LICHENOLOGY OF ICELAND 135 
species it occurs, but if it be found in about 20 species, that is no 
doubt all, and does not form even one-tenth of the species. The 
chief means of propagation of crustaceous lichens is, as we know, 
ascopores. 
In the foliaceous lichens it has been found in the species — 
about 9 — mentioned above, that is to say, in about one-sixth of 
the total number of species. 
In how many species of fructicose lichens it has been found, 
cannot be stated with any certainty, but doubtless, the number does 
not greatly exceed that of the foliaceous species. 
Whether any immigration has taken place by means of de- 
tached portions of thallus which have been conveyed by the 
wind, it is impossible to decide. It has been mentioned above that 
this mode of dispersal plays a considerable part within the boundaries 
of the country, with regard to many of the fruticose and foliaceous 
lichens. But whether portions of thallus, capable of germination, are 
really transported through the air from the surrounding countries, 
cannot, of course, be known, but the possibility is scarcely pre- 
cluded. 
Judging from the above, the rôle which we must assume that 
the wind has played in the immigration and emigration of Iceland’s 
species, is thus very considerable, as all the crustaceous lichens and 
the majority — perhaps even all — of the fructicose and foliaceous 
lichens have such means of dispersal (ascospores, pycnocodia, soredia 
and detached pieces of thallus) as justify us in believing that the 
wind in particular has transported them to the country. 
Water has played a far less considerable part as a means of 
dispersal, in fact, it can be assumed only with regard to the few 
submerged Verrucaria spp., and the emergent V. maura, that they 
have immigrated by this means. They occur doubtless, over nearly 
the whole of the Arctic, and over great parts of the adjoining climate- 
areas, on cliffs out in the sea. They are common on the coasts of 
Greenland, Iceland, Norway, the Ferées, Denmark and Great Bri- 
tain, consequently both in Arctic and in temperate regions. They 
constitute altogether not above 2—3 °/o of the flora of Iceland. 
What importance animals have had as regards immigration 
is quite unknown. Here again it must suffice us to frame questions 
which will, perhaps, in the future, be taken up and answered by others. 
Primarily it may be supposed that birds of passage which 
migrate backwards and forwards between Iceland and milder regions, 
