430 



THE AMERICAN NATURALIST 



[Vol. LVl 



any other cause. Xatural manner of dispersal must also 

 be carefully scrutinized as a preliminary step, for what 

 will prove a barrier to the extension of the range of a 

 plant with what I may term unadorned seeds m^y be 

 inoperative in the case of seeds adapted to dispersal by 

 the wind, and again, those whose covering is fitted for 

 adhesion to the coats of mammals will often be still more 

 widely scattered. 



In the case of an animal, the tirst thing to be considered 

 is the life-type to which it belongs, the chief divisions of 

 which are aquatic, fossorial, terrestrial, arboreal and 

 volant types, which are limited in varying degrees by 

 physical barriers. To an aquatic form, land masses are 

 insuperable obstacles, while to many terrestrial species, 

 especially such as live in very arid regions and are 

 totally independent of water, even a large river may 

 prove a delimiting agent. A strip of rocky country or 

 an extent of arid plain will prevent the spread of such a 

 fossorial mammal as the mole. Arboreal forms are 

 checked by large, treeless areas, and animals which are 

 adapted to a life on the phuns will usually shun the for- 

 ests. Volant types are the most independent of physical 

 barriers of all, and to some even wide stretches of ocean 

 are no obstacle, as in the case of the Pacific Golden 

 Plover (ChnirulnHs rlnuiiuinis fuh-us), m its annual 

 migrations b.-lwccu Alask:i and the Hawaiian Islands. 



While concrdiiig that tciiipci-ature is tlic most impor- 

 tant factor in the dist riltutinn (,r life, the writer is of the 

 opinion that not enonuh impoiiaucn' has h.'cn credited to 

 other agencies. 1)v. ('. If. AFcrriani was, I believe, the 

 first to formulate the theory that the northward range 

 of a species is governed by the mean amount of heat 

 present during the season of reproduction, while the 

 southward range of northern forms is restricted by the 

 mean temperature during the very hottest ])ortioii of the 

 year. Isotherms have been determined and our conti- 

 nent plotted and mapped into zones, called Arctic, llud- 

 sonian, Canadian, Transition, Upper Sonoran, Lower 



