THE GEOGRAPHICAL DISTRIBUTION OF MAMMALS 139 



or barriers that prevent the indefinite spread of terrestrial 

 mammals, so that the mammalian fauna of the whole earth, 

 and even of a single continent, is not uniform, but highly 

 variegated. The rate of multiplication of animals is so rapid 

 that, under normal conditions, the animal population is always 

 pressing hard upon the means of subsistence and every species 

 that is increasing in numbers must constantly extend its range 

 in search of food. Every species would increase indefinitely, 

 if there were no countervailing checks. Were all the young 

 to survive and breed in their turn, ''even large and slow-breed- 

 ing mammals, which only have one at a birth, but continue 

 to breed from eight to ten successive years, may increase 

 from a single pair to 10,000,000 in forty years " (Wallace). 

 Evidently, a species must spread from its place of origin until 

 stopped by insuperable obstacles, the most obvious of which 

 are wide seas. A few land mammals are not only excellent 

 swimmers, but will cross straits without hesitation, as the 

 Guanaco has been seen to swim the Straits of Magellan ; for 

 the great majority, however, a very few miles of sea form an 

 impassable barrier. As was shown above, a broad or deep river 

 is sufficient to limit many species, as the Santa Cruz River 

 in Patagonia marks the southern boundary of the armadillos. 



Important geographical changes, such as the joining of 

 lands that before were separate, or the dividing of continuous 

 lands by transgressions and incursions of the sea, must neces- 

 sarily have a profound effect upon the distribution of land 

 mammals. Separated land-areas, however similar may have 

 been their faunas at the time of separation, will, through the 

 operation of the divergent evolutionary process, grow more 

 unlike in proportion to the length of time that the separation 

 continues. Regions which have been severed within a short 

 time (in the geological sense of a short time) are zoologically 

 very similar or even identical, while those that have long been 

 isolated are correspondingly peculiar. Attention has already 

 been called, in another connection, to the contrasted cases 



