454 Zoological Society. 



are on the boundary of two distinct zoological districts, and we have 

 nothing to tell us whether the one came from the north or south of 

 the Amazon, or the other from the east or the west of the Andes. 

 Owing to this uncertainty of locality, and the additional confusion 

 created by mistaking allied species from distant countries, there is 

 scarcely an animal whose exact geographical limits we can mark out 

 on the map. 



On this accurate determination of an animal's range many interest- 

 ing questions depend. Are very closely allied species ever separated 

 by a wide interval of country ? What physical features determine 

 the boundaries of species and of genera 1 Do the isothermal lines 

 ever accurately bound the range of species, or are they altogether 

 independent of them ? What are the circumstances which render 

 certain rivers and certain mountain ranges the limits of numerous 

 species, while others are not ? None of these questions can be satis- 

 factorily answered till we have the range of numerous species accu- 

 rately determined. 



During my residence in the Amazon district I took every oppor- 

 tunity of determining the limits of species, and I soon found that 

 the Amazon, the Rio Negro and the Madeira formed the limits be- 

 yond which certain species never passed. The native hunters are 

 perfectly acquainted with this fact, and always cross over the river 

 when they want to procure particular animals, which are found even 

 on the river's bank on one side, but never by any chance on the 

 other. On approaching the sources of the rivers they cease to be a 

 boundary, and most of the species are found on both sides of them. 

 Thus several Guiana species come up to the Rio Negro and Amazon, 

 but do not pass them ; Brazilian species on the contrary reach but 

 do not pass the Amazon to the north. Several Ecuador species 

 from the east of the Andes reach down into the tongue of land be- 

 tween the Rio Negro and Upper Amazon, but pass neither of those 

 rivers, and others from Peru are bounded on the north by the Upper 

 Amazon, and on the east by the Madeira. Thus there are four 

 districts, the Guiana, the Ecuador, the Peru and the Brazil districts, 

 whose boundaries on one side are determined by the rivers I have 

 mentioned. 



In going up the Rio Negro the difference in the two sides of the 

 river is very remarkable. 



In the lower part of the river you will find on the north the Jac- 

 chus bieolor and the Brachyurus Couxhc, and on the south the red- 

 whiskered Pithecia. Higher up you will find on the north the 

 Ateles paniscus, and on the south the new black Jacchus and the 

 Lagothrix Humboldtii. 



Spix, in his work on the monkeys of Brazil, requently gives, 

 " banks of the river Amazon " as a locality, not being aware appa- 

 rently that the species found on one side very often do not occur on 

 the other, though the fact is generally known to the natives. In 

 these observations I have only referred to the monkeys, but the same 

 phaenomena occur both with birds and insects, as I have observed 

 in many instances. 



