THE FACTOES OF EVOLUTION 



CHAPTER I 



All the world, or at any rate all that portion of the 

 world which has a sufficient knowledge of the facts, is 

 practically agreed that the plant and animal kingdoms 

 have arisen by evolution ; but all the world is not agreed 

 as to what have been the factors in that evolution. It is 

 not now disputed that natural selection, by perpetuating 

 and accumulating inborn favourable variations, has been 

 a cause of evolution, but it is contended by many biolo- 

 gists, some of whom are of the greatest eminence, that 

 the accumulation of inborn variations has not been the 

 sole nor even the principal cause of evolution; but that 

 the accumulation of acquired variations has been an 

 additional and even the principal cause. To take an 

 example ; hares run swiftly, their rapid pace being due 

 to structural evolution from a slower ancestry. By the 

 one school of biologists it is held that the great speed 

 of hares is due to their having lived under conditions 

 which permitted only the naturally swift to survive, so 

 that while those animals that were naturally slow 

 perished, the others continued the race ; the other 

 school, while admitting that the evolution of speed in 



hares may have been partly so caused, contend that it 



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