LICHENOLOGY OF ICELAND 235 



hemisphere. The mutual relationship of the other belts, in this re- 

 spect, is somewhat doubtful. 



But let us now also suppose, that at some future time we shall 

 succeed in deciding the absolute number of species for each climate- 

 belt; there will nevertheless be highly important and interesting 

 details to investigate as regards these numbers; first and foremost 

 the mean number of species of the climate-belts, that is 

 the number of species per unit of area. 



For in itself it is very probable that a relatively small territory 

 as, for instance, the Antarctic region has a very small number of 

 species, whilst, for instance, the Tropical region, the superlicial 

 measure of which is many times larger than that of the Antarctic, 

 has a great number of species. If we compare the area of the 

 climate-belts with their number of species, dividing the number of 

 the species by the superficial measure (for instance, in geographical 

 square miles), we get fractions which give us a clear idea of the 

 abundance of species in proportion to the area of the climate-belt. 

 For if we imagine a climate-belt investigated, square mile after square 

 mile, and new species are constantly found, over and over again, in 

 every such small area, the sum total for the entire belt would be- 

 come very great. On the other hand, if we find in another belt, a 

 certain number of species in the square mile first investigated, and 

 thereafter the same species over and over again in the areas sub- 

 sequently investigated, the sum total for the whole climate-belt would 

 become rather small. It is exactly this circumstance which will be 

 recorded in the fraction, which results from the division of the 

 number of the species of a climate-belt by its area (e. g. in geogra- 

 phical square miles or kilometres). This fraction expresses the greater 

 or lesser monotony of the area as regards the occurrence of the 

 species. 



A third valuable means, wherewith to compare the abundance 

 of species of various climate-belts, is to take equally large (prefer- 

 ably very large) areas characteristic of the belts (that is to say areas 

 which contain all the plant-associations contained in each single 

 belt) and add up the number of their species, which then directly 

 indicates the comparison of them with regard to abundance of 

 species. This method is the most elucidatory of all three and has 

 therefore been made the subject of a fuller discussion in my "For- 

 beredende Undersegelser" (1913). In itself it is immediately evident, 

 that no other means of comparison is equal to this as regards reli- 



