No. 646] THE DISTRIBUTION OF LIFE 435 



eral and specific, is a powerful determining factor of dis- 

 tribution, and with this should be classed not only the 

 manner of feeding but the methods employed in secur- 

 ing sustenance. The search for a favorite food item will 

 even, in time, indirectly change a mammal from a ter- 

 restrial to an arboreal type, as it evidently has the tree 

 mouse, Plienacomys longicaudus, of the coasts of Oregon 

 and northern California, which, so far as kno^vn, feeds 

 exclusively upon the needles of coniferous trees. 



The question of enemies, it seems to me, should be 

 given much more weight in distributional problems than 

 it usually receives. This factor may be divided into ac- 

 tive and passive enemies. By the latter term is meant 

 competitive forms, as the more robust growth that chokes 

 out a tender seedling, or an organism which, being more 

 adaptable to a variety of conditions, forges ahead of less 

 plastic forms whose habits are competitive. It is the 

 opinion of the writer that such competition constitutes 

 the real remorseless struggle for existence which most 

 species are obliged to carry on in order to survive, rather 

 than their efforts to elude their active enemies. Al- 

 though these passive enemies are not spectacular and 

 are apparent only after scrutiny by an understanding 

 person, they are, nevertheless, always present and opera- 

 tive. 



Active enemies may be divided into irritating and ex- 

 terminating types, and in certain sections the former 

 may constitute a formidable barrier to dispersal. Few 

 of the larger parasites directly cause death, but the pres- 

 ence of great quantities of aphis, scales, ticks or intesti- 

 nal worms upon their respective hosts may so handicap 

 a species that it is forced to the wall by the competition 

 of more favored forms. Unusual numbers of horse flies 

 in a mountainous section may so liarass stock that they 

 utterly refuse to dwell in such ivuions. 



Exterminating agencies may cdii-lsi diroctly pre- 

 daceous organisms, such as caniivoros which consume 

 the flesh of their victims, or rodents whose presence in 



