Page - 414 - in The Origin of Species

414 ORIGIN OF SPECIES the same species on distant mountain-summits, we may al- most conclude, without other evidence, that a colder climate formerly permitted their migration across the intervening lowlands, now become too warm for their existence. As the arctic forms moved first southward and afterwards backwards to the north, in unison with the changing climate, they will not have been exposed during their long migrations to any great diversity of temperature; and as they all mi- grated in a body together, their mutual relations will not have been much disturbed. Hence, in accordance with the principles inculcated in this volume, these forms will not have been liable to much modification. But with the Alpine pro- ductions, left isolated from the moment of the returning warmth, first at the bases and ultimately on the summits of the mountains, the case will have been somewhat different; for it is not likely that all the same arctic species will have been left on mountain-ranges far distant from each other, and have survived there ever since ; they will also in all prob- ability, have become mingled with ancient Alpine species, which must have existed on the mountains before the com- mencement of the Glacial epoch, and which during the cold- est period will have been temporarily driven down to the plains ; they will, also, have been subsequently exposed to somewhat different climatal influences. Their mutual rela- tions will thus have been in some degree disturbed; conse- quently they will have been liable to modification; and they have been modified ; for if we compare the present Alpine plants and animals of the several great European mountain- ranges one with another, though many of the species remain identically the same, some exist as varieties, some as doubt- ful forms or sub-species, and some as distinct yet closely allied species representing each other on the several ranges. In the foregoing illustration I have assumed that at the commencement of our imaginary Glacial period, the arctic productions were as uniform round the polar regions as they are at the present day. But it is also necessary to assume that many sub-arctic and some few temperate forms were the same round the world, for some of the species which now exist on the lower mountain-slopes and on the plains of North America and Europe are the same; and it may be