4 ; ON GENERA AND SPECIES. 
stances of observation and discrimination, uncertainty often 
prevails. For instance, no class of plants has of late 
years been more written about than the Ferns of Great 
Britain; and yet much diversity of opinion still exists 
amongst British botanists regarding the number of species, 
although they have the opportunity of carefully and 
leisurely examining them under every circumstance con. - 
nected with the different aspects they assume in their 
various places of growth. This surely offers some pallia- 
tion for the errors committed by the general pteridologist, | 
more especially when we consider that the Ferns of Great ` 
Britain are in number as one to sixty of the Fern-flora of — 
the earth, 
Having had under my observation for nearly half a cen- 
tury the Fern collection in the Royal Gardens at Kew, 
distinct species, 
Another point which renders it difficult to arrive - 
species, is the botanical rule which prescribes that the 
specific name under which a plant is first described, is to 
be retained in whatever genus it may be referred to by 
