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. 



