. Such as it is, it is a sort of residual, and possibly irreducible, 

 conspicuousness ; for species, as gaudy as any, possess marked power 

 of adaptive color change. 



That many fishes, those from tropical waters in particular, may 

 vary greatly in color from moment to moment, has long been known. 

 A few moments' observation of the fishes in the tanks of the New 

 York aquarium, for example, even when the observer sees the crea- 

 tures for the first time and lacks more than the average intelligent 

 person's interest in them, will make clear their power of color change. 

 Under what conditions this power is exercised in nature, and whether 

 the color changes of unconfined fishes in their native surroundings 

 conform to law, has, however, until recently never been determined 

 on any comprehensive scale. 



This deficiency in biological knowledge is probably due to the fact 

 that for the most part the visits of biologists to tropical reefs, where 

 the changeable species are best to be observed, have chiefly been of 

 short duration, and, in so far as they have been concerned at all with 

 fishes, have been devoted to collection, rather than to the study in 

 life of local species. Under such circumstances the students, or col- 

 lectors, as the case may be, have gone chiefly to those places where 

 fishes most abound, to places, in other words, where the underlying 

 system their color changes follow is to be perceived with greatest dif- 

 ficulty. 



What it is that determines that color shall change in changeable 

 species, and how strictly utilitarian the changes are in their general 

 effect, appears most clearly when the creatures are observed at some 

 point where large areas of rather uniform character meet others from 

 which they differ sharply in color. Such, for example, are the 

 boundary lines between bare bottoms covered with clean white sand 

 and those covered by a dense mat of brown seaweeds or green marine 

 plants of one sort or another. If at such critical points one watches 

 passing fishes that are equally at home on either side of the line they 

 may be seen making appropriate changes as they pass from one to the 

 other. One has to wait largely upon the haphazard movements of 

 herbivorous species in order to secure demonstration of the law in 

 accordance with which their color changes occur; but with a broken 



