82 THE COLOUES OF ANIMALS 



perative necessity for the change to be extremely 

 rapid. 



In other cases it will be of advantage to the animal 

 to possess the power of changing twice in its life, once 

 for the peculiar surroundings of the caterpillar, and 

 once for the peculiar surroundings of the chrysalis. 

 There is indeed some ground for the belief that in 

 certain cases the colours of the perfect insect also may 

 be adjusted to correspond with the peculiar environ- 

 ment. 



Variable Protective Resemblance in Fishes 



Instances of the power of rapid adjustment are 

 very common, although most people are not aware of 

 them. Nearly all fishermen know that the trout 

 caught in a stream with a gravelly or sandy bottom 

 are light-coloured, while those caught in a muddy 

 stream are dark. It is also well known that the same 

 fish will soon change in colour when it passes from one 

 kind of background to the other. Thus Mr. E. D. Y. 

 Pode tells me that all the trout in a stream near Ivy 

 Bridge have become unusually light ever since the 

 pollution of the stream by white china clay. 



The same facts are true of many other freshwater 

 and sea fishes. The interior of a minnow-can is 

 painted white in order that the bait may become light- 

 coloured and thus conspicuous in the dark water where 

 the pike or perch is likely to be found. The change 

 of colour occupies an appreciable time, and the fisher- 



