102 BOTANY. 
209. 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 en- 
during generally die in a few centuries. The plants of the 
world are thus constantly dying off, and are as constantly 
being renewed. Occasionally the dying off in a particular 
species was more rapid than the renewal, in which case the 
species eventually became extinct: many such cases are 
known to paleontologists. On the other hand, it has fre- 
quently happened that new forms have appeared as the 
older ones have died off, so that the character of a particu- 
lar flora has thereby been gradually changed. 
210. 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- 
ern continent were then common in many parts of this 
continent, and tropical or sub-tropical species flourished 
abundantly in Nebraska and Dakota. 
211. Moreover, we learn by such a study that many of 
the plants of the present were not yet in existence in cer- 
tain geological periods. As we go back in geological time 
the vegetation is less and less like that of to-day. Thus, 
the higher flowering plants (Dicotyledons) were not in ex- 
istence earlier than the Cretaceous period, while the Lilies 
and their relatives date back to the Triassic. The great 
Carboniferous vegetation, from which our coal was derived, 
contained no plants with true flowers. There were no 
grasses or sedges, no lilies or orchids, no roses or violets, no 
