THE USES OF COLOUR 15 



a fish which was already difficult to detect on the 

 surface of the ocean on account of its transparency. 



II. Significant colours 



Colours may be useful in many ways, and are there- 

 fore always liable to be turned to account in one direc- 

 tion or another. They may be of direct physiological 

 value to the organism, or may assist in the struggle 

 for existence by deluding other species, or by aiding 

 the mdividuals of the same species, or they may be 

 intimately connected with courtship. 



1. The Direct Physiological Value of colour 



The colour of chlorophyll, which causes the green 

 appearance of vegetation, must be intimately con- 

 nected with the important changes which take place 

 in this substance in the presence of light. It is well 

 known that under these circumstances carbon dioxide 

 (popularly called * carbonic acid ') can be split up, 

 and its carbon made to unite with the elements of 

 water, forming organic substance. Although this 

 process has been much studied it is still very imper- 

 fectly understood. It is clear, however, that the 

 colour of chlorophyll, involving the special absorp- 

 tion of certain light-waves, has some direct bearing 

 upon the changes which occur. 

 ^ No equally clear instance has been proved to occur 

 in the animal kingdom, except in those few forms 



