230 THE POPULAR SCIENCE MONTHLY. 



while we sometimes possess of fossil animals entire skeletons, and 

 nearly always parts on which we can justly base a classification, 

 we ordinarily get of the ancient trees only isolated specimens of 

 leaves, or more rarely of fruits and seeds. We, nevertheless, gen- 

 erally succeed in determining these relics, and, by comparing them 

 with analogous living ones, in forming conclusions, the probability 

 of which carries conviction. Thus, with the aid of the data fur- 

 nished by stratigraphy, we can not only reconstitute the forests of 

 former days, but can also arrange them chronologically, grasp their 

 mutual relations, establish their filiations, and finally explain how 

 they have in the past been displaced and renewed. 



It is necessary to take account of the peculiarity that trees are 

 enrooted or fixed in the soil, so that only their seeds can leave 

 them and be carried away, but never to a very great distance. 

 This fixity is certainly one of the causes of the regularity and 

 relative slowness of the modifications to which arborescent vege- 

 tation has been subjected in the periods anterior to ours. The 

 new-comers of each region can never have rapidly traversed dis- 

 tances. It has been rather by slow steps, and by the aid of at first 

 partial introductions, that the flora of all the epochs has been trans- 

 formed. Instead of leaps, we meet with modifications aided by 

 time, which were worked out through a long duration before be- 

 coming definitive. An attentive examination of the vegetable 

 impressions collected over many successive levels and at points 

 distributed along the course formerly followed by the vegetation, 

 and marking its advance, should therefore enable us to recover 

 the partial terms of the presumed filiation of the types whose ori- 

 gin we are investigating. 



One phenomenon has been remarked in intimate relation with 

 this gradual and successive substitution of plants ; it is the cool- 

 ing of the globe, operating insensibly, but subject to a general 

 movement, the progress of which, although extremely slow, has 

 never been arrested. Plants have pursued their migrations under 

 the rule of this phenomenon, moving toward the south and grad- 

 ually abandoning the north, beginning with the extreme north, or 

 the immediate environs of the pole. The discovery of numerous 

 vegetable fossils at different points in the arctic regions, in Spitz- 

 bergen, Greenland, Grinnell Land, etc., has been sufficient to give 

 rise to terms of comparison and demonstrate what was the char- 

 acter of fossil vegetation when that of Europe more or less resem- 

 bled the present vegetation of countries near the tropics. Hence 

 it has been possible to establish with fair probability not only the 

 general march but also the filiation of a number of plants ; and it 

 has been ascertained that the direct ancestors of part of our trees 

 originally inhabited the interior of the polar circle, while many 

 others, confined now to southern countries, once had European 



