218 CHARACTERS OF TRIBES AND GENERA. 
belonging to this genus; they have a wide geographical 
range; extending from the arctic to the antarctic circles, 
and abounding in the tropics, ascending to great elevations, 
Its most common representatives are forms analagous to 
the Polypodium aculeatum, Linn. (Aspidiwm, Sw.), of the 
Eupropean flora, of which there are several states found in ` 
our own country sufficiently distinct to be considered 
separate species by some botanists, though others view 
them as only different states of one variable species. A 
similar difference of opinion exists regarding the foreign 
forms, the original discoverers having named and described 
plants from different localities as distinct species, and, 
indeed, in many instances they seem sufficiently distinct 
to be readily acknowledged as such, but on collating a 
general collection of specimens and regarding them from a 
Darwinian point of view, it is difficult to consider the great 
majority of them otherwise than as climatic conditions of a 
Single species; and the same may be said of the group 
represented by P. coriaceum. The difference in some of the 
extreme forms is, however, so obvious, especially when 
living examples are examined, that it seems reasonable to 
distinguish them by different names, and I therefore, after 
much consideration, think it best to enumerate them under 
the original names given by authors. 
As a genus, Polystichum is technically distinguished 
from Lastrea by its indusium being circular and peltate, 
instead of reniform, as in the latter. This character, 
. however, cannot be invariably depended upon, for in some 
Species with the habit of Polystichum have reniform in- 
dusium, and instances occur where peltate and reniform 
indusia are found on the same frond. To determine the 
genus, it is therefore necessary to study its habit. As a 
general rule, Polystichum may be known from Lastrea by 
