240 The Irish Naturalist. December, 



edition of Cybele Hibernica, written up to the opening of the 

 present year, we find that the index for Districts I. and XII. 

 is '22i, as against '297 for the divisions of South Kerry and 

 Antrim which are component parts respectively of I. and 

 XII. Similarly, comparing East with West, Districts V. and 

 VIII. give an index of '255 nearly, as against '328 for the 

 component divisions, Dublin and West Galway. Again, 

 Districts IX. and X. give an index of '225, while the com- 

 ponents, Roscommon and Fermanagh, give '25. And, finally, 

 to take the two most truly inland districts, III. and VII., we 

 find that they 3 T ield an index of -205 as against '272 for the 

 components, Queen's County and Westmeath. 



Having followed patiently this rather tedious discussion on 

 floral diversity, the reader may grow querulous. He may ask 

 what is the good of your index, and what does it prove ? To 

 be quite candid the index proves nothing ; it demonstrates no 

 new truth. Yet it may be claimed on its behalf that it serves 

 some humbler uses. It not only gives a simple and uniform 

 method of expression for complex groups of facts, but may 

 also be applied in some sort as a touchstone for the detection 

 of error. If, for instance, in the case of two areas subject to 

 closely similar conditions as to physical features, climate, 

 geographical position, and the various other factors which 

 affect plant distribution, we find the statistics giving a higher 

 diversity index than is yielded by two other areas where the 

 conditions are obviously and widely dissimilar, we imme- 

 diately infer some grave error or defect in the statistics. The 

 data are more closely scanned ; we find, perhaps, that the 

 flora of one of the areas has been explored by a judicially 

 minded botanist accustomed to the weighing of evidence, 

 while the investigation of another has fallen to the lot of an 

 uncritical enthusiast who has charitably given asylum to a 

 host of aliens and casuals. And thus the index has served as 

 a critical instrument. It is perfectly true that it has but led 

 us indirectly to results that might have been directly gained 

 by a study of the statistics themselves. But how confusedly 

 these results glimmer through lengthy catalogues of plant- 

 names, when compared with their clear and condensed 

 expression in a few decimals. 



Sandycove, Co. Dublin. 



