Vol. IX] HALL & GRIN NELL— LIFE-ZONE INDICATORS 41 



lower altitudes on a broad, massive mountain than they will 

 on a sharp, isolated peak. (See Grinnell and Swarth, 1913, 

 p. 216.) 



9. Rock surfaces. Rocky slopes, outcroppings, and talus 

 are usually warmer than surrounding areas, and the life- 

 zones are raised in consequence. 



10. Miscellaneous local influences. The explanation of 

 the apparently abnormal occurrence of a species at stations 

 far removed from the zone in which it is usually found 

 should be sought in a minute examination of local condi- 

 tions. The influence of man is especially important, and one 

 must be certain that the species is actually established. 



THE SELECTION OF LIFE-ZONE INDICATORS 



When all of the numerous disturbing "factors" are taken 

 into account we may be able to explain many of the apparent 

 incongruities in the position of life-zones as we find them in 

 nature. In the meantime, it is our clear duty to map these 

 zones in as definite a manner as possible, using for this pur- 

 pose the occurrence of stenothermic species of animals and 

 plants rather than any preconceived idea as to the tempera- 

 ture or other environmental factor. In selecting these indica- 

 tive species, or "life-zone indicators", as they are called, one 

 must have regard for a number of considerations. A few 

 of the criteria which the authors have used in making selec- 

 tions for the indicators enumerated beyond may be set forth 

 as follows : 



1. Only breeding records have been taken into account. 

 This applies to plants as well as to animals, for seeds are 

 sometimes carried above the normal position for the species. 

 The resulting seedlings may live through several or many 

 seasons and yet not be able to reproduce because of insufficient 

 summer heat. Conversely, plants and animals may be carried 

 to stations below their proper zone, there persist and even 

 reproduce during a few favorable seasons, but without be- 

 coming thoroughly established. It is thus evident that the 

 sporadic occurrence of a few individuals does not necessarily 



