xlii INTEODUCTION 



tolerably equal parts. The subdivision is an adaptation of a 

 suggestion made ( "Irish Nat.," ix. 44) by Mr. Hart, the author of 

 the county Flora. 



The whole of Ireland, 32,524 square miles, is thus divided into 

 40 portions of as nearly equal size as conditions will permit ; and 

 these divisions closely approximate in area to Watson's 112 "vice- 

 counties " of Great Britain. To be exact, the average size of the 

 British divisions is 804 square miles each, of the Irish 813 square 

 miles. On account of the variation in size of the counties, it 

 is not possible to attain the ideal — exact equality of area of 

 the 40 divisions. As a matter of fact, the divisions vary in 

 size from 1336 square miles (Mayo West) to 316 square miles 

 (Louth) ; but the majority approach reasonably near to the average, 

 so that the number of county-divisions in which a plant occm'S 

 affords a fair indication of its area of distribution. 



Numbering of the Divisions. — The divisions are numbered 

 according to a backwards-and-forwards line, commencing in the 

 south-west, ending in the north-east. This progression corresponds 

 with Watson's numbering of the British vice-counties, which also 

 runs from south to north, though with less regularity, since con- 

 secutive numbers are bestowed on the vice-counties included in 

 each of his eighteen "Provinces." The advantages of grouping 

 the divisions in a similar manner under the twelve Irish botanical 

 districts which were proposed by Prof. Babington and employed in 

 " Cybele Hibernica" are slight in comparison with the suggestive 

 facts of plant-distribution brought out by the adoption of a regular 

 progression from a lower to a higher latitude — from a higher to a 

 lower temperature. It will be observed that the lines of numbering 

 run NW. and SE., rather than W. and E. This is the direction 

 of the isotherms of winter and spring ; and a glance at the botanical 

 map in " Cybele Hibernica " will show that the distribution of the 

 characteristic Irish plants has the same general trend. The 

 advantage of following these natural lines is clear. 



It will be seen that according to the plan of numbering adopted, 

 the figures are themselves an indication of the range of a species. 

 If a plant is recorded from divisions 1 to 10, we know at once that 

 it is confined to the southern fourth of Ireland — the Province of 



