264 THE LOWER AMAZONS. Chap. VL 



similar to those where it exists under a constant 

 normal form. In these districts selection has not ope- 

 rated, or it is suitable to the conditions of life there pre- 

 vailing, that the species should exist under an instable 

 form. But in the adjoining moister forests, as the 

 result shows, the local conditions 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 entire absence of 

 these latter from an area whence they are now sepa- 

 rated only by a few miles, points to the conclusion that 

 they could not there maintain their ground. Those indi- 

 viduals of successive broods which were still better suited 

 to the new conditions would for the same reasons be 

 preferred over their relatives ; and this process going 

 forward for a few generations, the extreme form of H. 

 Thelxiope would be reached. At this point the race 

 became well adapted to the new area, which we may 

 suppose to have been at that epoch in process of forma- 

 tion as the river plains became dry land, at the last 

 geological changes in the level of the country. In the 

 higher and drier areas of Guiana and the neighbouring 

 countries, H. Melpomene has been the selected form ; 

 in the lower and more humid regions of the Amazons, 

 H. Thelxiope has been preferred. An existing proof of 

 this perfect adaptation is shown by the swarming abun- 

 dance of the species ; the derivation of H. Thelxiope 

 from H. Melpomene is made extremely probable by the 

 existence of a complete series of connecting links ; and 

 lastly, its permanent establishment is made evident by 

 its refusal to intercross with its parent form, or revert 



