GEOLOGICAL AND GEOGRAPHICAL DISTRIBUTION 229 
which had preceded it. While the comparative scarcity 
of Monocotyledons has been explained by the assump- 
tion that they are a later development than the Dicoty- 
ledons, a more plausible explanation is probably that the 
small number of types in which the tissues were firm 
enough to have been preserved, accounts for their 
scarcity in a fossil state. 7 
The geological history of the Dicotyledons shows, as 
might have been expected, that the earlier types are all 
Choripetale —largely the primitive Amentacez, willows, 
poplars, etc. These may have been preceded by herba- 
ceous forms like the living Piperinex, but of these no 
traces have been found. The Sympetale, which are 
the most specialized and numerous group at present, 
do not appear until much later, and the fossil record, 
so far as it goes, is quite in accord with the conclu- 
sions based upon comparative morphology. 
GEOGRAPHICAL DISTRIBUTION 
In considering the distribution of terrestrial plants, as 
they at present exist, many factors must be taken into 
account. First of all, we must examine the original 
distribution of the ancestors of the existing flora, as 
revealed to us by the study of fossil forms. There are 
next to be considered the factors affecting the distri- 
bution of these forms as they are found at the present 
time. The most obvious of these factors are climate, 
i.e. temperature and rainfall; direction of mountains 
and distribution of arid districts; currents of air and 
water; animals, including man. 
The distribution of plants during the Tertiary period, 
