go THE METHOD OF DARWIN. 



the appearance of organic beings on our earth, 

 and their disappearance from it, than any other 

 class of facts." 1 In passing southward over 

 the continent of South America, he recognized 

 another of Nature's classifications: he noticed 

 the frequent recurrence of the fact that a species 

 occupying a given region was replaced to the 

 southward by a closely related species, and this 

 serial arrangement impressed itself strongly 

 upon him. His visit to the Galapagos Islands 

 gave him an almost perfect example of sim- 

 plicity in the working of Nature's forces. The 

 conditions for one of Nature's classifications 

 were perfect. When Lawson, the Vice Gover- 

 nor, had declared to him that the tortoises from 

 the different islands differed from one another, 

 Darwin did not see the significance of the fact. 

 He mixed up his collections from the various 

 islands, and did not dream that there lay before 

 him one of the most remarkable facts that 

 Nature ever revealed to a naturalist. By some 

 happy accident he compared the many speci- 

 mens of mocking thrushes shot on Charles 

 Island with those from Albemarle Island, and 

 was astonished to find that they belonged to 

 different species. It was not the fact that 

 there were two species of mocking thrush liv- 



1 Naturalist's Voyage around the World, p. 173. 



