484 
ISLAND LIFE. 
[part II. 
extent without the aid of those powerful causes explained 
in our eighth chapter — causes which acted as a constantly 
recurrent motive-power to produce that “ continuous cur- 
rent of vegetation” from north to south across the wholo 
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 Enow 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 
