354 



♦ 



CHAPTER XXXIX. 



ON THE MIGRATION AND DIFFUSION OF TEEEESTEIAL ANIMALS 



MIGEATIOK OF QUADRUPEDS MIGRATORY INSTINCTS DRIFTING OF ANIMALS 



ON ICE-FLOES MIGRATION OF BIRDS MIGRATION OF REPTILES INVOLUN- 

 TARY AGENCY OF MAN IN THE DISPERSION OF ANIMALS. 



Migration of quadhufeds. — Before we consider the geo- 



graphical distribution of aquatic animals, it may be useful to 

 enquire what facilities the terrestrial species enjoy of spread- 

 ing themselves over the surface of the earth. The tendency 

 of each species to multiply is so great, that unless checked 

 it would soon extend its range over as wide an area as is ac- 

 cessible to it. Whether it feed on plants or prey on other 

 animals, it will not cease to enlarge the boundaries of its 

 habitation until its progress is arrested by some rival species 

 better fitted to the soil, climate, and organic conditions of 

 the country ; or by some lofty and unbroken chain of moun- 

 tains which it cannot scale, or by a desert, or the sea, or by 

 cold or heat, or some other barrier. 



Mr. Wallace and Mr. Bates have shown that largfe rivers 

 such as the Amazons and Eio Negro are capable of forming 

 effective barriers to the farther spread of many species of 

 monkeys. This happens even where the same kind of forest 

 occurs on the opposite banks. Mr. Darwin also mentions 

 that the biscacha, a rodent somewhat resembling a large 

 rabbit, which abounds in the Pampas, although it has crossed 

 the broader river Parana, has never been able to extend its 



range 



across the Uruguay. 



Geology 



teaches us that the 



present continents have been formed by the union of large 

 pre-existing islands ; and what were formerly straits of the 

 sea have often become, under a new arrangement of the land, 

 broad valleys and the channels of great rivers such as the 

 AmazonSj the Orinoco, and the La Plata. It is therefore 







i 



\ 



I 



I 



I 



