No. 480] 



EXTINCTION OF MAMMALIA 



837 



may have attacked various types of quadrupeds and resulted in 

 extermination before natural immunity was acquired. 



Living EnvironiMent. Competing and Hostile Mammalia 



From the struggle with physical environment, with the living 

 plant and insect environment, we now pass to the struggle with 

 other mammals. 



In the Tertiary of North America we witness: 



(a) the rapid multiplication of certain local or native mammals; 



(b) the repeated introduction by migration of new mammals, 

 coming either singly or in waves; 



(c) the slowly or rapidly sequent extinction of certain local 



Even considering the disastrous effects of glaciation and of 

 desiccation this competition, because it has worked more widely 

 and over longer periods of time, has been a tremendous agency 

 of extinction. 



Competition of Lower and Higher Types. — Of marsupials in 

 competition with rodents in Australia Spencer* observes: "In 

 the case of such smaller marsupials as, for example, species of 

 Sminthopsis in which the number of young produced at a birth is 

 from eight to ten and there are at least two broods in eacli year it 

 is a matter of considerable surprise tiiat they are not nuicli more 

 numerous than they are. The explanation is probably associated 

 with the fact that there is a consideaable length of time during 

 which not only does the capture of the mother result in her destruc- 

 tion and in that of all the young ones [by birds of prey, for example], 

 but that during this period she is severely handicapped by not 

 being able to reach shelter rapidly. It may perhaps be objectefl 

 to this that such an animal as a rabbit is handicapped by ha^•^ng 

 to carry the young ones in utero for a much longer time than the 

 marsupial does, but anyone who has seen the well-devel<)j)ed, 

 pouch young ones of a marsupial will realize how inncli more 

 cumbersome a burden they are than the uterine embryos of such 

 an animal as a wild rabbit." 



' Spencer, Baldwin. "Through Larapinta Land, A Narrative of the Horn 

 Expedition to Central Australia." Report of the Horn Expedition to Central 

 Australia, Sept. 1896, pp. 127-128. 



