The Botany of Iceland - 01.12.1930, Side 168

H. M0LHOLM HANSEN
158
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 botli 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 jaðar. 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, thére-
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 tlieir 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