VAEIABLE BESEMBLANCE IN VERTEBRATA, ETC. 87 



The power of varying the colour essentially protective 



The protective value of the change of colour in 

 normal trout was especially well seen when contrasted 

 with these hlind individuals. As it has been some- 

 times asserted that protection is not the meaning 

 of resemblance to the environment, I was anxious to 

 observe so striking a contrast for myself. Mr. Nicoll 

 kindly gave me the opportunity of seeing the fish in 

 his stream, and I can in every way confirm his state- 

 ment that a person unaccustomed to the observation of 

 animals would certainly fail to detect any trout except 

 the black ones. No one who had the opportunity of 

 comparing the changing colours of the normal fish — 

 ever harmonising with their surroundings, — with the 

 unvarying conspicuous darkness of the blind in- 

 dividuals, could hesitate for a moment in admitting 

 that concealment is the one object of the adjustment 

 of colour. 



The change of colour may also be voluntary or 

 may follow from mental excitement. Thus the colours 

 of fish often become much brighter while they are 

 feeding.^ 



The food of blind trout 



It may be objected that the dark fish still continue 

 to live in the same stream with the more perfectly 



* For a curious change of colour in the conger, see Bateson, Ix, 

 pp. 214-15. 



