226 THE FOREST LANDS OF NORTHERN RUSSIA. 



less already winters too marked, and summers of too little 

 warmth and too short, to open up for them access to the 

 Polar zone. 



' The contrast between the two seasons and the darkness 

 of that of winter ought necessarily, through the influence 

 of an annual period of enforced repose on vegetation, to 

 favour the development of species with caducous leaves. 

 Indeed, we are not far from admitting that the greater 

 part of the types of dicotyledons with caducous leaves 

 must have come originally out from the extreme 

 north, and that the cradle of some of them ought to be 

 placed in the interior of the Arctic zone, though it may 

 be that for some others of them it must be placed on 

 mountains and in the moist parts of the temperate zone. 

 It has been certainly thus with groups which comprise 

 at once species with caducous leaves and others with 

 persistent or semi-persistent leaves, such as the elms, of 

 which the sub-genus Microptelea represents the type with 

 non-caducous leaves ; the birches, of which the Betulaster 

 betokens the southern stock ; the oaks, divided into ever- 

 green oaks and common oaks ; and the chesnuts, of which 

 the Castanopsis and the Pasiana are the repetition in the 

 heart of the temperate zone. Every time that we can obtain 

 a duality of this sort, we are certain to meet in the Arctic 

 tertiary vegetation remains of the sub-type with caducous 

 leaves, whilst the other sub-type is alacking, and shows 

 itself at the same epoch by preference in Europe. Other 

 types, as those of the ginko, of the plane tree, of the lime 

 tree, &c., the prototypes of which, with persistent leaves, 

 have disappeared very long ago or are unknown, have 

 really come from the Polar region at a definite time, to 

 spread themselves then step by step across the northern 

 temperate zone. These kinds of trees, like the preceding, 

 had rayed out from the Arctic land, and their present 

 diffusion finds the occasion of its being in this anterior 

 emigration, by means of which they became free to advance 

 towards the south in one or in many directions. The 

 Liquidambar, the Betula alba, the Fayus sylvatica> the Taxus 



