8o THE SEA AS A BARRIER chap. 



company on the highlands of the Cordillera in Peru and Ecuador, 

 but as we go farther south the latter are found on the plains of 

 southern Argentina and Patagonia, as well as on the island of 

 Tierra del Fuego at the sea level. Here then is a clear proof 

 of the intimate connexion existing between temperature and 

 station ; the guanaco being an animal which can only live in 

 cold or temperate climates, finds suitable conditions for its 

 existence in tropical latitudes solely at a height of so many 

 thousands of feet, althougli farther south it is able to thrive at 

 the sea level." This, however, cannot be pushed too far — the 

 world cannot be mapped out into areas bounded by parallels of 

 temperature as was once attempted — since there are plenty of 

 cases like that of the Tiger, which is as much at home in a 

 tropical jungle as on the icy plains of Northern Asia. 



Seeing that there are in many cases no climatic barriers to 

 the spreading of a given race of animals over a larger area of 

 distribution than it actually occupies, it becomes important to 

 inquire why there are so many cases of restriction in range. 



It is possible to see, at any rate, three causes which are 

 responsible for a large number of such cases. In the first place, 

 a given species of animal must have originated at a certain spot ; 

 its multiplication in individuals must always be a slow matter, 

 since enemies, and untoward events generally, would conspire 

 to check the natural multiplication by geometrical progression, 

 A long time might therefore elapse before the species greatly 

 extended its range. A restricted distribution may therefore, 

 in some cases, mean a modern race. In the second place, 

 there are definite physical barriers which check the migra- 

 tion of species. The terrestrial Mammalia cannot cross wide 

 arms of the sea ; that they can and do swim for considerable 

 distances has been proved in several instances ; but, as has 

 been pointed out, it is unlikely that a purely terrestrial mam- 

 mal would voluntarily swim out into an unknown sea. And 

 then if it did, and successfully reached the opposite side, 

 nothing would happen unless it were a pregnant female ; or, if 

 not pregnant, till a male swam very soon afterwards in exactly the 

 same direction. Many travellers have told of floating islands, 

 formed of torn-up trees and brushwood, which have been seen 

 at the mouths of large rivers, with animal passengers upon them. 

 These are, however, so much at the mercy of currents and storms, 



