LICHENOLOGY OF ICELAND 241 
Rock-lichens. With regard to these it can be stated that 
tropical Africa has 182 species (24 °/o), Italy 729 (46 °/o), Denmark 
169 (39—40 °/o) and Iceland 157 (47 °/o). The figures indicate that 
the sub-tropics are rich, and the purely tropical regions poor, in 
species; whilst the temperate and Arctic regions are less rich in 
species than are the sub-tropics. 
On comparing more particularly Denmark with Iceland, we find 
that the number of species is greatest in Denmark, although Iceland 
is much larger in area. Remembering, moreover, that Iceland has 
bare rock-substrata, the superficial extent of which is so great that 
in Denmark we can form no conception of it, whilst our Danish 
species are limited to the very modest granite-surfaces on Bornholm, 
and to the loose stones found scattered about in fields and in fences, 
the small number connected with Iceland appears extremely eluci- 
datory. It is impossible to explain this as anything else than a 
direct result of the climate, because Iceland has so many kinds of 
rock-substrata, that there would be plenty for the lichens to choose 
among, if the climate had otherwise been favourable to them. 
We can consequently briefly sum up the above in the following 
few sentences: — 
(1) Iceland (as also the Arctic countries) has on the whole a 
lichen-vegetation poor in species in proportion to its area, poorer 
than have the temperate and sub-tropical countries. 
(2) The Bark-lichens meet with the most favourable condi- 
tions in the tropics — that is to say, they are rich in species there 
— in the Sub-tropics and in the Temperate regions they are poorer, 
whilst in the Arctic countries and in Iceland they are poorest of 
all; this should probably be correlated with the abundance of sub- 
strata present. 
(3) The Epiphyllous lichens find the most favourable con- 
ditions in the Tropics, less favourable in the Sub-tropics, and least 
favourable of all in the Temperate regions; in the Arctic countries 
and in Iceland they are entirely wanting. 
(4) The Earth-lichens meet with very unfavourable conditions 
in the Tropics, better in the Sub-tropics, better still (probably) in 
the Temperate regions, and best of all in the Arctic regions — as 
regards conditions concerning competition. The climate, on the other 
hand, appears to be directly unfavourable to them in the Arctic 
regions and in Iceland. 
(5) The Rock-lichens meet with very unfavourable conditions 
The Botany of Iceland. Vol. IL 16 
