946 CHARLES DARWIN 



brilliantly white sea of clouds was stretched out beneath our 

 feet, shutting out the view of the equally level Pampas. We 

 soon entered the band of clouds, and did not again emerge 

 from it that day. About noon, finding pasture for the ani- 

 mals and bushes for firewood at Los Arenales, we stopped 

 for the night. This was near the uppermost limit of bushes, 

 and the elevation, I suppose, was between seven and eight 

 thousand feet. 



I was much struck with the marked difference between 

 the vegetation of these eastern valleys and those on the 

 Chilian side: yet the climate, as well as the kind of soil, is 

 nearly the same, and the difference of longitude very trifling. 

 The same remark holds good with the quadrupeds, and in 

 a lesser degree with the birds and insects. I may instance the 

 mice, of which I obtained thirteen species on the shores of 

 the Atlantic, and five on the Pacific, and not one of them 

 is identical. We must except all those species, which habitu- 

 ally or occasionally frequent elevated mountains; and cer- 

 tain birds, which range as far south as the Strait of Magel- 

 lan. This fact is in perfect accordance with the geological 

 history of the Andes; for these mountains have existed as 

 a great barrier since the present races of animals have 

 appeared ; and therefore, unless we suppose the same species 

 to have been created in two different places, we ought not to 

 expect any closer similarity between the organic beings on 

 the opposite sides of the Andes than on the opposite shores 

 of the ocean. In both cases, we must leave out of the ques- 

 tion those kinds which have been able to cross the barrier, 

 whether of solid rock or salt-water. 6 



A great number of the plants and animals were absolutely 

 the same as, or most closely allied to, those of Patagonia. 

 We here have the agouti, bizcacha, three species of arma- 

 dillo, the ostrich, certain kinds of partridges and other birds, 

 none of which are ever seen in Chile, but are the character- 

 istic animals of the desert plains of Patagonia. We have 

 likewise many of the same (to the eyes of a person who is 



5 This is merely an illustration of the admirable laws, first laid down 

 by Mr. Lyell, on the geographical distribution of animals, as influenced 

 by geological changes. The whole reasoning, of course, is founded on the 

 assumption of the immutability of species; otherwise the difference in the 

 species in the two regions might be considered as superinduced during a 

 length of time. 



