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 ddbris 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. 
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 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 would have been 
increased by the depression of the ocean which must have 
