GENERAL ADAPTATION 23 



live in darkness have no sight ; and those which 

 have it may lose it, as happens with a certain 

 class of crawfish, whose organ of vision gets 

 atrophied to the point of losing it through living 

 outside the influence of light. 



The sense of sight is extremely curious and 

 instructive with regard to adaptation, and it is very 

 important for our object, if one bears in mind that 

 it is the sense with the most dynamic force, and 

 therefore it plays a great part in the cerebral 

 structure and the intelligence. 



With regard to the genesis of this sense, Eamon 

 y Cajal well observes that he is not satisfied with 

 Herbert Spencer's explanation of the formation of 

 the eye and its nerves by adaptation and selection 

 alone. And, in fact, as the first of these authors 

 says, how is the point of departure of this evolution 

 to be explained ? 



Thus, for example, we do not understand why 

 the coloured spots or rudimentary eyes of worms 

 (Turbellaria, Trematodes, etc.) are round, in couples, 

 and are exactly on that part of the skin which 

 covers the supra-oesophagic ganglia, and not upon 

 other corresponding nerve-centres ; or why, in the 

 course of time, there appeared in front of the 

 coloured spot and the underlying nerve nothing less 

 than an epidermic lenticular thickening, whose 

 radius, index of refraction, etc., seem calculated to 



