394 Professor Henslow's Catalogue of British Plants. 
consideration, or the degree in which the species may have claim to 
be held indigenous to Britain, are fourfold, and are indicated by 
particular marks, namely, species " possibly introduced by the agency 
of 7waw"t " Naturalized species, certainly not indigenous"* oc- 
casionally found wild, but not even naturalized, extinct, or erroneously 
introduced, and which ought to be excluded from our Floras" (o) ; 
admitted natives. The species included in the Flora Cantabrigiensis 
are distinguished by the addition of a letter (c,) and the goodness 
of their titles to be considered natives of the district, to which this 
Flora relates, is also indicated. 
On the very debateable question touching the identity or distinct- 
ness of species, it is in vain to expect uniformity of judgment, while 
mere opinion, in contradistinction to fixed rules, is the only guide for 
our decisions. No fixed rules have yet been discovered or admitted, 
and we might add, probably with equal truth, that no fixed rules 
ever will be discovered. The subdivisions of genera, or (what is 
the same thing,) the union of varieties into species, must rest on the 
ground of expediency, so long as absolute rules are wanting; and, 
viewing the question in this light, we unhesitatingly recommend the 
catalogue of Professor Henslow as the best guide for the botanist of 
Britain ; agreeing with the Professor's suggestions in most instances, 
though inclined to sink the majority of his questionable species into 
the grade of mere varieties. We do not, however, so fully agree 
with the manner of classifying the species not indigenous or not 
found at all in Britain. It appears a more judicious course to join 
the species " occasionally found wild," with the " naturalized spe- 
cies," both classes being " certainly not indigenous ;" and the differ- 
ence between them being one of degree only, where the degrees are 
so very difficult to distinguish, that no uniformity of opinion can be 
hoped for. The classes of extinct and mistaken species are readily 
separable from the preceding, since these species are not found wild 
in any sense of the term. They are, indeed, equally different from 
each other, yet cannot be completely separated ; it being now quite 
impossible to decide whether several of the species, the names of 
which are still included in our Floras, have become extinct ; or whe- 
ther such names were introduced originally through some mistake 
about the plants to which they were applied. It would be well to 
expunge from our descriptive Floras the names of all species not now 
found wild. The remainder would unite into two classes, the wild, 
but certainly introduced ; the established, oj uncertain origin. The 
subdivisions of these two groups would be merely those of degree. 
In the former, the extent to which the species have become natu- 
