422 FRANCIS B. SUMNER 



the minute white particles (shell and tufa) of the sand. There was, 

 nevertheless, nothing which could be regarded as very specific in 

 this resemblance. It was a harmony of the same sort that we meet 

 with frequently, both among land and water animals. 



2. A fine gravel of a predominantly gray tone, apparently com- 

 posed of about the same materials as the last, but much coarser, 

 and containing a larger proportion of the paler ingredients. For 

 the latter reason it was, on the whole, of a considerably lighter 

 shade. The particles ranged in size from a few millimeters to 

 about a centimeter in diameter, and in shade from pale gray or 

 white to black. The general appearance was thus diversified, and 

 this effect was heightentd by the presence of yellow and red frag- 

 ments. Upon gravel of this type the adaptation of the fish was 

 often very striking (figs, lb, 2e, 5). Here is it plain that no single 

 color or shade, disposed homogeneously throughout the animal's 

 surface, would have rendered it inconspicuous to any such extent 

 as this. It was necessary that the diversity of the background 

 should be matched by a corresponding diversity on the part of 

 the fish. The way in which this was brought to pass is difficult to 

 analyze in detail, but the results are indicated in the figures. It is 

 evident that the annuli have come conspicuously into view, the 

 intra-annular areas being pale, but commonly broken up more or 

 less by darker shading. The brown spots of the second type (p. 

 415) have also become rather conspicuous, and the general ground 

 color of the skin, which had earlier been dark and fairly uniform, 

 is now diversified by various sized patches of contrasting shades. 

 One of the striking features of the present skin pattern is the curi- 

 ous appearance of transparency which is often manifested, as 

 if we were actually looking through the body of the fish and saw the 

 gravel underneath. 2" This effect is heightened, in some cases, by 

 the appearance of certain of the dark spots, which are darkest at 

 the center, and shade off into a paler margin. These spots con- 

 vey the impression of depressions among the pebbles which seem 

 to surround them. This appearance may be merely one of those 



=" This, of course, is true of the marginal fins, but is not at all true of the body 

 proper. Moreover, even the marginal fins undergo pigment changes which con- 

 tribute to the general harmony of appearance (Fig. 2e). 



