246 MAMMALS. 



Alpine species. There is a cui'ious circumstance in its geographical distribution, mentioned by 

 Mr. Darwin, viz. " That it has never been seen, fortunately for the inhabitants in Banda Oriental, 

 to the eastward of the river Uruguay ; yet in that province there are plains which appear admirably 

 adapted to its habits. That river has formed an insuj)crable obstacle to its migration, although 

 tlie broader barrier of the Parana has been passed, and the Viscacha is common in Entre Rios, 

 (the province between the two rivers), dii-ectly on the opposite shore of the Uruguay. Near 

 Buenos Aj'res these animals are exceedingly common."* 



OcTODONTiNA. These animals are not distantly allied to the Chinchillas, and their habitat is on 

 the whole similar ; their range being chiefly on the Andes, or along theii- base. Unlike most of the 

 Rodents which we have hitherto encountered, however, a goodly j)roportion of them are inhabitants of 

 Chili, on the west side of the Andes. That great chain has proved a barrier to the passage of a 

 large portion of the South- American Mammals ; and where sj^ecies of the same family occur on both 

 sides of it, Alpine species also usually occur upon it too. This is the case with the Octodontina ; 

 but it is not so with the Chinchilla, the only species in that group which is found beyond the 

 limits of the moimtains being the Viscacha, and it is found only to the east of the Andes. 



The distribution in this respect must throw some light upon the original habitat of such species, 

 but it still leaves it a very complicated question ; for, in the first place, the species may have first 

 taken its place on the movmtains, as in the case of species pushed towards the Equator by the 

 glacial epoch, and afterwards have sent species to the right or to the left, or to both ; or the original 

 ancestors may have first ajjpeared on the low ground on one side of the mountains, and sent ofi" 

 species up them, which may either have gone no further, or may have been developed into fresh 

 species on the other side, and afterwards either the original or the mountain off-shoot, or both, 

 may have been extinguished. In such uncertain ground it may help us in our conjectures if we can 

 lay down any general principles to guide us in the application of the facts. When different species 

 of the same family occur, both in lofty cold regions and in neighbouring warm lowlands, which 

 habitat is most likely to have been the original ? whether woidd the sj)ecies most probably spread, 

 from the cold to the hot or from the hot to the cold.? In the first place, I do not believe that 

 animals ever spread far or change their abodes unless on compulsion ; and, least of all, from a 

 warm to a colder one ; all inhabitants of cold climates have, I suspect, had the change forced 

 upon them either by a general change of climate, or by a gradual uj)heaval of the ground on which 

 they rested. The fiict that imtil a comparatively recent stage in the geological history of South 

 America the lowlands were under water, and that their appearance is due to the upheaval of the 

 Andes, which therefore must have been dry first, is an item of proof tending in the same 

 direction. 



But supposing my idea in this respect to be erroneous, is there any probabilitj' more in one 

 direction than the other ? or is it a matter of equal chance to either ? Other things being equal, 

 I should think it more likely that the change woidd proceed from the animal in the cold country 

 than from that in the warm. More provision of apparatus in the way of fat, fur, &c., is needed 

 to fit for life in a cold country than a warm one, and more energy and vis appears to be required 

 to make the change. Less decided action has to be taken, as it were, to enable life to be suited 

 to the warm country. It may seem that this must depend upon the constitution of the animal ; 



* Darwin, " Joxu-nal of a Naturalist," p. 124. 



