CHAP. XXIII ARCTIC PLANTS IN NEW ZEALAND 



517 



accompanying accumulations of d^hris and of alluvial soil, 

 affording innumerable stations in which solitary wind- 

 borne seeds might germinate and temporarily establish 

 themselves. 



This lowering and rising of the snow-line each 10,500 

 years during periods of high excentricity, would occur in 

 the northern and southern hemispheres alternately ; and 

 where there were high mountains within the tropics the 

 two would probably overlap each other, so that the 

 northern depression would make itself felt in a slight 

 degree even across the equator some way into the southern 

 hemisphere, and vice versa ; and even if the difference of 

 the height of perpetual snow at the two extremes did not 

 average more than a few hundred feet, this would be 

 amply sufficient to supply the new and unoccupied 

 stations needful to facilitate the migration of plants. It 

 is well known that all great mountain ranges have 

 undergone such fluctuations, as proved by ice-marks below 

 the present level of snow and ice. 



But the differences of temperature in the two hemi- 

 spheres caused by the sun being in perihelion in the 

 winter of the one while it was in aphelion during the same 

 season in the other, would necessarily lead to increased 

 aerial and marine currents, as already explained ; and 

 whenever geographical conditions were such as to favour 

 the production of glaciation in any area these effects would 

 become more powerful, and would further aid in the 

 dispersal of the seeds of plants. 



Changes of Climate Favourable to Migration. — It is clear 

 then, that during periods when no glacial epochs were* 

 produced in the northern hemisphere, and even when a 

 mild climate extended over the whole polar area, alternate 

 changes of climate favouring the dispersal of plants would 

 occur on all high mountains, and with particular force on 

 such as rise above the snow-line. But during that long- 

 continued, though comparatively recent, phase of high 

 excentricity which produced an extensive glaciation in 

 the northern hemisphere and local glaciations in the 

 southern, these risings and lowerings of the snow-line on 

 all mountain ranges would have been at a maximum, and 



M M 



