6 ON GENERA AND SPECIES. 
relative importance assigned to them. For example, while 
| some, as Presl, Fée, Moore, and myself, break up the old 
Linnean genera, Polypodium, Aspidium, &c., into a greater 
or lesser number of smaller genera, upon characters derived 
from differences in their anatomical structure and modes of 
growth; others, as Hooker and Mettenius, prefer adhering - 
to the Linnean genera, without greatly altering their 
characters, and adopting the modern generie names as 
sectional ones for such divisions as they find themselves 
compelled to make. 
On reviewing what I have now stated it may naturally 
be asked, What is a species or genus ? or, by what law of 
nature can this be determined? As generally under- 
stood by naturalists, a species is an organised structure | 
specially created and endowed with an essence or quality 
peculiar to itself, possessing the power of increase and 
transmitting its primitive essence and anatomical structure 
and form without change, to its progeny for successive | 
generations. But the difficulty of defining species becomes ` 
evident on taking a general view of the numerous forms 
. which connect one with another, Tt will then be found be- — 
. yond human power to ascertain whether the several grada- - 
tions of allied forms are descendants of primitive specific 
creations, or are, according to the Darwinian theory of the 
" origin of species," only deviations from a few primordial 
creations, endowed with a protean principle which becomes 
manifest during the lapse of ages, and controlled by the 
different climatic and local influences under which the 
progeny of the original have become established, and 
which now form the flora of the earth. If the latter is 
admitted to be the case, and we are led to believe that 
. intermediate forms originate during the slow progress € 
time,.then all must be uncertainty, and the number « 
