146 



and therefore, unless we suppose the same species to 

 have been created in two different places, we ought not 

 to expect any closer siiflilarity 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*." 



Conceding, therefore, this distinction between barriers 

 of a primseval and more recent character, it is not diffi- 

 cult to understand why the opposite sides of an alpine 

 chain, as well as countries separated by the sea, shoiild 

 display different phsenomena from each other. On the 

 contrary indeed, j£ we could feel satisfied that no means 

 of accidental transportation had operated to take them 

 there, and that the animals themselves were incapable of 

 enduring great diversities of temperature, and other con- 

 tingencies ; we should be startled to di-scover creatures 

 specifically identical in such regions, — so long at least as 

 the doctrine of unique centres of radiation formed part 

 of our zoological creed. We must not, however, be too 

 hasty ia questioning (if I may be pardoned for the com- 

 pletion of a metaphor of which I thoroughly disapprove) 

 this article of our faith, through the occurrence of simi- 

 lar beings in areas between which there exist barriers, 

 both primary and weU-defined; for the methods of 

 diffusion are so complicated and numerous, that, even 

 where human agency (that most important of elements) 

 * Journal of Researches, pp. 326, 327. 



