64 INTRODUCTION. 
by the formula; namely, because the other half of the aggregate 
cannot also be so shown apart. 
The names and numbers of the plants will correspond with those 
used in the ‘London Catalogue of British Plants,’ sixth edition, 
published in 1867, which thus becomes an arranged Index to the 
present volume. The nos. will not be found to run quite regu- 
larly ; and less so in this volume than in the Catalogue itself. 
Plants peculiar to Ireland or the Channel Islands are enumerated 
in the Catalogue, as also various Alien species and some segre- 
gates, which must here be omitted from the regular series. Hence, 
the omission of their appropriate nos. in this volume. Moreover, 
since the first edition of the London Catalogue was printed, 
various changes have been made in it, in order to adapt each 
successive edition to the actual knowledge and views of British 
botany at its date; and these changes necessitated a doubling or 
repetition of some of the nos. and a transfer of others ; so that the 
numeration of the Catalogue itself has ceased to be quite regular. 
As the same Catalogue was followed in the original Cybele Britan- 
nica, the several volumes of that work corresponding with the 
successive editions of the Catalogue, the sequence of names will 
still be nearly the same in the present Compendium, and usually 
also the same nos. will precede them. This co-relation of 
Cybele and Catalogue was one among the several cogent reasons 
for continuing the original numeration of the latter through six 
editions ; it being found altogether far more convenient to alter a 
few nos. in each edition, than to have six editions differently 
numbered throughout; which must otherwise unavoidably have 
been the case with respect to all nos. after the first changes, which 
would each time have occurred almost at the beginning of the list, 
in the genera Thalictrum and Ranunculus. 
No species is admitted into the series unless the Author has 
actually seen a specimen alleged on good grounds to be of British 
origin. So many errors have got into our botanical literature, and 
have been transferred from book to book, as merely false names or 
otherwise incorrect reports, that some such rule for non-admittance 
scems to have become highly desirable. And after his many years 
