ON THE VARIATION OF SPECIES. Vo 



entire absence of these latter from an area whence they are 

 now separated by only a few miles, points to the conclusion 

 that they could not there maintain their ground. Those in- 

 dividuals 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, tlie extreme form of S. 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 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 adap- 

 tation is shown by the swarming abundance of the species ; 

 the derivation of H. Thelxiope fi-om H. 3Ielpomene 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 to its former shape when brought by natural redis- 

 tribution into contact with it. 



If this explanation of the derivation of Hellconius 

 Thelxiope be true, the origination by natural process of a 

 host of now distinct allied species of this genus, as well as, in 

 fact, all other genera containing numerous closely related 

 species, will have to be admitted. 



A species allied to //. Thelxiope, namely, II. Vesta, seems 

 to have been derived also from H. Melpomene, for amongst 

 the numerous varieties already mentioned are many examples 

 intermediate between the two. There is this difference, how- 

 ever, between H. Thelxiope and H. Vesta; the former is 



