i9or- Coi^GAN. — Notes on Irish Topographical Botany. 235 



which affect plant distribution. Perhaps the most potent of 

 these conditions, at least for areas of considerable extent in 

 latitude, is temperature, as any large variation in this 

 condition must limit the number of ubiquitous species by 

 constituting mutually exclusive northern and southern floral 

 groups increasing in size with increase in the difference of 

 the regional temperatures. Other things being equal, tem- 

 perature varies with latitude, and the range in latitude of 

 Great Britain being twice that of Ireland, we have in the 

 consequent wider range of temperature alone good reason for 

 expecting a much smaller proportion of ubiquitous species in 

 the larger than in the smaller island. A further reason for 

 such an expectation will be found in the far greater number 

 of divisions in Great Britain than in Ireland, since the risk of 

 a particular species failing to find in all divisions the precise 

 conditions of soil and surface which it demands, increases 

 with the increase in the number of divisions. 



If we assume then for the purpose of the comparison it is 

 desired to make here, that the proportion of ubiquitous 

 species to the total flora in Great Britain should be only half 

 the proportion of such species to the total flora in Ireland, we 

 can hardly be charged with undue severity toward British 

 field botanists. To remove, however, the smallest suspicion 

 of any such severity, let us reduce the number of British vice- 

 counties from 112 to 109 by excluding the three purely insular 

 divisions of the Orkneys, the Shetlands, and the Outer 

 Hebrides, as, quite apart from climatic considerations, many 

 of the ubiquitous species of a mainland may fail to reach an 

 adjacent insular area. Ireland, it may be noted, has no 

 comital divisions purely or even largely insular. Taking 

 then as ubiquitous for Great Britain such species as occur in 

 not less than 109 of its vice-comital divisions, and assuming 

 that the per-centage of such species in the British flora should 

 be one-half of the per-centage of such species found in the 

 flora of Ireland, we are in a position to compare the results of 

 British and of Irish research in matters of topographical 

 botany. 



From Mr. Praeger's Table we find that the percentage of 

 ubiquitous species in Ireland is 25-5 of the total flora, or 260 

 out of 1,020, so that, on our assumption, the British flora should 



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