492 X. GENERAL REMARKS. 



usually found also extending into the next succeeding 

 zone ; though maybe less abundantly there, in accordance 

 with the general rule that distribution fines out gradually, 

 and cannot be traced to abrupt limits. 



On the other hand, in relying upon the presence of 

 certain species, not passing into the next zone, we are 

 resorting to a less obvious characteristic, by taking the 

 partial or even local species, those which are becoming 

 less general, instead of the predominating. But this 

 latter test, — that of the presence of certain species, con- 

 jointly with the absence of other species, — is the only 

 available method for defining the zones of plants rela- 

 tively to each other. Those adopted in this work were 

 accordingly characterized briefly in volume first, page 40, 

 b}' the presence and absence of a few shrubs, as here 

 repeated : — 



6. Super-arctic zone — Salix herbacea, without Culluna. 



5. Mid-avctic zone — Calluna vulgaris, without Erica. 



4. lufer-arctic zone — Erica Tetralix, without Pteris. 



3. Super-agrarian zone — Pteris aquilina, without Rhamuus. 



2. Mid-agrarian zone — Rhaninus catharticus, without Clematis. 



1-. lufer-agrarian zone — Clematis, Rubia, Cyperus lougus. 



And the other plants were then assigned to the zones 

 according to their positions relatively to the limits of 

 those mentioned as characteristic ; in the higher region, 

 those ascending above the upper limit of the Calluna 

 being referred to the superarctic zone, — those ascending -^^ 

 above the Erica, but not passing the Calluna, being 

 referred to the midarctic zone, — those ascending above 

 the Pteris (or, more properly, above the cultivation of 

 cereals), but not passing the Erica, being referred to the 

 inferarctic zone. The cessation of cereal cultivation was 

 made the chief test of distinction between the agrarian 

 and arctic regions, And it was explained that the three 

 zones of the former region were scarcely distinguishable 



