CHAPTER XII - GEOGRAPHICAL DISTRIBUTION
Dispersal during the Glacial Period
The identity of many plants and animals, on mountain-summits,
separated from each other by hundreds of miles of lowlands, where
Alpine species could not possibly exist, is one of the most striking
cases known of the same species living at distant points without the
apparent possibility of their having migrated from one point to the
other. It is indeed a remarkable fact to see so many plants of the
same species living on the snowy regions of the Alps or Pyrenees,
and in the extreme northern parts of Europe; but it is far more
remarkable, that the plants on the White Mountains, in the United
States of America, are all the same with those of Labrador, and
nearly all the same, as we hear from Asa Gray, with those on the
loftiest mountains of Europe. Even as long ago as 1747, such facts led
Gmelin to conclude that the same species must have been
independently created at many distinct points; and we might have
remained in this same belief, had not Agassiz and others called
vivid attention to the Glacial period, which, as we shall
immediately see, affords a simple explanation of these facts. We
have evidence of almost every conceivable kind, organic and inorganic,
that, within a very recent geological period, central Europe and North
America suffered under an arctic climate. The ruins of a house burnt
by fire do not tell their tale more plainly than do the mountains of
Scotland and Wales, with their scored flanks, polished surfaces, and
perched boulders, of the icy streams with which their valleys were
lately filled. So greatly has the climate of Europe changed, that in
northern Italy, gigantic moraines, left by old glaciers, are now
clothed by the vine and maize. Throughout a large part of the United
States, erratic boulders and scored rocks plainly reveal a former cold
period.
The former influence of the glacial climate on the distribution of
the inhabitants of Europe, as explained by Edward Forbes, is
substantially as follows. But we shall follow the changes more
readily, by supposing a new glacial period slowly to come on, and then
pass away, as formerly occurred. As the cold came on, and as each more
southern zone became fitted for the inhabitants of the north, these
would take the places of the former inhabitants of the temperate
regions. The latter, at the same time, would travel further and
further southward, unless they were stopped by barriers, in which case
they would perish. The mountains would become covered with snow and
ice, and their former Alpine inhabitants would descend to the
plains. By the time that the cold had reached its maximum, we should
have an arctic fauna and flora, covering the central parts of
Europe, as far south as the Alps and Pyrenees, and even stretching
into Spain. The now temperate regions of the United States would
likewise be covered by arctic plants and animals and these would be
nearly the same with those of Europe; for the present circumpolar
inhabitants, which we suppose to have everywhere travelled
southward, are remarkably uniform round the world.
As the warmth returned, the arctic forms would retreat northward,
closely followed up in their retreat by the productions of the more
temperate regions. And as the snow melted from the bases of the
mountains, the arctic forms would seize on the cleared and thawed
ground, always ascending, as the warmth increased and the snow still
further disappeared, higher and higher, whilst their brethren were
pursuing their northern journey. Hence, when the warmth had fully
returned, the same species, which had lately lived together on the
European and North American lowlands, would again be found in the
arctic regions of the Old and New Worlds, and on many isolated
mountain-summits far distant from each other.
Thus we can understand the identity of many plants at points so
immensely remote as the mountains of the United States and those of
Europe. We can thus also understand the fact that the Alpine plants of
each mountain range are more especially related to the arctic forms
living due north or nearly due north of them: for the first
migration when the cold came on, and the re-migration on the returning
warmth, would generally have been due south and north. The Alpine
plants, for example, of Scotland, as remarked by Mr. H. C. Watson,
and those of the Pyrenees, as remarked by Ramond, are more
especially allied to the plants of northern Scandinavia; those of
the United States to Labrador; those of the mountains of Siberia to
the arctic regions of that country. These views, grounded as they
are on the perfectly well-ascertained occurrence of a former Glacial
period, seem to me to explain in so satisfactory a manner the
present distribution of the Alpine and arctic productions of Europe
and America, that when in other regions we find the same species on
distant mountain-summits, we may almost 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 migration to any great diversity
of temperature; and as they all migrated 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 productions, 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 probability, have become mingled
with ancient Alpine species, which must have existed on the
mountains before the commencement of the Glacial epoch, and which
during the coldest period will have been temporarily driven down to
the plains; they will, also, have been subsequently 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 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 doubtful 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 asked
how I account for this degree of uniformity in the sub-arctic and
temperate forms round the world, at the commencement of the real
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 whole Atlantic Ocean and by the northern part of the
Pacific. During the Glacial period, when the inhabitants of the Old
and New Worlds lived farther southward than they do at present, they
must have been still more completely separated from each other by
wider spaces of ocean; so that it may well be asked how the same
species could then or previously have entered the two continents.
