22 LIFE ON THE EARTH. 



and the Snow-flake is equally local. Gagea lutea 

 is a rare plant of the Oxfordshire hills, yet it re- 

 appears in the Himalaya ; Gentiana lutea is found 

 on the high slopes of the Puy de Dome, but it is 

 deficient over the greater part of central France, and 

 is rare even in the direction of the Alps till we reach 

 the mountains where it occurs more frequently. In- 

 stances like these which have been now enumerated, 

 seem to be satisfactory in proof that the actual dis- 

 tribution of any given species is the final result of 

 many long periods of general effort to extend, and 

 of some particular influences to restrain, its diffusion. 



Remarkable examples are recorded by botanists 

 of plants having sprung up unexpectedly in the 

 course of cultivation, in spots where such had not 

 been growing for very long periods. Perhaps the 

 well-known case of the upspringing of white clover 

 on the burnt surface of the heaths of Yorkshire, 

 when lime has been added, is one of the most strik- 

 ing. The heath thus displaced has been growing 

 there for many centuries ; the little clover is rarely 

 seen in the district ; yet no sooner is the heath ex- 

 pelled and the calcareous element added, than it 

 grows and covers the surface. 



The distribution of particular groups of animals 

 is quite as limited as that of plants, and this is true 

 not only for land-animals but for the inhabitants of 



