i9oi. Coi<GAN. — Notes on Irish Topographical Botany. 237 



by 1, lesser degrees of diversity being indicated by fractions 

 receding further and further froin this maximum as the floras 

 approach more and more nearly to identity of constitution. 



A few instances drawn from Mr. Praeger's Table of Dis- 

 tribution and from his estimates of the total floras of the 

 various divisions, will serve to show the application of this 

 method of comparison. First, let us take two divisions 

 which a priori might be expected to yield the highest index 

 of diversity for Ireland, No. 1, South Kerry, situated in the 

 extreme south-west, and No. 39, Antrim, in the extreme north- 

 east. Of the 680 species in the flora of South Kerry, we find 

 that 78 are peculiar to that division as compared with 

 Antrim, that is to say, do not occur in the latter division. 

 Deducting these peculiar species from the total flora of South 

 Kerry, we have 602 species common to both South Kerry and 

 Antrim, and deducting the number of these common species 

 from the Antrim total, 777, we leave 175 as the number of 

 peculiar species in Antrim. The total of distinct species, 

 then, for the two divisions combined will be obviously 

 the common species plus the peculiar species, that is, 

 602 + 78 + 175, or 602 + 253 = 855, and the index of diversity 

 for South Kerry as compared with Antrim will be fff, or 



•296. 



The diversity just shown may be taken as largely, perhaps 

 mainly, climatic in its origin, since the physical features of 

 the two divisions are not very dissimilar. To a like origin we 

 should chiefly attribute the diversity for the extreme south- 

 west as compared with the extreme north-west ; but the 

 degree of this diversity we should expect to be less than in 

 the case of the south-west and north-east divisions, since, 

 while the physical features are fairly similar, the climatic 

 difference between South Kerry and West Donegal (35) is less 

 than that between South Kerry and Antrim. This expecta- 

 tion is justified by the Table of Distribution. Comparing 

 the floras of the two divisions, 1 and 35, each being 

 estimated by Mr. Praeger at 680 species, we find that of 

 the South Kerry species 92 are peculiar, leaving 588 

 common both to South Kerry and to West Donegal. The 

 floras of the two divisions being equal, West Donegal, 

 too, must have 92 peculiar species ; so that the total flora 



