122 BOTANY. 



of South America (II"), and the South African Region 

 (II'"). The Calif ornian Region (IV) is in many respects 

 similar to the Mediterranean Region (IV) and the Chile- 

 Andean Region of South America (IV"). 



211. The accompanying map (Fig. 58) shows one of the 

 ways of dividing the earth into botanical regions. Each 

 region is capable of subdivision into districts. The plants 

 of a region or district constitute a flora; thus we may 

 speak of the Prairie Flora, or the flora of the Upper Mis- 

 sissippi district, or the flora of Iowa. 



212. Distribution of Plants in Time. — Most plants 

 are short-lived. By far the greater number perish in a 

 year or two, as is the case with our annuals and biennials. 

 Some shrubs and trees may live for a considerable number 

 of years, but even the most enduring generally die in a few 

 centuries. The plants of the world are thus constantly 

 dying oil, and are as constantly being renewed. In the 

 past ages of the world death and renewal occurred as in 

 the present. Occasionally in the past the dying ofE in a 

 particular species was more rapid than the appearance of 

 new plants, with the result that the species eventually be- 

 came extinct : many such cases are known to palaeontolo- 

 gists. On the other hand, it has frequently happened that 

 new forms have appeared as the older ones have died off, 

 so that the character of a particular flora has thereby been 

 gradually changed. 



213. By a study of the fossil plants of any period in the 

 world's history we may learn that the flora of each region 

 has undergone great changes. The flora of North America 

 in the Tertiary period was very different from what it is 

 now, while the Cretaceous flora was still more unlike that 

 of the present. Plants that now are confined to the east- 



