838 



THE AMERICAN NATURALIST 



[Vol. XL 



As regards this principle, Wallace observes: "There is good 

 reason to believe that the most effective agent in the extinction of 

 species is the pressure of other species, whether as enemies or 

 merely as competitors." Lyell ^ observes : " Extension of the range 

 of one species alters that of others. In reference to the extinction 

 of species it is important to bear in mind, that when any region 

 is stocked with as great a variety of animals and plants as its 

 productive powers will enable it to support, the addition of any 

 new species to the permanent numerical increase of one previously 

 established, must always be attended either by the local extermi- 

 nation or the numerical decrease of some other species." 



Exception must be taken to the sweeping character of these 

 statements: First, because the eliminating action of a change in 

 plant life may have been the real cause of extinction in several 

 cases where competition with other mammals is the apparent 

 cause. For example the extinction of the Titanotheriidse and 

 P^lotheriidje may have been entirely due to changes in vegetation, 

 rather than to competition with any other Herbivora. 



Second, because the survival of the opossums (Didelphiida*) 

 in North America shows there may be striking exceptions to this 

 principle. 



The conclusion drawn from such exceptions is that of Darwin, 

 namely, that the keenest competitors are the animals of Tnost nearly 

 similar feeding habits. There are, however, exceptions to Darwin's 

 conclusion also, as the following instances prove. 



Destruction of Food Supply by Smaller Browsing Animals. — 

 The enormous changes in the quadruped life of the district en- 

 circling the eastern and southern sides of the Mediterranean are 

 popularly attributed to secular changes of climate. Haan shows, 

 liowever, that evidence for secular change of climate within the 

 historic period is insufficient or actually negative. Parts of these 

 n><i:i()ns were tornicrly iiihal)itc(l by some of the larger quadrupeds 

 which hiivc since disappeared through the agency of man; it is 

 (■<Hially true that the country could not support the life of these 

 (|uadrupcds at the present time. There can be little doubt that 

 the change in soil and vegetation has been indirectly caused bv 



' Lyell. C. Principles of Geology, vol. 2, 1872, p. 451. 



