Praeger — On Ti/pes of Distribution in the Irish Flora. 3 



Between " English- Atlantic " and "Atlantic-English" no wide diffe- 

 rence exists, and it can be readily imagined that the distribution of a 

 species may place it between the two. Especially in such cases, the 

 finding of a plant in a couple of new counties might turn the scale. 

 Many such discoveries have been made since Watson defined the " type 

 of distribution" of each British plant in 1870, yet the " types " have 

 not been revised. Therefore, for our pui'poses, it will be better to use 

 pure types only, where possible. 



The maps are constructed, according to a uniform plan, in five 

 ■depths of shading. The units of area employed are the forty county- 

 divisions of " Irish Topographical Botany " and the standard used as a 

 list of the Irish flora, and its distribution, is taken from the same work, 

 posted up to date. For the construction of the maps, the distribution 

 in the forty divisions of the component species of each group has been 

 tabulated. In order to balance the statistics, and maintain their 

 scientific integrity, sub-species (i.e. those printed in italics in "Irish 

 Topographical Botany") are not reckoned, nor records of doubtful 

 value (i.e. those of which the accuracy is doubted, or to which the 

 marks signifying "probably introduced" or " certainly introduced " 

 are applied). Erom the totals thus obtained for the county-divisions, 

 giving the number of plants of the type present in each, the lowest and 

 highest figures are taken, and the intervening space divided into five 

 «qual portions. The forty totals are grouped according to these five 

 portions, and the map shaded accordingly in the order : — 



(1) white, (2)= (3) =1=1=1=, (4) ^\i\i\t, (5) black. 



An example will make the process clear. Say we find that of the 

 plant-group in question the maximum number of species occurring 

 in any one of the county-divisions is 30, and the minimum 11. Divid- 

 ing this difference into five equal portions, we get as our series : — 



11 to 14, 15 to 18, 19 to 22, 23 to 26, 27 to 30 

 white m =1=1- ^|^|^ black. 



■ It is to' be distinctly understood that the shading of each division 

 represents the number of plants of the group which occur in it, not 

 their distribution in the division. Eor instance, in the Highland type 

 map, the actual distribution of the species in many divisions would 

 show as little more than a few dots on the map ; instead of which 

 an even shading is spread over the whole of each division according to 

 the number of Highland plants growing within it. 



£2 



