378 MY LIFE 



occur on both banks of the river. In only one case do you 

 specially mention a species being found only on the north 

 bank. In other cases, except where the insect is local and 

 confined to one small district, no one can tell whether they 

 occur on one or both banks. Obydos you only mention once, 

 Barra and the Tunantins not at all. I think a list of the 

 species or varieties occurring on the south bank or north 

 bank only should have been given, and would be of much 

 interest as establishing the fact that large rivers do act as 

 limits in determining the range of species. From the localities 

 you give, it appears that of the sixteen species of papilio 

 peculiar to the Amazon, fourteen occur only on the south 

 bank ; also, that the Guayana species all pass to the south 

 bank. These facts I have picked out. They are not stated 

 by you. It would seem, therefore, that Guayana forms, hav- 

 ing once crossed the river ? have a great tendency to become 

 modified, and then never recross. Why the Brazilian species 

 should not first have taken possession of their own side of 

 the river is a mystery. I should be inclined to think that 

 the present river bed is comparatively new, and that the 

 southern lowlands were once continuous with Guayana ; in 

 fact, that Guayana is older than north Brazil, and that after 

 it had pushed out its alluvial plains into what is now north 

 Brazil, an elevation on the Brazilian side made the river 

 cut a new channel to the northward, leaving the Guayana 

 species isolated, exposed to competition with a new set of 

 species from further south, and so becoming modified, as 

 we now find them. . . . The whole district is, I fear, too 

 little known geologically to test this supposition. The moun- 

 tains of north Brazil are, however, said to be of the cretaceous 

 period, and if so their elevation must have occurred in 

 tertiary times, and may have continued to a comparatively 

 recent period. Now if there are no proofs of such recent up- 

 heaval in the southern mountains of Guayana, the theory 

 would thus far receive support. I regret that your time was 

 not more equally divided between the north and south banks, 

 but I suppose you found the south so much more productive 

 in new and fine things. 



