1 will try to illustrate what I mean by an example drawn from 
our own immediate neighbourhood. No doubt we are all acquainted 
with the genus or sub-genus Polystichum among the British ferns ; 
now the forms of this genus, occurring in Great Britain, have been 
variously described by those to whom we look up as authorities, as 
being divided into four species — three species — two species, three 
sub-species, and one variety — three species, fourteen varieties, and 
about fifty distinct forms. But from a tolerably intimate acquaint- 
ance with this genus as it is found in Britain, having observed it 
in most of its states, ranging from the tiny mountain form of 
Louchitis, of a few inches, to the luxuriant Devonshire forms of 
Angulare, of seven feet long ; I venture to suggest that, after all, 
they are but one species, for on the heavy lands lying west of 
Norwich, may be collected some twenty to thirty different forms, 
from one closely resembling the mountain simply pinnate form, 
through varying shades of division, to the almost tripinnate form, 
which is the most divided state known in the British Isles. There 
they are by hundreds, growing side by side, shading off so gradually 
from the simple to the compound form, that it is virtually impos- 
sible to say where one begins and the other ends ; and here we 
have, within the limits of the species thus considered, the utmost 
laxity of variation, but this laxity goes no further than the species ; 
interspersed with Polystichum are numbers of other ferns, of very 
closely allied sub-genera — Lastrea, for instance, but among them 
all, you will not find one single individual plant that holds an 
intermediate place between the genera ; there is no attempt at 
fusion, they grow, and doubtless have grown thus together for an 
immense number of generations, yet they remain perfectly distinct. 
But if we are in doubt in defining the limits of a species, how 
much will our difficult}- increase when we consider that if evolution 
in its fullest sense is to be accepted, we shall have to come to the 
conclusion that there is really no such thing as species at all ; and 
that “ the same question, in a wider sense, and taking into con- 
sideration a much longer time, would be applicable to genera and 
families.” Of course the very essence of the theory of evolution is 
the, to our comprehension, almost unlimited time, supposed to be 
