ORGANIC EVOLUTION — THE FACTORS 97 



advantages to the individuals distinguished by them." 

 Did such a variation occur as an abnormality it would 

 soon be swamped by interbreeding. But if under stress 

 of scarcity the herd were compelled to supplement its 

 diet with the leaves of the hitherto uneaten plant, it is 

 easy to understand that, other things equal, evolution 

 would occur through the continual survival of those 

 that digested the plant best — and we have no reason to 

 suppose that the other things, keenness of scent, agility, 

 &c., would not on the average be equal, but every 

 reason to suppose that they would. Peculiarities like 

 the above do not spring into the world like Minerva, 

 fully developed, but arise by a slow process of growth. 



Mr. Spencer next discusses the question of retro- 

 gression — such retrogression, for example, as is exhibited 

 by the eyes of the proteus, an animal now inhabiting 

 dark caverns, but evidently descended from ancestors 

 that lived in the light of day, and possessed well-deve- 

 loped eyes. As usual there are two principal theories 

 whereby it is sought to account for this retrogression ; 

 one of which attributes it to the cessation of selection, 

 the other in whole or part to the cessation of use ; the 

 one to the progressive lapsing, in consequence of the 

 withdrawal of the preserving influence of selection, of 

 those inborn variations, the accumulation of which, as 

 is supposed by the supporters of this theory, resulted in 

 the fully-developed ancestral eye ; the other in whole 

 or part to the progressive lapsing, in consequence of 

 the withdraAval of the preserving influence of use, of 

 those acquired variations, the accumulation of which, 

 as is supposed by the supporters of this theory, resulted 

 in the fully-developed eye. These are the two main 

 positions, but a subsidiary question is — Has Natural 

 Selection — reversed Natural Selection, as it is called in 

 this case — aided Cessation of Selection, or Cessation of 

 Use, in causing retrogression in the eyes of animals to 



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