226 ESSAYS. 
now and then, in the lapse of time, by the chance conveyance 
of seeds across oceans, or even from one mountain to another. 
The plants of the top of the White Mountains and of Labra- 
dor are mainly the same; but we need not suppose that it is 
so because birds have carried seeds from the one to the other. 
I take it that the true explanation.of the whole problem 
comes from a just genera] view, and not through piecemeal 
suppositions of chances. And I am clear that it is to be found 
by looking to the north, to the state of things at the arctic 
zone, — first, as it now is, and then as it has been. 
North of our forest-regions comes the zone unwooded from 
cold, the zone of arctic vegetation. In this, as a rule, the 
species are the same round the world ; as exceptions, some are 
restricted to a part of the circle. 
The polar projection of the earth down to the northern 
tropic, as here exhibited, shows to the eye—as our maps do 
not — how all the lands come together into one region, and 
how natural it may be for the same species, under homogene- 
ous conditions, to spread over it. When we know, moreover, 
that sea and land have varied greatly since these species ex- 
isted, we may well believe that any ocean-gaps, now in the 
way of equable distribution, may have been bridged over. 
There is now only one considerable gap. 
What would happen if a cold period was to come on from 
the north, and was to carry very slowly the present arctic 
climate, or something like it, down far into the temperate 
zone? Why, just what has happened in the Glacial period, 
when the refrigeration somehow pushed all these plants before 
it down to southern Europe, to middle Asia, to the middle 
and southern part of the United States ; and, at length reced- 
ing, left some parts of them stranded on the Pyrenees, the 
Alps, the Apennines, the Caucasus, on our White and Rocky 
Mountains, or wherever they could escape the increasing 
warmth as well by ascending mountains as by receding north- 
ward at lower levels. Those that kept together at a low level, 
and made good their retreat, form the main body of present 
arctic vegetation. Those that took to the mountains had their 
line of retreat cut off, and hold their positions on the moun- 
