266 INTRODUCTION TO EVOLUTION 



transported from island to island across intervening ocean. Methods by 

 which such transport might occur are discussed in the following chapter. 

 "Island hopping" involves a large element of chance. The chances are 

 against the dispersal of any terrestrial animal in this manner. As we have 

 seen (p. 191 ), marsupial and placental mammals arose at about the same 

 time. Why did the marsupials and not the placentals reach Australia 

 originally? Simpson (1953) has suggested that this outcome was determined 

 by chance. Once having chanced to arrive, the early marsupial immigrants 

 were afforded opportunity for the remarkable adaptive radiation described 

 above. 



Until man began his introduction of placental mammals, among the 

 latter only fliers, bats, and such accomplished stowaways as rodents had 

 been able to reach the island continent. Even the rodents were few in num- 

 ber. Simpson (1943) stated that their immigration could not have occurred 

 before the Oligocene; most probably it began in the Miocene, with occa- 

 sional rodent immigrants arriving at later dates. At any rate, these rodent 

 immigrations must have been unaided by any direct land connections. 



Some reader may suspect that the reason higher mammals were not 

 more abundant in Australia was because this continent was not suited to 

 them. We should point out, therefore, the marked success some species of 

 placental mammals have had following introduction by man. Rabbits, for 

 example, introduced by man, have in some regions increased in numbers 

 so prodigiously as to become a serious economic liability, as well as to cause 

 extinction in those regions of marsupials dependent upon the food supply 

 the rabbits have successfully monopolized. A similar situation prevails in 

 New Zealand, where animals introduced originally for sport have thrived 

 so mightily that the forest is being destroyed. The worst offender is the 

 European red deer. Clark (1949) estimated that in 1942 there were at least 

 90,000 of them on South Island, another 10,000 grazers being composed col- 

 lectively of fallow deer, Virginia deer, thar (a beardless wild goat from 

 the Himalayas), chamois, wapiti, and moose. Wild pigs, goats, and sheep 

 add to the destruction. In some regions overpopulation is so great that the 

 forest resembles a trampled cattle yard, all young growth being destroyed 

 and even adult trees sufi'ering devastation. Evidently, then, inaccessibility 

 rather than unsuitability is the key to explanation of the original unique 

 fauna in the Australian region. 



To a considerable extent the same explanation applies to the peculiarities 

 of the South American fauna. The connection of this continent to North 

 America is the tenuous Isthmus of Panama. At various times in geologic 

 history that isthmus has been submerged, leaving South America isolated 



