SECTION VII 



THE EVOLUTION OF COLOUR VISION 

 CHAPTER I 



INTRODUCTION 



A priori we should expect some light to be thrown upon the fully 

 developed colour sense of man by a knowledge of the stages through 

 which that colour sense has evolved. The sources of our information 

 on the evolution of colour vision are few and the methods of in- 

 vestigation difficult and arduous. Only recently have the researches 

 been carried out in a scientific manner and yielded valuable results. 



Positive evidence is derived from three chief sources. In the first 

 place we naturally appeal to the visual sensations of lower animals. 

 These are extremely difficult to investigate since we are almost wholly 

 dependent upon observation of motor responses which the animals 

 make to various light-stimuli, though some deductions can be made 

 from the structure of the visual organs. In the invertebrata little can 

 be done beyond recording the phototropism of the animal, i.e., its 

 attraction or repulsion by lights of different wave-length and intensity, 

 as exhibited by its movements towards (positive phototropism) or away 

 from (negative phototropism) the light 1 . As we ascend the animal 

 scale the increase in complexity of the nervous system and of the visual 

 organs is accompanied by a corresponding increase in complexity of 

 the motor responses, associated with a greater difficulty in their inter- 

 pretation. On the other hand, as we descend the animal scale from 

 man there is an unwarranted tendency to interpret the apparently 

 purposeful responses of the animal in an anthropomorphic manner 

 which is not necessarily justified on neurological and psychological 

 grounds. For example, we have little knowledge of the psychology 



1 Mast, Light and the Behavior of Organisms, Now York, 1911. 



