THE ORIGIN OF FOREST-GROUPINGS. 239 



als, distinguished by shades which were gradually effaced on 

 approaching modern times. 



The principal requisite for tracing the presumed origins, 

 whether of the type at its birth, of supposed ancestors, or of the 

 direct antecedents of existing species, is to have in mind the 

 exact succession of the periods and stages, giving the relative 

 date of each of the determined first appearances and the order of 

 the constitutive elements of the march which the forms of which 

 we meet the traces have followed through time and space. The 

 succession of ages, represented by beds deposited in a constant 

 order, or stratigraphic geology, makes this known to us. It is 

 also necessary to take account of the changes that have been im- 

 pressed upon the whole vegetable kingdom during this long 

 series of periods. The changes have been too profound, and 

 attended by too complete renovations, for us to be able to find the 

 cradles of existing plants in the primitive periods. 



Three great plant-periods may be distinguished, starting from 

 the moment when the surface of the globe first began to be cov- 

 ered with aerial vegetation : the primary or paleophytic period, 

 or cryptogamic era, which derives its name from the domination 

 of cryptogams; the secondary period, or mesophytic, during 

 which gymnosperms — conifers and cycads — obtained the predomi- 

 nance, while foliage-trees were still absent ; and the tertiary or 

 neophytic period, also called angiospermic, from the presence of 

 the higher plants and particularly of the foliage-trees. 



The last of these periods, which began with the chalk and is 

 still in continuance, is characterized by the appearance and exten- 

 sion of the higher plants, and was also coincident with the first 

 signs of polar refrigeration and with the more and more marked 

 decrease of terrestrial temperature with increase of latitude. This 

 fact, at first hardly sensible, then gradually accented, exercised 

 an influence within the polar circle before extending its action 

 beyond, into the temperate zone, which was for a long time hot, 

 and afterward warm, while the regions around the pole were 

 already frozen. A remarkable relation certainly seems to exist 

 between the beginning and the course of the climatological de- 

 pression of the northern regions and the progress of vegetation, 

 which perfected itself in a parallel line. Indeed, it closed the en- 

 tire cycle of its definitive evolution by the adjunction of the an- 

 giosperms, the most perfect plants ; and these acquired prepon- 

 derance as rapidly as the cooling of the arctic regions went on. 

 These regions appear to have been exempt till this time from the 

 rigors of a cold season, and, by that fact, subtracted from the 

 effects of the winter rest. 



It is certain that the vegetable kingdom, as soon as it had ac- 

 quired all the elements of which it is still composed, began to dis- 



