XI. CLI]VIATAL ZONES. 491 



in instances formerly cited, the smaller floras (or areas) 

 are found in connexion witli proiDortionately greater dif- 

 ferences among the species ; that is, the orders are rela- 

 tively more numerous. The differences may be shown 



thus : — 



Inferagrarian, 1225 species, 95 orders, 12.9 average. 



Midagrarian, 1070 „ 



Supeiagrarian, 760 „ 



Inferarctic, 293 „ 



Midarctic, 244 „ 



Superarctic, 111 „ 



So that, with one trifling difference in a decimal figure, 

 the average number of species to an order decreases with 

 the decreasing floras. But it may be quite correct also 

 to attribute some share in this result to the differences of 

 climate ; for the many closely similar species, so often 

 observed in the floras of temperate climates, seem to 

 condense (so to express it) into fewer species in colder 

 climates. 



Widely different as the floras of the zones must un- 

 doubtedly be, where the ordinal changes are so great, it 

 is far from easy to describe or characterize them in such 

 manner that a botanist may be enabled at once to see, by 

 observing the plants around him, where and when he has 

 passed from one zone to another, either in travelling hori- 

 zontally or in ascending a mountain. Such divisions are 

 natural, however, in the sense of .being accordant with 

 actual differences in the flora and vegetation, — how then 

 are such divisions to be distinguished among themselves ? 



The most obvious characteristics of any zone are the 

 species which there predominate in numbers or conspicu- 

 ousness. But unfortunately those floral features are 

 seldom or never available as precise tests. The pre- 

 dominating species in and through any one zone are 



