236 The Irish Naturalist. December, 



yield 1275 per. cent, of such species, or 191 out of a total of 

 1,500. But an examination of the ninth edition of the London 

 Catalogue shows that in 1895, after twenty years' work in this 

 field of inquiry, British botanists had ascertainedthe occurrence 

 of only 179 ubiquitous species in Great Britain, while we may 

 fairly assume the disparity in the areas of the two islands to 

 have been counterbalanced by the greater number of com- 

 petent workers in the larger. These considerations will, it is 

 hoped, enable students of Irish Topographical Botany to 

 appreciate the really wonderful amount of sustained and skil- 

 fully directed energy which its author has brought to bear on 

 the botanical exploration of Ireland. 



A discussion on ubiquitous species naturally raises the 

 whole question of what may be called floral diversity, a term 

 which requires some explanation. It is almost an axiom in 

 distribution that perfect identity in the floras of two distinct 

 areas cannot occur. This is true even if the areas be con- 

 tiguous and of no larger extent than a parish, or even a town- 

 land ; it is true a fortiori if the areas compared be large and 

 widely separated. Should the number of species in one area 

 happen to be precisely equal to the number in the other, yet 

 the species of one will not be always those of the other ; each 

 area will have some species peculiar to itself as compared 

 with the other. This greater or less departure from identity 

 of species in the floras of two distinct areas is what is meant 

 here \>y floral diversity ; it is simply an expression of the vary- 

 ing intensity and complexity of the influences which operate 

 in plant distribution. 



The existence of such a book as Irish Topographical Botany 

 enables us to discuss this interesting question in greater 

 detail than was possible with the data supplied by the second 

 edition of Cybele Hibemica. In such a discussion, it is first 

 of all necessary to fix on some uniform method of expressing 

 these floral dissimilarities, of calculating, in short, an index of 

 diversity for each pair of areas to be compared. It seems to 

 me that the most satisfactory index will be found in the ratio 

 which the total of species ?iot common to both areas bears to the total 

 flora of the two areas combined. By this method total floral 

 diversity, such as exists, say, between the Co. Dublin and a 

 strip of virgin forest on the Amazons, would be represented 



