EXCURSIONS OF AN EVOLUTIONIST 



any species, and never will be. The habitable 

 surface of the earth has been perpetually chang- 

 ing for a hundred million years, and the relations 

 between the countless groups of organisms that 

 have covered its surface have been perpetually 

 changing in endless degrees of complexity ; and 

 in such a world, under the working of natural 

 selection, there can be no such thing as " fixity 

 of species." 



Having arrived at these grand conclusions, it 

 became comparatively easy for Mr. Darwin to 

 go on and trace the workings of natural selection 

 in many special instances. In these inquiries, 

 upon which he brought to bear a knowledge of 

 the details of organic life more vast and multi- 

 farious than has ever been possessed by any 

 other man, he occupied nearly a quarter of a 

 century before it seemed to him that the time 

 had come for making his discovery known to 

 the world. In 1 844, he wrote out a brief sketch 

 of the conclusions which, as he modestly says, 

 " then seemed to me probable ; " and this sketch 

 he showed to his friend Hooker, perhaps also 

 to Lyell. But fifteen years more, rich in ob- 

 servation and reflection, passed away, and still 

 the world had heard nothing about the origin 

 of species by means of natural selection. How 

 much longer this silence might have lasted, had 

 not an unforeseen circumstance come in to 

 break it, one cannot say. But no doubt it would 

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