1916] Dice: Land Vcrtcbnifes of Soiitlicasfcnt Washington 839 



due to differences in temperature until the effect of difference in habi- 

 tat has been eliminated. 



We believe that the true significance of the facts considered in the 

 life-zone method of studying distribution in North America would be 

 better presented by an extension of the zoogeographical method of 

 Lydekker and Allen. This method would recognize the zones of life 

 found in the various jiai'ts of the continent. It would recognize the 

 relation of the zones on the higher parts of the mountains to tlie belts 

 of life in the North by placing these higher zones in subdivisions of 

 the Holarctic region. Under this system there would be no compulsion 

 to recognize a certain number of life-zones in each i-egion, but the 

 number of divisions could be varied to fit the circumstances. The 

 effect of temperature as a barrier to distribution in places where that 

 is important would be shown, and the effects of other climatic barriers 

 could also be emphasized. 



The Ecological Method 



The ecological method of studying the distribution of animals and 

 plants brings out chiefly the relations of the organisms to their en- 

 vironments. It makes as simple as possible the comparison of environ- 

 ments and of adaptational structures and habits in different species 

 and in different localities. The different associations and formations 

 of any region can be compared with associations and formations in any 

 other part of the world. At present our knowledge of associations and 

 formations in general is too slight to point out the significance of each 

 ecological division in southeastern Washington, but we feel certain 

 that the study of the distribution of the species of animals in relation 

 to the distribution of different kinds of environments will lead to re- 

 sults of the highest value. 



The classification of the habitats of a region and the placing of the 

 species in associations, which to some extent at least are arbitrary 

 divisions, may be objected to on tlie ground that such a system ap- 

 parently indicates a discontinuity in nature which does not exist. 

 However, it is thought that the use of terms showing the relative 

 abundance of each species in the different habitats, prevents the asso- 

 ciations from assuming more of a definite character than they actually 

 possess. 



The different systems used for describing animal distribution are 

 used for convenience in classifying the complexly related facts in- 



