Distribution of the Flora in the Alpine Zone. 39 
three districts, and absent from the two others, are common sub- 
alpine or woodland species, whose presence in one of the three 
\ 
districts to the exclusion of the two others is explained by the 
particular topographical configuration and by the greater or less 
continuity of the districts with the adjoining sub-alpine and lower 
zones. 
About thirty are fairly common species in the Valaisian Alps, 
but are sometimes absent from considerable stretches of country, 
though it is impossible to explain such absence in one or another 
of the three districts by any special ecological conditions. 
Finally, of the 660 species of the three districts, scarcely 
40 are really rare, or are strictly localised either in the Alps 
composed of crystalline rocks (Pennine Alps and the massif of 
Mont Blanc), or in the calcareous Alps (western portion of the 
Bernese chain). 
It seems then that the great majority of species (more than 
nine-tenths) might be met with in all three districts. But, as the 
annexed table shows, this is far from being the case. 
Total numbers of species occurring in pairs of districts. 
Trient and Dranses together .... 645 
Wildhorn and Trient „ .... 525 
Wildhorn and Dranses „ .... 647 
Percentage common 
Species common to two districts. to two districts. 
Trient and Dranses - - 390 60 
Wildhorn and Trient - - 295 56 
Wildhorn and Dranses - 327 50 
Each of the districts W, T and D thus possesses, besides the 
rare species mentioned above, a considerable number of species 
which are absent from one of the other two. 
The ratio of the number of species common to two districts 
(T and D for example) to the total number of species collected in 
the two districts together (T + D), i.e., their coefficient of com¬ 
munity ,' varies in the three cases given in the table between 50 
and 60 per cent. 
Thus in spite of their proximity and the similarity of their 
ecological conditions, the florulae of our three districts possess 
very different compositions, and the comparative study of these 
1 Number of species common to the two districts 
Total number of species in the two districts 
x 100 
