92 ON THE VARIATION OF SPECIES. 



1)0 remarkable modification throughout all that area, some 

 slight variations only occurring at the extreme points of it. 

 If local conditions, acting directly on individuals, had orio-i- 

 nally produced this race or species, they certainly would 

 have caused much modification of it in different parts of this 

 region ; for the upper Amazons country differs greatly from 

 the district near the Atlantic in climate, sequence of seasons, 

 soil, forest clothing, periodical inundations and so forth. 

 These differences, moreover, graduate away, so that the 

 species is subjected to a great diversity of physical conditions 

 from locality to locality, and ought in consequence to present 

 an endless series of local varieties, on the view mentioned, 

 instead of one constant form throughout. Besides, how 

 should we explain the fact of H. Thelxiope and H. Melpo- 

 mene both existing under the same local conditions ; and 

 how account for the diversified modifications presented in 

 one and the same locality, as at Serpa and on the Tapajos? 



There is evidently, therefore, some more subtle agency at 

 work in the segregation of a race than the direct operation of 

 external conditions. The principle of natural selection, as 

 lately propounded by Darwin, seems to offer an intelligible 

 explanation of the facts. According to this theory, the 

 variable state of the species exhibited in the districts above 

 mentioned would be owing to JTe/iconius 3fefpomene having^ 

 been rendered vaguely instable by the indirect action of local 

 conditions dissimilar to those where it exists under a constant 

 normal form. In these districts selection has not operated, 

 or it is suitable to the conditions of life there prevailing that 

 the species should exist under an instable form. But in the 

 adjoining moister forests, as the result shows, the local con- 

 ditions were originally more favourable to one of these 

 varieties than to the others. The selected one, therefore, 

 increased more rapidly than its relatives; and the fact of the 



