1835.] PORTILLO PASS. olo 



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 habitually or occasionally frequent elevated mountains ; and 

 certain birds, which range as far south as the Strait of Magellan. 

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

 whether of solid rock or salt-water.* 



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 armadillo, the ostrich, 

 certain kinds of partridges and other birds, none of which are ever 

 seen in Chile, but are the characteristic animals of the desert plains 

 of Patagonia. We have likewise many of the same (to the eyes of 

 a person who is not a botanist) thorny stunted bushes, withered 

 grass, and dwarf plants. Even the black slowly crawling beetles 

 are closely similar, and some, I believe, on rigorous examination, 

 absolutely identical. It had always been to me a subject of regret, 

 that we were unavoidably compelled to give up the ascent of the S. 

 Cruz river, before reaching the mountains : I always had a latent 

 hope of meeting with some great change in the features of the 



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

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

 l>y 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. 



