622 ON THE DISTINCTION OF SPECIES. 
all other respects “as like as two peas in a pod.” But in the 
whole range of botanical characters we could scarce find 
better examples of inconstancy, than are seen in the form and 
toothing of the perigone in the genus Atriplex, which are, 
nevertheless, made grounds for distinction between book 
species. ; 
It would be easy to extend these remarks on the distinc- 
tion of species in books and in nature, by suggesting other 
rules, applicable in some degree, as tests of the validity 
of book species; but they are already longer than was 
wished or anticipated, when I began to put them on paper. — 
It is hoped that they may prepare the way for proper 
inferences, (as I coneeive them to be), respecting the limits of 
certain species, in which variations have been observed from 
the characters commonly assigned to them in books; more 
particularly in the two recent works on the plants of Britain, 
to which I have already several times alluded. Such varia- 
tions must be reserved for another occasion ; though some of. 
them, it may be seen, have incidentally been mentioned — 
here, as illustrations of my grounds for rejecting some species 
and some specific characters which are now admitted by | 
other botanists. 
In conclusion, I may i well confess at once, that I have 
been urged to write upon this subject, chiefly on account of 3 
the impediments which mere book species prove to me, 
in prosecuting a favourite department of botanical science; 
namely, inquiries concerning the geographical relations of | 
plants. True lists of species, both as regards their names 
and their distinctness in nature, are essential to such in- | 
quiries ; and yet it is impossible to make them true, while 
describers of plants are so continually changing both names | 
and species in their books. Fortunately, they cannot change - 
the species in nature also, if permanently distinct species do | 
certainly exist. 
