54 Instinct and Intelligence 



to the fact that the movements made by 

 Euglena are influenced by the stimulus of light; 

 it is, however, uncertain how far the structures 

 forming the eye-spot determine these move- 

 ments ; but in the case of Volvox each cell pos- 

 sesses a well-defined refracting lens behind 

 which is a layer of coloured sensitive matter. 

 The movements of this organism are clearly 

 influenced by the stimulus of light, and its eye- 

 spots are essential to these movements. 



It is by no means difficult to follow the transi- 

 tional stages of the structures constituting the 

 eye-spots of Euglena and Volvox into the 

 forms these tissues have assumed in the ocelli of 

 the Medusoids and the eyes of Mammalia. The 

 pigmentary elements in each of these orders of 

 beings give precisely similar reactions to light, 

 and their dioptric arrangement is built up on 

 the same general plan ; so that we are disposed 

 to hold they originally proceeded from a 

 common ancestral stock. 1 The regular evolu- 

 tion of structures of this description could not 

 have been effected, much less maintained, by 



1 Comparative Anatomy of Animals, by Gilbert C. Bourne, 

 Vol. I., p. ig<t. 



