DISPERSAL OF SPECIES: 19 



statement of native hunters, that the river does limit ranges, and out of many other instances 

 gives the case of two remarkable species of monkey, Brachyurus CALvrs and B. RrBicuxDrs, 

 which are found abundantl)' in Ygajio Forest, north of the Ujjper Amazons west of Japura, and 

 altliough there are hundreds of miles of the same sort of forest on the south bank, no instance 

 is known of either of these monkeys having been seen there. 



Mr. Darwin mentions that the Bizcacha has never been seen to the eastward of the Eiver 

 Uruguay. "Yet in this province there are plains which appear admirably adapted to its 

 I'.abits. The Uruguay has formed an insviperablc obstacle to its migration, although the broad 

 barrier of the Parana has been passed, and the Bizcacha is common in Entre Rios, the province 

 between these great rivers."* 



Even birds are subject to the same law, althoiigh it cannot be expected that rivers should often 

 limit their ranges. Mr. Wallace mentions several instances, some of which, as the birds are of a 

 kind whose flight is short, are not so remarkable, but others are not open to that objection ; for 

 example, — 



" The fine blue Macaw (Macrocercus Maximili.\nus) inhabits the borders of the hilly country 

 south of the Amazon, from the sea-coast probably up to the Madera. Below Santarem it is sometimes 

 found close up to the banks of the Amazon, but is said never to cross that river. Its head-quarters 

 are the U2)per waters of the Tocantins Xingu and Tapagoz rivers. 



" iijiother instance of a bird not crossing the Amazon is the beautiful Curl-crested.Aracari (Ptero- 

 GLOssus Beauharnasii), which is found on the south side of the Upper Amazon, opposite the Rio 

 Negro, and at Coari and Ega, but has never been seen on the north side. The green Jacamar 

 of Guiana, also (Galbula viridis), occurs all along the north bank of the Amazon, but is not found 

 on the south, where it is replaced by the G. cyaxocollis and G. maculicauda, both of which occur 

 in the neighbourhood of Para." f 



Other facts to the same effect are mentioned by 'Mr. "Wallace and Mr. Bates regarding insects 

 and the lower animals. Similar instances can be given from the Niger, the Lena, and other great 

 rivers. But enough has been said to show that the fact is not exceptional. 



These are cases showing that under the most favourable and iu^itLng circumstances, colonization, 

 immigration, or occasional visitation, has not taken place. If it has failed in these, with what logic 

 can we be asked to admit its being the ride under less favoured circumstances? 



In like manner moimtains, deserts, woods, and plains, all act as barriers of separation or 

 bridges of communication, according as they are fitted or not for the occupancy of the animal. 



On the other hand, where the sea between neighboiiring lands is bridged over in ^-inter by 

 ice, it often, although not always, ceases to be a barrier. In Captain M'Clintock's journal, 

 various incidental notices occur, showing that a general migration of the polar animals over the 

 ice takes place on the breaking up of winter.* 



* Darwin, "Journal of Researches." Second edition, be some land in the north-east or east. Jfay 0th. — 



1845, p. 124. Dehghtful weather; tufts of moss and the tops of 



t Wallace, Alfred, " Narrative of Travels on the Ama- stones are gradually peeping up through the snow, and 



zon and Rio Negro." London, 1853. animals begin to appear. I picked up two catcrpil- 



X The following are examples of this. "April 21st, lars to-day. Fox-tracks are very numerous; the ma- 

 temperature 8°. Whilst crossing a bay, and about a jority of these tracks are crossing to the north-westward, 

 mile oft" the laud, we passed four separate tracks of probably following the ptarmigan. Lemmings are abun- 

 lemmings, travelling in for the laud. If these little crca- dant, and these wonderful little creatures, which consti- 

 tures are migrating across the ice, there would seem to tute the chief support of foxes, ravens, wolves, owls, and 



