306 FIRST FORMS OF VEGETATION. 
distinct species have been found and described in 
Britain alone ; while more than twenty thousand 
species altogether are known to the scientific 
world. In round numbers it may be said that 
fungi form about a third of the flowerless plants. 
To show how numerous and varied are their 
forms, it may be mentioned that the British species 
are distributed in 368 genera,—an unusually large 
proportion ; only eight species on an average being 
included in each genus. A large number of these 
species constitute separately distinct genera. In 
no family of plants, indeed, are there so many 
single forms, which, owing to the absence of affini- 
tive characters, cannot be associated together,— 
so many genera consisting of only one species. 
While, on the other hand, there are no other plants 
which have such immense genera, containing, some 
of them, hundreds of species. The genus Agari- 
cus, for instance, in this country alone has up- 
wards of 450 species, so closely allied to the com- 
mon mushroom of our tables, that many of them 
are continually confounded with it, and yet ex- 
hibiting specific differences in colour, shape, size, 
etc., so distinct as to be easily distinguished by an 
educated eye. The two genera, Spheria and 
Peziza—whose ideal forms, in the former case a 
simple round ball furnished at the apex with a 
minute orifice, and filled internally with minute 
