FLIGHT OF PLANTS AND ANIMALS. 429 



Eanges, and another so very like the Sequoia gigantea of the 

 Sierra Nevada, that, if such fossil twigs with leaves and cones 

 had been exhumed in California instead of Europe, it would 

 confidently be affirmed that we had resurrected the veritable 

 ancestors of our two giant trees. Indeed, so it may probably 

 be. Cwlum non animam mutant, etc., may be applicable even 

 to such wide wanderings and such vast intervals of time. If 

 the specific essence has not changed, and even if it has suffered 

 some change, genealogical connection is to be inferred in all 

 such cases. . . . 



I take it that the true explanation of the whole problem 

 comes from a just general 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 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 homogeneous conditions, to spread 

 over it. When we know, moreover, that sea and land have 

 varied greatly since these species existed, 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 consid- 

 erable gap. 



What would happen if a cold period were to come on from 

 the north, and were very slowly to carry the present arctic cli- 

 mate, 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 receding, left some 

 part 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 northward at lower levels. Those 



