ME. R. SPETJCE ON INSECT-MIGRATIOlSrS IN SOUTH AMERICA. 351 



whicli grows everywhere on lands flooded by the Amazon, and, 

 from its accessibility and the readiness with which its wood burns 

 while green, supplies a gi'eat part of the fuel consumed by the 

 steamers that navigate the Amazon. It is almost equally common 

 on some of the white-water tributaries ; I have seen it, for in- 

 stance, far away up the Huallaga to the south, and up the Pastasa 

 to the north. Two of the commonest river-side Ingas of the 

 Amazon ( Z splendens, "W., and I. corymbifera, Benth.) reappear to- 

 gether on the Upper Casiquiari and Orinoco ; and similar instances 

 might be multiplied indefinitely. 



Streams of black or clear water have also their proper riparial 

 vegetation, some species being apparently repeated on all of them. 

 For example, many of the trees of the inundated margins of the 

 Tapajoz (some of them undescribed when I first gathered them) 

 I found afterwards on the E.io Negro up to its very sources — 

 although none of them inhabit the shores of the Amazon, either 

 between the mouths of those two aj09.uents or elsewhere. A few 

 recur on the TleSe and other black- water streams entering still 

 further to the west, and even on- similar affluents of the Orinoco. 



Here, at least, would seem to be a case of the vegetation de- 

 pending on the distribution of the running waters ; but in reality 

 both the kind of water and the vegetation nourished by it depend 

 entirely on the nature of the soil, those rivers which run chiefly 

 through soft alluvial bottoms being turbid, while those that have 

 a hard rocky bed run clear ; and the two classes of rivers are re- 

 peated over and over throughout the length and breadth of the 

 Amazon region. Into the black Eio Negro runs that whitest of 

 rivers, the E-io Branco, and imparts to the vegetation of the for- 

 mer, for a little way below their confluence, quite an Amazonian 

 character*. The two largest tributaries of the Casiquiari, namely 

 the Pacimoni and the Siapa, run nearly parallel through a longish 

 course, and at rarely more than fifteen miles apart ; yet the former 

 has clear dark water, and the latter is excessively muddy. More- 

 over, when I explored the Pacimoni to its very sources, I found 

 it' divide at last into two nearly equal rivulets, whereof the one 

 had white and the other black water. The true riparial vegeta- 

 tion in all these, and in hundreds of other cases, is invariably 

 modified after the same fashion by the colour of the waters. How 

 it became what it is, and how it came there at all, are questions 

 not to be discussed here. 



* Here, for instance, is. the only locality throughout the Rio Negro for Boni' 

 bax Munguba, a fine silk-cotton tree abounding on the Amazon. 



