138 FAMILIAR TREES 



migrate rather from north to south, and from §ast to 

 west, than in the reverse directions. In explanation of 

 the first of these hnes of passage Darwin pointed out 

 that, as there is more land in the north, the plants of 

 those regions may have existed in greater numbers, 

 and so have attained under competition a higher state 

 of perfection or dominating power ; but no one has yet 

 explained the meaning of Bishop Berkeley's dictum 

 that " Westward the course of empire takes its 

 way," at least in so far as it is true in the vegetable 

 world. 



Among the most ancient known assemblages of 

 fruit-bearing — i.e. " angiospermous " — plants in the 

 world is that in the Lignites, or Brown-coal, of the 

 Dakota group, on the plains of Eastern Kansas and 

 Nebraska, a group apparently intermediate in geol- 

 ogical age between our Chalk and the Thanet Sands 

 that overlie it ; and here, among many other trees, 

 occur what are perhaps the oldest-known Maples. 

 In rocks far more modem, and yet of immeasurable 

 antiquity — the Miocene beds of (Eningen, in Switzer- 

 land — as many as nineteen well-marked species of 

 Maple have been discovered, a greater number than 

 occurs in any one district at the present day. The 

 plants with which they are associated have a North 

 American " f^cies," or general character, and the whole 

 of this Miocene flora is believed to have come from 

 what is now the United States, across Asia, the greater 

 part of it retreating along the same line in a reverse 

 direction, at a later period, before the southerly 

 advance of the increasing cold of the Glacial Epoch. 

 A TuUp-tree in China, the Magnolias of Japan, and a 



