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 superficial 
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 Undersøgelser” (1913). In itself it is immediately evident, 
that no other means of comparison is equal to this as regards reli- 
