of Geological Periods. 351 



tropical types fiaally disappeared, the subtropical ones still per- 

 sisting ; but the predominance from that time attained by the 

 European types tends to become more and more exclusive, while 

 the temperature, following the same movement, gradually de- 

 creases so as to become more and more like that which we now 

 have. 



Thus, to sum up, the temperature has formerly undergone 

 oscillations which it is difficult to define ; but, notwithstanding 

 these variations, it preserved a degree of elevation nearly equal 

 to that which now exists under the tropics, until after the middle 

 of the Tertiary period. 



It is only after that point (that is to say, about the epoch 

 at which the Swiss Mollasse was deposited) that it began to de- 

 cline ; and yet, long before this age, continual transformations 

 had taken place in the midst of the vegetation of ancient Eu- 

 rope — changes correlative with a progress which may be spoken 

 of as regular through all the periods. We must therefore be 

 careful not to confound the effects of temperature with those of 

 organic evolution, which brought about the first appearance and 

 then the development of the various types of plants. 



The two phenomena are far from standing towards each other 

 in the relation of eflfect and cause. At the utmost the modifica- 

 tions of temperature have constituted occasional circumstances 

 with which certain evolutions may have corresponded. It is 

 impossible at this distance to conjecture the nature of the cir- 

 cumstances which must have occurred; but in assuming the 

 presence of certain truly tropical genera as a proof of elevation 

 of temperature, we see that the first ascertained types only par- 

 tially correspond with this supposed elevation ; whilst, on the 

 other hand, the appearance of the European types by no means 

 coincides with any lowering of the primitive temperature. We 

 see also that these types, or at least several of them, were 

 already fixed at a very distant epoch, and have not since varied, 

 even as regards the consistence of the foliaceous tissue, which 

 must have been membranous and caducous at that time as at 

 present. 



With regard to the progress proper to all these genera, we 

 must distinguish two kinds of evolution, — one peculiar to such 

 genera as Alnus, Carpinus, and Ulmus, of which the physiognomy 

 is uniform, and which include a rather small number of species. 

 The species of these genera, similar in time to what they are 

 now in space, occur from their origin with their present phy- 

 siognomy; they are only scarcely diversified impressions of a 

 not very variable type. The other kind of evolution applies to 

 more numerous and heterogeneous groups, which, like Quercus, 

 include species of very diverse forms and aptitudes. Here the 



