516 ISLAND LIFE 



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 ef 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 Snow 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 w^ere 

 .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 vegetation, and, what is perhaps more important, 

 the depression would necessarily produce a great extension 

 of the area of these zones on all high mountains, because 

 as we descend the average slopes become less abrupt, — 

 thus affording a number of new stations suitable for such 

 temperate plants as might first reach them. But just 

 above and below the snow-line is the area of most 

 powerful disintegration and denudation, from the alternate 

 action of frost and sun, of ice and water ; and thus the 

 more extended area would be subject to the constant 

 occurrence of land-slips, berg-falls, and floods, with their 



