CAUSES OF VARIATION 133 



formed sensitive spots, while heat vibrations did not 

 do so? We know what light is, and we can judge 

 of its action, but we cannot imagine how it could 

 have originated the optic nerve nor the eye. How 

 could vibrations of the aether build up the lens, the 

 retina, and all the other parts of the eye? Eyes 

 have originated independently in the Arthropoda, 

 the Mollusca, and in the Vertebrata ; and in all 

 three they are situated as near the brain as possible. 

 This look as if the brain had something to do with 

 it. The brain must have taken advantage of the 

 vibrations and built up an organ capable of recog- 

 nising their different lengths. This was a mental 

 operation , and looks as if it were the result of intelli- 

 gence. For how else could three different branches 

 of the animal kingdom have found out that images 

 could be produced by a convex lens.? 



Also, could the action of water, either directly or 

 indirectly, originate or increase air-spaces in the 

 stems and leaves of water-plants ? Could electricity 

 originate electric organs in animals? Or could the 

 absence of light originate phosphorescent organs in 

 the deep-sea fishes? 



In alpine countries the plants often have small, 

 thick concave leaves, either closely pressed round 

 the stem, or with the margins recurved ; and these 

 are unquestionably adaptations to the climate. But 

 can we say that they were produced by the climate? 

 In the same alpine regions the rocks stand out in 

 high bare pinnacles, due to the wind and rain re- 

 moving all debris. But we do not say that the rocks 

 are adapted to the conditions ; because by the word 



