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 léaves 
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 
Lnquidambar, the Betula alba, the Fagus sylvatica, the Taaus 
