CHAP. VII.] DISTRIBUTION OF PLANTS. 203 



continents islands are, the larger is the proportion which 

 ferns bear to Phanerogamic plants. 



The family of ferns has probably more fossil represen- 

 tatives than any other group of plants, and occur in all 

 fossiliferous formations from the Devonian upwards, 

 being especially abundant during the Carboniferous 

 period. The fossil species present the same broad 

 features as those existing at the present day; no fossil 

 ferns have as yet been discovered that cannot be referred 

 to some existing type. 



The distribution of Phanerogams may be studied from 

 two standpoints ; latitude and altitude, the predomi- 

 nating factor in determining the distribution being in 

 both cases temperature, and a series of changes can be 

 traced in the lowland flora from the equator to the poles, 

 each zone being characterized by the predominance of 

 certain groups of vegetation that often give a feature to 

 the landscape. These zones of vegetation do not follow 

 the parallels of latitude, but are undulatory, correspond- 

 ing with the isotherm of the particular month in which 

 there is the greatest development of vegetable life. 



In very high latitudes, lichens and mosses are only 

 met with, then follow grasses, species of saxifrage, 

 buttercup, woodrush, etc., species of rhododendron with 

 showy flowers are also present in the Arctic Zone. 

 Amongst trees the birch predominates, and extends 

 nearly to the North Gape; firs and pines are also 

 present. 



In the 8ub-arctic zone, including the northern parts of 

 Norway, Siberia, Iceland, and the Faroe Islands, firs 

 and willows are the predominating trees, and in some 

 localities are very much dwarfed, but in Siberia very 



