ADJUSTMENT OF FLATFISHES 423 



'accidents' of nature, of which we hear so much, but it is hard to 

 suppress the beUef that it is an integral part of the process of 

 adaptation which we are considering, 



3. A coarse gravel, composed of stones ranging from 2 to 3 

 or 4 cm. in diameter. This is not strictly to be included 

 among the natural types of background, since much mud and 

 sand had to be sifted away in order to obtain it. The stones 

 ranged in shade from nearly white to nearly black, and the diver- 

 sity was increased by the occurrence in small numbers of reddish 

 or brown pieces. In most of my experiments, white shells and 

 smooth fragments of marble were added in order to heighten the 

 contrasts. This type of bottom was used with only four specimens, 

 but rather impressive results were obtained from two or three of 

 »these. In the most striking of these (fig. le) most of the annuli 

 became inconspicuous, as well as the finer elements of the skin 

 pattern generally, and certain large dark areas came into view, 

 giving to the fish a coarsely marbled appearance. This effect 

 rapidly disappeared, in large degree, when the fish was transferred 

 to a background of white glass, but reappeared upon its being 

 returned to the coarse gravel. A similar appearance was mani- 

 fested by this specimen upon certain of the other backgrounds 

 (see below), and the same large dark areas may be distinguished 

 in a number of the figures.-^ But the 'marbled' effect came into 

 view clearly only in cases in which it was appropriate, i. e., upon a 

 field having a coarsely blotched appearance. 



Another specimen, which had been kept for 11 days upon 

 the coarse gravel became in time — though rather gradually — quite 

 strikingly adapted. Unfortunately the fish escaped from the jar 

 and died, before any photographs were taken, but my notes state 

 that: ''for past few days mottled effect had become quite strik- 

 ing, showing a fusion both of darker and lighter areas into larger 

 spots."" One of the other specimens, on the contrary, assumed an 

 appearance which though plainly a mottled and 'gravelly' one, was 

 more nearly adapted to the fine gravel than to the coarse. As 



-1 This specimen seemed predisposed to the production of these effects, since the 

 coarse blotching was dimly evident when the fish was first received. 



22 This is probably not a very accurate account of the origin of these larger spots. 



