VARYING COLOURS IN FISHES. 79 



can be effected upon the hues of fishes in a way 

 which must be widely different from the foregoing, 

 has repeatedly been demonstrated by actual experi- 

 ments. Thus, to refer to the last which have been 

 published, Mr. Shaw, Drumlanrig, procured two 

 large earthen-ware vessels, the one nearly white 

 inside, and the other nearly black. He then placed 

 a healthy Parr in each, while a constant supply of 

 fresh water w^as maintained. The fishes when put 

 into the vessels were of their natural colour ; but 

 they had not remained in their new position more 

 than four minutes w^hen each gradually assumed a 

 colour nearly approaching to that of the respective 

 vessel into which it had been introduced. He then 

 frequently exchanged the position of the two fishes, 

 and the result uniformly followed ; the fishes chang- 

 ing their colour according to the surface around 

 them. He next placed both fishes in one basin, 

 when the contrast for a short time was exceedingly 

 striking. With the view of ascertaining what effect 

 the light had in producing the extraordinary change, 

 the fish were allowed to remain in the white basin 

 till they effectually attained the pale tint ; the light 

 was then excluded by covering it with a thick mat ; 

 on removing which, a few minutes afterwards, the 

 fish were again changed to a dark colour; which 

 disappeared gradually on exposure to light : the 

 cliange being produced alike under a bright and 

 cloudy sky. Though at the time Mr. Shaw was 

 unacquainted with the fact, it must have been gene- 

 rally known tliat Dr. Stark of Edinburgh had several 



