
DISTRIBUTION OF PLANTS = 
It must be remembered that in each of the regions given in 
the above table, where only latitude is considered, the vegeta- 
tion will vary with the altitude. The three climatic zones are _ 
present in each region, provided a certain height above the 
sea-level is reached. Thus in the tropics, the plants character- 
istic of temperate and arctic regions will be met with at certain 
elevations ; similarly, arctic plants are found in the temperate 
zones. 
6 ‘The fact that alpine and arctic species are 
imatic 
Changes in found in tropical and temperate regions can be 
explained only by a reference to past geological 
"changes. Geologists speak of Primary, Secondary, 
Tertiary Ages, and they tell us that Flowering Plants are known 
to belong to the Tertiary period, whilst non-flowering plants, 
such as Algz and Ferns, are much older, and have been found 
in the oldest, or Primary, rocks. It is possible that, as fossili- 
ferous rocks all the world over are more and more examined, 
Flowering Plants will be found to be older than is at pr esent 
believed. During the Tertiary period there were great climatic 
changes on the earth’s surface. At the beginning of the period 
the climate seems to have been very much warmer than at the 
present time, so that what we now call the polar regions had 
plants similar to those found to-day in sub-tropical countries. 
Then the climate gradually grew colder, and new species 
adapted to the new conditions appeared in the north, and 
extended southwards, giving rise, it is thought, to the flora of 
the temperate regions of the present time. The climate went 
on getting colder, and in time arctic species appeared. ‘Then 
came the Glacial Epoch, when tropical plants were driven 
south to the torrid zone, and arctic species reached as far 
south as the Pyrenees. Then, as the cold of the glacial period 
lessened, those arctic species which had migrated southwards 
remained on the mountain ranges, hence the presence of arctic 
plants on the tropical mountains of the world. A similar | 
glacial period in the southern hemisphere would account for 
the appearance of alpine plants in the southern continents. 
The following table shows the chief subdivisions of the 
