24 GEOGEAPHICAL DISTKIBUTION. 



water barriers project themselves into a given region. Thus, it will 

 not rarely be found that a genus of animals is represented by one or 

 several species on one side of a long mountain-slope, and by entirely 

 distinct species on the other. And, similarly, distinct species of a 

 genus may be encountered on opposite sides of a river-bed, although 

 instances of such a nature among the higher animals are probably 

 not of very frequent occurrence. Mr. "Wallace cites the case of cer- 

 tain species of Saki monkey (Pithecia), found on either side of the 

 Amazon River, whose range either southward or northward appears 

 to be limited by that stream. The same naturalist instances among 

 birds species of jacamar (Galbula) and trumpeter (Psophia) which 

 exhibit a similar limitation, particularly the latter, where five dis- 

 tinct species are relegated to as many distinct, but contiguous, geo- 

 graphical areas, separated from each other by the Amazon and some 

 of its tributaries (Negro, Madeira, Tocantins). Of about twelve 

 species of armadillo (separated by some naturalists into several dis- 

 tinct genera), most of which are inhabitants of Brazil, it would 

 seem that not a single species is common to Brazil and the Argen- 

 tine Republic, or the Argentine Republic and Paraguay, the Parana 

 River, with its tributaries, evidently forming an insurmountable 

 barrier to the passage of this animal. The Uruguay River appears 

 in the same way to limit the eastward progression of the viscacha 

 (Lagostomys trichodactylus), an animal allied to the chinchilla, 

 although, as has been pointed out by Mr. Darwin, the trans-Uru- 

 guayan plains are fully as well adapted to the animal as those of 

 its native home. 



Just as the boundaries of land-animals are in many instances 

 defined by the dominant river-courses, so, in a like manner, but in 

 a much more marked degree, the domains of fresh-water forms are 

 frequently circumscribed by the land surfaces bordering the waters 

 inhabited by them. This fact is beautifully exemplified in the geo- 

 graphical distribution of two American families of fluviatile mol- 

 lusks, the Strepomatida3, or American melanians, and the UnionidsB, 

 the fresh-water mussels, where the species of several genera, at 

 least in the Southern United States, are restricted in their habitats 

 to certain individual streams, to the exclusion of all others. In- 

 deed, it would appear that even in such aquatic forms a large river 

 may constitute an almost insuperable barrier to migration, as is 

 shown in the case of the Strepomatida? by the Mississippi (south of 



