of Geological Periods, 341 



The following is an enumeration of these genera, of course 

 restricted to the principal ones — that is to say, to those which 

 play an important part in the general vegetation of the northern 

 temperate zone and are at the same time most frequently ob- 

 served in the fossil state in several successive stages : — 



Alnus, Toumef. Populus, Toumef. 



Betula, Toumef. Salix, Toumef. 



Ostrya, Mich. Fraxinus, Linn. 



Carpinus, Toumef. Hedera, Linn. 



CoryluSy Toumef. Cornus, Toumef. 



Quercus, Linn. Liquidamhar, Linn. 



Fagus, Tournef. Liriodendron, Linn. 



Castanea, Toumef. Acer, Linn. 



Ulmus, Linn. Juglans, Linn. 



Celtis, Tournef. Crat<^gus, Linn. 



Platanus, Linn. Cercis, Linn. 



This list includes twenty-two genera, of which eighteen still 

 grow naturally in Europe; three {Platanus, Liquidambar, Ju- 

 glans), without being spontaneous in Europe, inhabit the neigh- 

 bouring parts of Asia as well as North America ; one only (Li- 

 riodendron) is no longer met with except in the New World. 

 But these last only quitted our soil at an epoch very nearly ap- 

 proaching our own ; so that, by going a little backward, one 

 might say that all these genera equally characterize the northern 

 temperate zone, the limits of which they overstep only excep- 

 tionally*, and solely by means of certain mountainous regions 

 in which altitude compensates the climate. As regards their 

 polar limits these genera show great diversities. The willows 

 and birches advance furthest towards the north, since they reach 

 Iceland, but certainly with repent species. The oak does not 

 pass Stockholm, with a mean temperature of 5° C. ( = 41° F.) ; 

 the ash stops at Gothenburg, with 7°-9 C. (= 46°-2 R). The 

 chestnut and the plane do not go so far. But these differences 

 depend rather on the inherent aptitudes of the species than even 

 on those of the genus, especially in the frequent cases where the 

 latter is represented by a small number of species, or even by a 

 single one ; the area of the genus then depends upon that of 

 the species, and is confounded with it. Whatever be the nature 

 of these diversities, they may and must relate to anterior causes, 

 and especially to those which we now proceed to study. 



In ascending the course of ages we shall pass through the 

 same periods that we have just traversed, but inversely, and 

 commencing with the most recent. The eighth of our horizons, 



* The genus Fagus reappears in the southern hemisphere, but in forms 

 sufficiently distinct from those of the boreal zone to constitute another 

 type. 



