158 H. MOLHOLM HANSEN 
species from class I to class VII is given by the following values: 8.3, 
12.7, 14.3, 13.2, 11.1, 7.9, and 1.8 and the mean number of species in 
the corresponding classes by 25, 32, 36, 39, 33, 19, and 5. It applies 
both to the number and density of species that they attain a maxi- 
mum in both scales of external factors, whence they decrease more or 
less in both directions. The position of this maximum corresponds 
to the conditions of environment normally prevalent in the country, 
and practically coincides for both series of figures. The maximum 
for the scale of snow-covering lies in class II, corresponding to the 
mo, for the scale of moisture it lies in classes III—IV, corresponding 
to moist mo and jadar. These types of vegetation must be regarded 
as the climax vegetation of the country. 
The rule thus seems to hold good that those parts of the country 
where the environment is typical of the country and which, there- 
fore, bear the climax vegetation of the country, have the 
vegetation which is richest and densest in species. No 
matter in what direction the external factors are changed, 
whether in the direction of greater drought or greater 
moisture, or in the direction of a deeper or a slighter 
snow-covering, the result will always be a diminution 
both of the number and of the density of species, and 
the greater the change of environment, the greater the 
diminution. 
Our investigation of the distribution of the species, species 
groups, and life-forms in the formations, arranged according to in- 
creasing prevalence of one and the same external factor, has here- 
with been brought to a close as far as the Icelandic scales of 
moisture and snow-covering are concerned. Besides extending the 
investigations to an increased number of external factors, it will 
likewise be appropriate to divide the flora into groups according to 
the distribution of the species from Atlantic to more continental 
regions. It would also seem of interest to divide the flora into 
groups according to the geographical distribution of the genera, 
as well as according to the quantitative distribution of the 
species within their areas. Judging by the investigations given 
above, such an extension of view-points would lead to a more 
thorough understanding of the distribution of plants in Iceland, the 
knowledge of which is essential partly for the question of the genesis 
