78 PORTILLO PASS. 



not again emerge from it that day. About noon, 

 finding pasture for the animals and bushes for fire- 

 wood 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 dif- 

 ference of longitude very trifling. The same re- 

 mark 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 oi 

 occasionally frequent elevated mountains, and cer- 

 tain 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 or- 

 ganic 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 



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

 down by My. 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 spe- 

 cies ; otherwise the difference in the species in the two regions 

 might be considered as superinduced during a length of time. 



