DSB OF COLOUEING OFTEN UNKNOWN. 387 



their tentacles or pierce their skin with their microscopic dart- 

 like stinging-threads. It is impossible — ^so far as our present 

 knowledge extends — to discover the faintest trace of usefulness 

 in the brilliant colours of the polypes, and it is highly probable 

 that they are in fact perfectly unimportant as regards the 

 selective influence caused by their reciprocal relations with 

 other animals. To this example of a mode of colouring which 

 is insignificant with regard to selection, we might add many 

 others, particularly of invertebrate animals ; and the question 

 even arises whether in many cases the distribution of colours, to 

 which we are at present disposed to attribute a marked value 

 in the process of selection, may not be considerably overrated 

 in this respect. 



But it follows from all this that not the colour or pigment 

 itself merely, but its distribution — i.e. the "markings of the animal 

 — may under certain circumstances have been produced by other 

 causes than those on whose effects selection seems to depend. 

 It is perfectly evident that under no conceivable circumstances 

 can the pigment, the colouring-matter itself, originate from 

 selection ; this point has already been gone into in Chapter III. 

 (see pp. 99 and 115). It was there shown that the origin of the 

 pigment must depend on physiological processes acting in the 

 body of each individual, and which seem to be of the greatest 

 importance to the healthy life of each. Hence the particular 

 mode of its distribution throughout the skin must in the first 

 place be the result of causes acting entirely in the animal itself, 

 perfectly regular from the very first, or, it may be, wholly 

 irregular ; and this will depend on whether the internal physio- 

 logical causes have determined the deposition of the colouring- 

 -amatter in the skin in a certain regular order or no.'^^ If this 

 order is very sharply defined, the distribution of colour must, of 

 course, be extremely regular, and many of the characteristic 

 markings in Actinise, corals, and the shells of Mollusca may 

 have arisen in this way. But, on the other hand, selection 

 can control this colouring of the skin, and can confirm any par- 

 ticular arrangement which is especially advantageous to the 

 creature in the struggle for existence, can make it more regular 

 or enhance its brilliancy. The possibility that selection may 



