Iv ON PLANT LIFE 163 
IMPERFECTION OF OUR KNOWLEDGE 
The total number of living species of 
plants may be roughly estimated at 500,000, 
and there is not one, of which we can 
say that the structure, uses, and life-history 
are yet fully known to us. Our museums 
contain large numbers which botanists have 
not yet had ‘time to describe and name. 
Even in our own country not a year passes 
without some additional plant being discov- 
ered ; as regards the less known regions of 
the earth not half the species have yet been 
collected. Among the Lichens and Fungi 
especially many problems of their life-history, 
some, indeed, of especial importance to man, 
still await solution. 
Our knowledge of the fossil forms, more- 
over, falls far short even of that of existing 
species, which, on the other hand, they must 
have greatly exceeded in number. Every 
difference of form, structure, and colour has 
doubtless some cause and explanation, so that 
the field for research is really inexhaustible. 
