246 TRANSACTIONS—PERTHSHIRE SOCIETY OF NATURAL SCIENCE. 
able to judge, the evidence for a great submergence on the one hand, 
or against it on the other, is at the present time about equally bal¬ 
anced, for while these high-level shell beds found in Ayrshire seem 
to be undoubtedly m situ, and intercalated between the boulder clay, 
this would necessitate a submergence of over a thousand feet to 
account for their position there. It is, however, a singular thing that 
if this submergence took place all over Scotland the traces of it in 
other localities should be so scanty, in fact, that they should be prac¬ 
tically non-existent at all. 
Leaving, however, this difficult geological problem, the question 
arises,., is it more probable that these Alpine plants should have 
migrated to this country during such a submergence as we have seen 
is contended for by some geologists and botanists, or during an eleva¬ 
tion of the land as maintained by others. To us it seems the latter 
condition would be by far the most favourable to the immigration of 
the Scandinavian flora, for, though it were quite possible that they 
could have been carried on icebergs from the mainland, yet the 
evidence derived from the fauna as well as the flora goes to show 
that it is more probable that their dispersal took place during a land 
connection between Britain and the Continent. 
Having thus discussed the probable conditions of the immigration 
of these plants into our country, and having seen that in all likelihood 
they must have at one time covered the whole of the lower grounds 
of these islands, it now remains for us to consider the manner in 
which they have been exterminated, so as to account for their present 
limited distribution upon these mountains which we have indicated 
at the beginning of this paper. As we have seen in the case of the 
maritime plants, it must be conceded at once that environment is 
the principal factor to which we must look for an explanation of the 
present distribution of these plants if it can be shown that on such 
mountains as Ben Lui, Craig-na-Caillach, Ben Lawers, or the Clova 
range there are any particular conditions of environment which would 
give these plants an advantage, so that in the gradual recession from 
the lowlands they should be able, because of these advantages, to 
make their final stand upon this ridge. As we have seen at the 
outset of this paper, altitude alone is not in itself sufficient to account 
for the present distribution of these Alpine plants on our Perthshire 
mountains; for we have many instances of mountain summits rising 
to an equal height and almost barren of Alpine plants, while others 
in the Highlands rise to even greater heights and are poor in Alpine 
plants, so that we must look to some other cause than altitude alone 
for a solution of the problem. 
Axel Blytt, in his paper on the immigration of the Norwegian 
Flora, has attempted to show That certain meteorological or physical 
