274 FIRST PRINCIPLES OF NATURAL CLASSIFICATION. 
as we have before explained*, may be accounted for in 
two ways, either by our imperfect acquaintance with 
the productions of nature, or by the extinction of those 
animals which would render such groups pevfect. 
- (335.) Having now laid before the reader a few of 
those facts which serve to verify the general truth 
of the five propositions with which this division of our 
volume commenced, we must revert to a subject in- 
timately connected with the definite character of natural 
groups, and of which they are, in fact, composed ; we 
mean species and varieties — those individuals, in short, 
which constitute the assemblages in question, and whose 
variation leads to a knowledge of all higher combin- 
ations. We alluded toa theoretic belief, even now com- 
mon among naturalists, that species are the only absolute 
divisions of nature. So far, however, from such being 
the fact, we believe that the truth consists in this posi- 
tion being reversed ; in other words, that if there are 
any absolute natural divisions, they are to be found in 
the different gradations of groups and types here pointed 
out, but that in numberless cases it is utterly impossible 
to discriminate species from varieties ; species, in short, 
being, to human apprehension, the most indeterminate 
of all the links in the chain of being. This opinion is 
borne out by the sentiments of one whose peculiar line 
of study renders him, on this subject, one of the highest 
authorities in this country.t Setting aside, however, 
those exceptions which give rise to these opinions, and 
where the discrimination of species from varieties is 
impossible, we shall now proceed to describe those pe- 
culiarities which generally constitute a species; and we 
do this fully, because we think the subject has not re- 
ceived that attention, in introductory works, which it 
merits. 
(336.) A species, in popular language, may be de- 
fined as “ a natural object, whose differences from those 
most nearly related to it are, as far as observation has 
* See Preliminary Discourse on Nat. Hist. p. 213. 
+t J, F. Stephens’s Catalogue of British Insects, preface, p. Xvi. 
