VIII. LATITUDE. 459 



singleness. If the slightly suspected Leucojum (by several 

 botanists), Aconis (by W. A. Bromfield), and Stratiotes 

 (by A. De CandoUe) are decided not to be true natives, 

 then the fact becomes simply that 30 geneva, each repre- 

 senting an order, are absent from one or other of the 

 three latitudinal divisions ; all of them being so absent 

 from North Britain, except Eriocaulon, which is found 

 there only. 



In thus passing from orders to genera, we find the 

 same prevailing tendency to northerly decrease or cessa- 

 tion among the genera of larger orders also ; many being 

 quite absent from Middle, and more especially from North 

 Britain. As in the case of orders, however, exceptions 

 are found also among genera ; and these exceptions are 

 not restricted to small genera. Some few of the largest 

 genera decrease in their number of species in a southerly 

 direction ; of which the troublesome genera Salix and 

 Hieracium are examples. And among some other smaller 

 genei-a, the appearance of alpine or arctic species on the 

 northern mountains either augments the total number, or 

 compensates for any cessation of more southern species. 

 Pyrola, Luzula, Juiicus, Cerastium, Arenaria, Saxifraga, 

 Gnaphalium, Pinguicula, and Erigeron are generic exam- 

 ples of numerical increase northwards ; and, conversely, 

 of decrease southwards. 



Certain smaller genera are confined to the flora of 

 North Britain, though much too few in number to ba- 

 lance those which are peculiar to the more southerly 

 divisions. Cherleria, Mulgedium, Menziesia, Azalea, 

 Goodyera, Eriocaidon, Hierochloe, and Pseudathyrium are 

 thus restricted ; and all of them are genera of a solitary 

 species each in this country, unless the P. flexile may 

 prove to be a true natural species, and so add a second to 

 the genus last named. 



