THE ORIGIN OF FOREST-GROUPINGS. 235 



Changes insensible at each single step have in the long run re- 

 sulted in modifications of the aspect of the landscape and replace- 

 ments of the types and species composing the floral carpet at a 

 given movement by forms different from them, and also from 

 those before -which they were themselves destined to retire at a 

 later period. 



The impression which one feels in the midst of a deep forest is 

 one of perennial duration. Except man, what is there to uproot 

 those giants that have lived through centuries ? What action 

 can be conceived of that will exclude them from the ground 

 which they possess so completely ? The first impression would 

 almost make these forest masses coeval with the globe, its natural 

 product and spontaneous dress from the days of its youth. Such 

 an impression would be a mistaken one. The forests have not 

 been perpetuated in the same order from the beginning, but have 

 changed much in the course of ages. Those which we now see 

 have taken the place of other more ancient ones, and these substi- 

 tutions have occurred many times, sometimes through partial 

 modifications and sometimes also under such conditions that the 

 old order has only indirect relations with the present one, or is 

 even wholly foreign to it. 



Since a serious mind can not suppose that at every revolution 

 of plant-life there has been a total destruction of the anterior ele- 

 ments, followed by a creation conceived anew in all its details, we 

 are forced to seek in the order which precedes the reason for the 

 existence of that which has replaced it. This view implies an 

 endless chain of causes and effects, of ancestral and derived forms, 

 stretching along, now spreading, now continuing themselves, to 

 spread out again, and — in what more particularly concerns the 

 types of the vegetable kingdom — emigrating in a determined di- 

 rection. This direction is found to have consisted, for plants, in a 

 march from north to south in search of more favorable regions 

 and stations better fitted to the exigencies of acquired adaptation, 

 as rapidly as the terrestrial temperature declined from its pris- 

 tine conditions, as latitudes took on their individual characteris- 

 tics, and as the arctic zone, which had been temperate, grew cooler 

 and became more and more differentiated. The polar circle was 

 thus constituted a barrier that became more pronounced, less ac- 

 cessible, and was finally closed to arboreal vegetation, while under 

 the operation of the same movement the present temperate zone 

 became cooled in an equivalent measure, was impoverished, and 

 gradually stripped of a considerable part of its floral wealth. 

 The remnants that escaped this elimination in those successive 

 and numerous retreats that filled the second half of the Tertiary 

 period still occur, scattered and dwarfed, in the southern part of 

 that zone, and upon points where the less sensible depression of 



