CLIMATIC ZONES 27 



the early and middle Tertiary a wide-spread flora, which like 

 the arctic and sub-arctic floras of today was very similar through- 

 out, and occupied much the same range as the present arctic 

 and sub-arctic vegetation. 



It is clear, however, from the plants composing this "Arcto- 

 tertiary" flora, that the climate of the far north was very much 

 milder than at present. Trees like magnolias, laurels, gums, and 

 other denizens of warm temperate climates lived during the mid- 

 dle Tertiary in Greenland and Spitsbergen where now only the 

 hardiest of arctic plants can survive. 



The arcto-tertiary flora showed a division comparable to 

 the arctic and sub-artic zones of the present. North of 75°, the 

 plant remains seem to be strictly boreal types, like willows, 

 poplars, oaks, etc., but in the southern zone are found remains 

 of palms, laurels, myrtles, now characteristic of only the warmest 

 parts of the temperate zones. 



With the increasing cold of the late Tertiary and Pleistocene, 

 the arcto-tertiary flora which included the ancestors of most 

 of the vegetation now occupying the north temperate regions, 

 migrated southward, and came to occupy much lower latitudes. 



This retreat was greatly accelerated by the advancing glaciers 

 of the ice-age, and in many regions, especially in Europe and 

 northern Asia, many forms became quite extinct, but survived 

 in the more favorable conditions of eastern Asia and America. 



With the amelioration of the climate in post-glacial time, 

 there was a northward movement of vegetation following the 

 retreat of the ice-sheet, and developing the modern arcto-glacial 

 flora. 



The Arctic Zone 



The region surrounding the north pole is, of course, very 

 incompletely explored; but where land occurs, it is completely 

 ice-clad and probably quite destitute of any vegetation, except 

 certain humble sea-weeds. 



However, an unexpectedly large number of flowering plants 

 manage to exist within a few degrees of the pole, a considerable 

 number of species being recorded as far north as 83°. 



From this region southward to where tree growth begins is 

 the true arcto-glacial and tundra zone. Within this zone topo- 



