100 THE INFLUENCE OF INANIMATE SURROUNDINGS. 



have already been made which indicate the possibility of an 

 early solution of it, they are at present far from being perfect 

 enough for us to discuss them in this place. One thing only 

 may here be briefly observed. If the Darwinian principles are 

 indesd the true ones, we must assume that the pigment itself — 

 not by its variable distribution — together with its subsequently 

 acquired utility in the maintenance of the species by the 

 selective influence of the conditions of life, must either have 

 some direct primary function in the normal life of the indi- 

 vidual, or else be the inevitable secondary product of some in- 

 dispensable physiological process. In a few rare cases this last 

 is known to be the case, and they have been classed by 

 Darwin as cases of correlational colouring. But we may hope 

 that the time is not far oflF when the presence of every kind of 

 pigment will be as intelligible to us — as easily referred, that is, 

 to definite causes — as are certain variations of colour, which, 

 under the chromatic function, are now recognised as being 

 directly and absolutely dependent on the effects of light on the 

 eye of the animal.^" 



