484 



ISLAND LIFE. 



[part II. 



extent without the aid of those powerful causes explained 

 in our eighth chapter — causes Vvhich acted as a constantly 

 recurrent motive-power to produce that " continuous cur- 

 rent of vegetation" from north to south across the whole 

 width of the tropics referred to by Sir Joseph Hooker. Those 

 causes were, the repeated changes of climate which, during all 

 geological time, appear to have occurred in both hemispheres, 

 culminating at rare intervals in glacial epochs, and which have 

 been shown to depend upon changes of excentricity of the 

 earth's orbit and the occurrence of summer or winter in 

 aphelion, in conjunction with the slower and more irregular 

 changes of geographical conditions; these combined causes 

 acting chiefly through the agency of heat -bearing oceanic 

 currents, and of snow- and ice-collecting highlands. Let us 

 now briefly consider how such changes would act in favouring 

 the dispersal of plants. 



Elevation and depression of the Snoiv Line as aiding the 

 migration of Plants, — We have endeavoured to show (in an 

 earlier portion of this volume) that wherever geographical or 

 physical conditions were such as to produce any considerable 

 amount of perpetual snow, this would be increased whenever 

 a high degree of excentricity concurred with winter in aphelion, 

 and diminished during the opposite phase. On all mountain 

 ranges, therefore, which reached above the snow-line, there 

 would be a periodical increase and decrease of snow, and 

 when there were extensive areas of plateau at about the 

 same level, the lowering of the snow-line might cause such an 

 increased accumulation of snow as to produce great glaciers 

 and ice-fields, such as we have seen occurred in South Africa 

 during the last period of high excentricity. But along with 

 such depression of the line of perpetual snow there would 

 be a corresponding depression of the alpine and sub-alpine 

 zones suitable for the growth of an arctic and temperate vege- 

 tation, and, what is perhaps more important, the depression 

 would necessarily produce a great extension of the area of these 

 zones on all high mountains, thus affording a number of new 

 stations suitable for such temperate plants as might first reach 

 them. Bat just above and below the snow-line is the area of 



