VARIABLE RESEMBLANCE IN VERTEBRATA, ETC. 85 



pigment cells. It is, however, now well known that 

 the action is extremely indirect; certain kinds of 

 reflected light act as specific stimuli to the eye of the 

 animal, and differing nervous impulses pass from 

 this organ along the optic nerve to the brain. The 

 brain being thus indirectly stimulated in a peculiar 

 manner by various kinds of reflected light, originates 

 different impulses, which pass from it along the nerves 

 distributed to the skin, and cause varying states of 

 concentration of the pigment in the cells. The highest 

 powers of the microscope, assisted by all the varied 

 methods of histology, have failed to detect the con- 

 nection between the nerves and the pigment cells in 

 the skin, and yet such connection appears to be ren- 

 dered certain by the fact that light falling on the eye 

 modifies the distribution of the pigment granules. 



The pigment cells in the skin are often of various 

 colours, and are arranged in layers, so that very 

 different effects may be produced by concentration in 

 certain cells, leading to the appearance of those of 

 another colour, or to a combined effect due to the 

 colours of two or more kinds of cells. 



Blind animals cannot vary their colour protectively 



It has been shown by experiment that blinded 

 frogs have no power of altering their colour so as to 

 correspond with surrounding tints. The same facts 

 also have been proved in a most interesting manner 



