46 TRANSACTIONS OF THE [S 



Sess. lxxx 



the Burma frontier is continued ; for while it seems pro- 

 bable that the Himalayas (except perhaps Bhotan) will 

 not yield many new Primulas, there are probably a large 

 number still to be found in the Burma-Yunnan area, the 

 difficulties in the exploration of which can hardly be 

 exaggerated. 



The richness of the Burma-Yunnan area is shown as 

 much by the fact that it has nine sections conhned to it 

 as by the increase of Himalayan forms there, while the 

 isolation of the North China area is shown by the fact that, 

 in spite of its comparative poverty in Primulas, it also has 

 seven sections confined to it, several of which are unique 

 in the genus. This is an argument in favour of the belief 

 that the North China area has not been recently in com- 

 munication with the Burma-Yunnan area, or at least not 

 as recently as has the Himalayan area, which it seems 

 possible to me may still be in some sort of connnunication 

 with it. 



A consideration of these facts seems to show then that, 

 so far as the broad distribution of the genus is concerned, 

 they fit in with the theory of a Sino-Himalayan range 

 which has been breached, the eastern end being isolated 

 and the western end remaining more or less in communica- 

 tion with the south, at least till a much later date, via a 

 series of curved ranges, wherein, partly owing to its sources 

 of supply and partly owing to physical conditions, changes 

 of climate, soil, and so on, a new and richer Primula area 

 has come into existence, still further augmented by the 

 infiux of eastern forms driven backwards and southwards 

 by the ice. Mr. Farrer says that crosses between Primulas 

 occur most frequently, if not exclusively, between extreme 

 species of the same secticjn — in other words, between species 

 of difi'erent subsections within the limits of a single section. 

 For example, in the section Candelabra, which forms two 

 colour-groups, we might expect one of the yellow group to 

 cross with one of the purple group, but not a purple with 

 a purple or a yellow with a yellow. 



Now at a time wiien the fiora of the earth was more 

 uniform than it is at present — say, in early Tertiary times — 

 the fiora of such a continuous range as the Sino-Himalayan 

 would show no very great variation, and Primula itself 



