CHAP, xxiii.] ARCTIC PLANTS IN NEW ZEALAND. 



485 



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 accompanying accu- 

 mulations of cUhris 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 ncrthern 

 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. 



But the differences of temperature in the two hemispheres 

 caused by the sun being in perihelion in the winter of the one 

 while it was in aiihelion during the same season in the other, 

 wxuld 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 favour alle 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 jjolar 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 would have been 

 increased by the depression of the ocean wiiich must have 



