i9Ui] Dice: Land Vcrtcbrafrs of Southeastern WasJrington 333 



Routes of migration have been opened and closed at irregular times, 

 and many groups have become differentiated only to become exter- 

 minated. Climatic barriers are hard to determine and different 

 species show various degrees of limitation by such barriers. It i.s a 

 difficult matter to divide any region into satisfactory zoogeographical 

 areas. It has been pointed .out that the zoogeographical divisions of 

 the globe are different for each group of animals and that in any one 

 group these divisions indicate roughly the length of time the different 

 sections of the group have been separated (Gadow, 1913, p. 13-15). 



The zoogeographical method has many limitations and it is unwise 

 to attempt to apply it too closely. Species are limited in distribution 

 by various factors or complexes of factors. The zoogeographical 

 divisions are founded on comparative statistics and there will always 

 be exceptions. In some cases the exceptions will almost equal the 

 number following the rule. The zoogeographical divisions are more 

 or less arbitrary and there are sure to be many places of uncertain 

 position. Still, ,the system greatly simplifies the consideration of the 

 facts of distribution. 



The Life- Zone System 



The significance of the life-zone method lies in its indication of 

 climatic barriers on the continent of North America, and the origin of 

 the faunas of the several life-zones. Although this method of con- 

 sidering distribution has come into rather general use, it has a number 

 of disadvantages and difficulties, and a considerable amount of criti- 

 cism has been directed at the system. It seems advisable therefore to 

 consider its history and some of the objections which have been raised 

 against it. 



On the high mountains of the western United States there are dif- 

 ferent zones of vegetation at different levels, and with each of these 

 vegetational zones there are associated particular species of animals. 

 Merriam (1890, pp. 7-11) found seven such zones of life on San Fran- 

 cisco Mountain, Arizona. Beginning at the top he gave these the 

 names of Alpine zone, Subalpine or Timberline zone, (Central) Hud- 

 sonian or Spruce zone, (Central) Canadian or Balsam Fir zone. 

 Neutral or Pine zone, Piiion zone, and the Desert Area. He showed 

 that some species in the fauna and flora of the uppermost four of 

 these zones were characteristic of much more northern regions. On 

 the mountains of central Idaho, Merriam (1891, pp. 21-2.5) distin- 



