Chap. XI. 
DURING THE GLACIAL PERIOD.- 
369 
first at the bases and ultimately on the summits of 
the mountains, the case will have been somewhat dif¬ 
ferent ; for it is not likely that all the same arctic spe¬ 
cies will have been left on mountain ranges distant from 
each other, and have survived there ever since; they 
will, also, in all probability have become mingled with 
ancient Alpine species, which must have existed on 
the mountains before the commencement of the Grlacial 
epoch, and which during its coldest period will have 
been temporarily driven down to the plains; they will, 
also, have been exposed to somewhat different climatal 
influences. Their mutual relations will thus have been 
in some degree disturbed; consequently they will have 
been liable to modification; and this we find has been 
the case; for if we compare the present Alpine plants 
and animals of the several great European mountain- 
ranges, though very many of the species are identically 
the same, some present varieties, some are ranked as 
doubtful forms, and some few are distinct yet closely 
allied or representative species. 
In illustrating what, as I believe, actually took place 
during the Glacial period, I assumed that at its com¬ 
mencement the arctic productions were as uniform 
round the polar regions as they are at the present day. 
But the foregoing remarks on distribution apply not 
only to strictly arctic forms, but also to many sub-arctic 
and to some few northern temperate forms, for some of 
these are the same on the lower mountains and on the 
plains of North America and Europe; and it may be 
reasonably asked how I account for the necessary de¬ 
gree of uniformity of the sub-arctic and northern tem¬ 
perate forms round the world, at the commencement of 
the Glacial period. At the present day, the sub-arctic 
and northern temperate productions of the Old and 
New Worlds are separated from each other by the 
