INTRODUCTORY REMARKS. 
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fore becomes a question what is tbe limit of form or of 
structure that constitutes a species. Generally under- 
stood, 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 ge- 
nerations 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 devia- 
tions from one original, brought into existence during the 
lapse of ages by the different climatic and local influences 
they have been subjected to. It is well known that 
pheenogamous plants assume different forms and aspects, 
effected by the agency of man and by various natural 
causes ; the difference from the original types being often 
so great, that if evidence of the change were not on re- 
cord, the botanist of the present day would be justified in 
describing them as distinct originally created species. With 
Ferns we possess but little evidence of new forms having 
come into existence, the chief examples being found in 
several intermediate states in the genus Gymnogramme , 
which of late years have made their appearance in gardens, 
and seem to have as good right to be regarded as species 
as the original typical forms first known. If such changes 
do actually take place, and we are to deduce from them 
that races of intermediate forms originate in the progress 
of time and through the causes above alluded to, then great 
