82 ISLAXD LIFE 



relation to that of the nearest continent ; but there are 

 other phenomena presented by the dispersal of species and 

 genera of plants over very wide areas, especially when 

 they occur in widely separated portions of the northern 

 and southern hemispheres, that are not easily explained 

 by such causes alone. It is here that transmission along 

 mountains chains has probably been effective ; and the 

 exact mode in Avhich this has occurred is discussed in 

 Chapter XXIII., where a considerable body of facts is 

 given, showing that extensive migrations may be effected 

 by a succession of moderate steps, owing to the frequent 

 exposure of fresh surfaces of soil or dSbris on mountain 

 sides and summits, offering stations on which foreign 

 plants can temporarily establish themselves. 



Antiquity of Plants as affecting their Distonhution. — We 

 h?.ve already referred to the importance of great antiquity 

 in enabling us to account for the wide dispersal of some 

 genera and species of insects and land-shells, and recent 

 discoveries in fossil botany show that this cause has also had 

 great influence in the case of plants. Eich floras have 

 been discovered in the Miocene, the Eocene, and the Upper 

 Cretaceous formations, and these consist almost wholly of 

 living genera, and many of them of species very closely 

 allied to existing forms. We have therefore every reason 

 to believe that a large number of our plant-species have 

 survived great geological, geographical, and climatal 

 changes; and this fact, combined with the varied and 

 wonderful powers of dispersal many of them possess, ren- 

 ders it far less difficult to understand the examples of wide 

 distribution of the genera and species of plants than in the 

 case of similar instances among animals. This subject 

 wdll be further alluded to when discussing the origin of 

 the New Zealand flora, in Chapter XXII. 



