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FERNS : BRITISH AND FOREIGN. 
of their association with a host of species of quite dis- 
tinct characters. It also leads to investigation, and, 
accordingly, to a better knowledge of the structure of 
Perns. 
With regard to the characters that define the limits 
of species, as much uncertainty prevails amongst 
authors as with genera. This is owing to several 
causes ; such as many species being normally hetero- 
morphous, presenting at the same time different forms, 
which again vary at another period of growth ; and in 
many instances authoi’S have described the different 
states as distinct species ; and in some cases different 
fronds of the same plant, and even portions of the 
same frond, have been placed under separate genera. 
It also frequently happens that two or more presumed 
species present so many intermediate gradations of 
form, that only the most extreme states appear suffi- 
ciently distinct to warrant their adoption as species, 
the numerous intermediate forms seeming to set 
specific distinctions at defiance. It therefore becomes 
a question what is the limit of form or of structure that 
constitutes a species. Generally understood, a species 
is an organized structure endowed with an essence or 
quality peculiar to itself, and possessing the power of 
multiplying and transmitting its type to new genera- 
tions without change, ad infinitum. Admitting this 
definition as correct, it seems to be beyond human 
power to ascertain whether the serial gradations of 
form are genuine descendants of original creations, or 
only deviations from one original, brought into exist- 
ence during the lapse of ages by the different climatic 
and local influences they have been subjected to. It 
is well known that phaenogamous plants assume differ- 
