1/6 



THE LOWER AMAZONS 



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. Melpomene both existing under the same local 

 conditions ; and how account for the diversified modi- 

 fications 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 opera- 

 tion 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 

 Heliconius Melpomene having been rendered vaguely 

 instable by the indirect action of local conditions dis- 

 similar 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 moist er 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 separated only by a few 

 miles, points to the conclusion that they could not there 

 maintain their ground. Those individuals of successive 

 broods which were still better suited to the new con- 

 ditions 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 formation 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 



^ As the action of external influences would be on the early 

 states of the insects and not on the adults, it is well to mention 

 that the broods of the Heliconii appear to be social ; the larvae 

 feeding together and undergoing their last transformation on 

 the same tree. This I observed with regard to the H. Erato, 

 a species closely allied to H. Thelxiope. 



