42 CALIFORNIA ACADEMY OF SCIENCES [Pkoc. 4th Ser. 



indicate the presence of a zone of which the species is 

 characteristic. 



2. In the case of plants, perennials are usually preferred 

 to annuals (Coville, 1893, p. 17). 



3. The more abundant a species the greater is its value 

 as a life-zone indicator. Rare or local species may be 

 restricted by other factors than those which influence zone 

 position. A dearth of facilities for dispersal may account for 

 the restricted occurrence of a species which would otherwise 

 inhabit two or more zones. Conversely, organisms with 

 effective provision for wide dispersal may be considered as 

 having been already distributed and tried out over a number 

 of zones, so that their absence from any one of them may be 

 taken as evidence that they are there unable to persist. 



4. Certain indicators may be absolutely constant as to 

 their zonal position in one portion of their range but quite 

 unreliable when the entire range of the species is taken into 

 account. This may be due to a variety of causes, chief of 

 which is perhaps the possible development of hardy strains in 

 one portion of the range and not in another. Allowance 

 should also be made for the presence of biotypes which may 

 be so similar in external characters as to escape detection by 

 the taxonomist but which react differently to their environ- 

 ment. Whatever the cause, it should be borne in mind that 

 in a few cases a particular indicator may be of no value 

 when widely separated faunas or floras are to be compared. 



5. Since the delimitation of life-zones as outlined by 

 Merriam is accepted in the main by the present authors, these 

 zonal limits are determined as far as possible by means of 

 indicators listed by Dr. Merriam himself, especially in his 

 later publications (Merriam, 1899, etc.). Since the nomen- 

 clature first proposed by INlerriam has now become well estab- 

 lished we consider it highly undesirable that any other should 

 be promulgated. Uniformity in the use of terms applied to 

 the various zones is essential to ready intelligibility and to 

 scientific accuracy. However, the present authors do not 

 commit themselves as to the exact temperature factors limit- 

 ing the various life-zones and the distribution of species as 

 laid down in some detail by Merriam (Merriam, 1898). 



