80 MAMMALS. 



those of the Amazon, there is nothing to hinder them, or more jDroperly their descendants (for the 

 journey would be one beyond the lives of many Monkeys), from doing so ; but as they journeyed 

 they must have been gradually rising above the level of the sea, for a;lthough the ascent is gentle 

 and imperceptible, still in hundreds of miles a small rise tells. By the time they have reached 

 the upper waters of the Eio Negro, they have got into the granitic high lands which extend 

 from Guiana across the soiirces of the Rio Negro towards the Andes, and having entered into 

 new conditions of existence, the alteration would begin to have its usual effect, and induce a change, 

 which would gradually end in producing a new sjDecies ; and if the species thus transmogrified 

 pursued its journey down the other bank, the change from high land to low land would again 

 operate, and a second change would take place, but, unlike our old metamorphosed friends in the 

 " Arabian Nights," the changeling would not be disenchanted back into its old form, but woidd 

 undergo a now change into a third species. So that, although the Guiana species may reach 

 the opposite shore by turning the sources of the river, it would have ceased to be itself by the 

 time it had done so. The facts which have been observed seem quite consistent with this hj-- 

 pothesis. Mr. Wallace says, " Towards the sources, rivers do not form a boundary between distinct 

 species ; but those found there, though ranging on both sides of the stream, do not often extend 

 down to the mouth."* And as instances he mentions the Ixiet that on the Upper Rio Negro and 

 its branches, are found the Callithrix torquatus, Nyctipithecus trivirgatus, and a species of 

 Jacchus, none of which inhabit the Lower Rio Negro or Amazon. Where they do extend on 

 both sides of the river the circmnstance may be due to one of Mr. Darwin's exceptional modes 

 of accidental colonization. 



There are two points relating to the limits of the South American Quadrumana, in which many 

 of our physical- geography maps are incorrect. Dr. Sclater f has' pointed out one of these, viz. the 

 inaccuracy in the limit of their northern range. This is given in Johnston's Phj^sical AtlasJ by a 

 line drawn across Honduras, which is supposed to mark the northern limit of Mycetes seniculus. 

 Dr. Sclater has shown by reference to authorities both published and unpublished, the most important 

 of which is a communication from M. Auguste Salle, that the line must bo drawn considerably 

 to the north of this, viz. in the neighbourhood of Tampico, or about 23^ of north latitude : in other 

 words, the tropic of Cancer, which is as nearly as jjossible the northern limit of the quadrumana in 

 the Old World. WTiat species are found in this part of their range is not exactly known, but 

 some of them would appear to be the species which are most widely distributed to the south. 

 Dr. Sclater has thrown out the suggestion that some singidar new species may yet be found in 

 these regions of Central America, on the ground that in birds he has foimd it " a general rule that 

 this northern portion of the great South American (his neotropical) region possesses specifically 

 distinct rejDresentatives of all the more important groups which characterise the ornithology of 

 tropical South America, and that it not unfrequently happens that these northern outliers of the 

 genus are the finest in colouring, and the most outre or exaggerated in form, of the whole group." 

 No confirmation of this conjecture has yet been obtained, but it must add an intelligent interest 

 to future explorations in that country to see whether the rule observed by Dr. Sclater in birds 

 also holds good in other classes of beings. As yet we can scarcely say that it holds good in mammals. 



The othei' point which I do not observe correctly stated in any Pliysical Atlas is the western 



* Wallace, op. cit. J The map in Schmakija's " Geographisclie Verbrei- 



t Sclater, in "Nat. Hist. Rev." Oct. 18C1, p. 507. tung," is more correct. 



