Relation of Climate to Vegetation. | 111 
this we perceive that while a study of climate will enable 
us to pretty accurately determine the character of the vege- 
tation for a given area, conversely, the critical examination 
of a given flora will enable us to arrive at tolerably exact 
conclusions relative to the climatic conditions under which 
it flourishes. Therefore, while geographical botany enables 
us to solve many questions of importance so far as the pre- 
sent is concerned, it renders it possible, by comparing 
similar types of the present and the past, to accurately de- 
termine the climatic conditions which must have obtained 
in the various geological periods since vegetation first made 
its appearance. And finally, we may note that, as plants 
are influenced in their distribution, so will their regularity 
of development depend upon uniformity of climatic condi- 
tion—periodicity in the latter enforcing periodicity in the 
former. 
In instituting inquiries of the nature of those with which 
we are now dealing, we first of all naturally seek informa- 
tion respecting the number of plants known to man. Bot- 
anists in all parts of the world are bringing hitherto un- 
known species to our knowledge, and in some of the more 
imperfectly explored parts of the globe, the number thus 
constantly added is very considerable. It will therefore 
appear that we are wholly unable at the present time to 
make any exact statement relative to the number of existing 
species. Meyen in 1846, estimated the whole number of 
species at somewhat more than 200,000. Duchartre’s es- 
timate places the figure between 150,000 and 200,000, while 
De Candolle and Gray estimate somewhat more than 
120,000 species of flowering plants alone. 
But the distribution of this enormous number of plants is 
nowhere uniform. Hach species or genus has its centre 
of distribution where the number of individuals is greatest, 
from which there is a more or less rapid diminution in all 
directions until the extreme limits are reached. This law 
may be illustrated in our own flora. Of the North American 
oaks, there are thirty-six species. These have their cen- 
tre of distribution within a narrow radius centering upon 
