THE GEOGRAPHICAL DISTRIBUTION OF MAMMALS 143 



from the south or from the Old World, which were unable to 

 gain a permanent foothold and speedily died out. At this 

 distance of time it is seldom, if ever, possible to explain why 

 a species which succeeded in reaching this continent could 

 not maintain itself, though the most probable assumption is 

 that the forms already in possession of the land were an in- 

 superable obstacle to the intruders. 



The rate of dispersal of a species into new areas may be 

 fast or slow, according as the conditions are more or less favour- 

 able. Newly introduced insect-pests, like the Gypsy and the 

 Brown-tailed Moths in New England, often spread with por- 

 tentous rapidity; and introduced mammals have frequently 

 taken possession of vast areas in a surprisingly short time. 

 One of the most remarkable of these cases is cited by Darwin. 

 "In the time of Sarmiento (1580) these Indians had bows and 

 arrows, now long since disused ; they then also possessed some 

 horses. This is a very curious fact, showing the extraor- 

 dinarily rapid multiplication of horses in South America. 

 The horse was first landed at Buenos Ayres in 1537 and the 

 colony being then for a time deserted, the horse ran wild ; in 

 1580, only forty-three years afterwards, we hear of them at 

 the Strait of Magellan ! " (" Voyage of a Naturalist," pp. 232- 

 233.) In this example, something must be allowed for human 

 agency, but even so, it is very surprising. 



In the case of lands newly raised above the sea and con- 

 necting formerly separated areas, it is necessary that they should 

 first be taken possession of by vegetation, before they can 

 become passable by animals, for the migration of mammals 

 from continent to continent is an entirely distinct phenomenon 

 froiji the annual migration of birds. The latter, though a fact 

 famiUar to every one, is an unexplained mystery, and it is some- 

 what unfortunate that the same term should be used for the 

 completely different process of the spread of mammals into 

 newly opened land. This spread is purely unconscious and is 

 due to the pressure of increasing numbers upon the means of 



