PRINCIPLES OF SCIENTIFIC ARRANGEMENT. 123 
cific form in the succession of generations, that even 
the shadow of a deviation from the typical distinction is 
scarcely to be discovered: a reason for this it is hard to 
surmise. We may, nevertheless, conclude it to be cer¬ 
tain that true species are ever distinct, and can no more 
coalesce, however closely they may approach together, 
than can asymptotes. 
Specific differences result from many characteristics,— 
from colour, clothing, size, and sometimes from pecu¬ 
liarities of structure; but these last are usually of a 
higher order, tending to indicate an aberration, slight 
though it be, from the normal generic character which 
holds the group together, thus implying a distinctive 
economy. This is sometimes called a subgeneric attri¬ 
bute, and there might be a reason, certainly, for not 
elevating such species to the full rank of genera, were 
genera equivalents, which they are not, and it merely 
remains an evasive admission of the doubt that attaches, 
except for the sake of convenience, to any subdivision, 
but the specific. 
The species is thus the very last term of subdivision, 
the very elemental principle itself, which unites toge¬ 
ther as one, solely for the purposes of perpetuation, the 
two sexes of similar individuals, and without whose in¬ 
tercourse the kind or species would die out. 
That some species greatly abound in individuals, as 
before observed, whilst others appear to be extremely 
limited, is an absolute fact, and not merely suggested 
by a defective observation of their occurrence, resulting 
from their rapid dispersion. It is verified by being 
noticed to occur where we know they would resort, as is 
exemplified in the case of some of the parasitical species 
of the insects herein treated of, and which are sometimes 