The explanation, I believe, lies in the nature of the climate before
the commencement of the Glacial period. At this, the newer Pliocene
period, the majority of the inhabitants of the world were specifically
the same as now, and we have good reason to believe that the climate
was warmer than at the present day. Hence we may suppose that the
organisms which now live under latitude 60, lived during the
Pliocene period farther north under the Polar Circle, in latitude
66-67; and that the present arctic productions then lived on the
broken land still nearer to the pole. Now, if we looked at a
terrestrial globe, we see under the Polar Circle that there is
almost continuous land from western Europe, through Siberia, to
eastern America. And this continuity of the circumpolar land, with the
consequent freedom under a more favourable climate for intermigration,
will account for the supposed uniformity of the sub-arctic and
temperate productions of the Old and New Worlds, at a period
anterior to the Glacial epoch.
Believing, from reasons before alluded to, that our continents
have long remained in nearly the same relative position, though
subjected to great oscillations of level, I am strongly inclined to
extend the above view, and to infer that during some still earlier and
still warmer period, such as the older Pliocene period, a large number
of the same plants and animals inhabited the almost continuous
circumpolar land; and that these plants and animals, both in the Old
and New Worlds, began slowly to migrate southwards as the climate
became less warm, long before the commencement of the Glacial
period. We now see, as I believe, their descendants, mostly in a
modified condition, in the central parts of Europe and the United
States. On this view we can understand the relationship with very
little identity, between the productions of North America and Europe,-
a relationship which is highly remarkable, considering the distance of
the two areas, and their separation by the whole Atlantic Ocean. We
can further understand the singular fact remarked on by several
observers that the productions of Europe and America during the
later tertiary stages were more closely related to each other than
they are at the present time; for during these warmer periods the
northern parts of the Old and New Worlds will have been almost
continuously united by land, serving as a bridge, since rendered
impassable by cold, for the intermigration of their inhabitants.
During the slowly decreasing warmth of the Pliocene period, as
soon as the species in common, which inhabited the New and Old Worlds,
migrated south of the Polar Circle, they will have been completely cut
off from each other. This separation, as far as the more temperate
productions are concerned, must have taken place long ages ago. As the
plants and animals migrated southwards, they will have become
mingled in the one great region with the native American
productions, and would have had to compete with them; and in the
other great region, with those of the Old World. Consequently we
have here everything favourable for much modification,- for far more
modification than with the Alpine productions, left isolated, within a
much more recent period, on the several mountain-ranges and on the
arctic lands of Europe and N. America. Hence it has come, that when we
compare the now living productions of the temperate regions of the New
and Old Worlds, we find very few identical species (though Asa Gray
has lately shown that more plants are identical than was formerly
supposed), but we find in every great class many forms, which some
naturalists rank as geographical races, and others as distinct
species; and a host of closely allied or representative forms which
are ranked by all naturalists as specifically distinct.
As on the land, so in the waters of the sea, a slow southern
migration of a marine fauna, which, during the Pliocene or even a
somewhat earlier period, was nearly uniform along the continuous
shores of the Polar Circle, will account, on the theory of
modification, for many closely allied forms now living in marine areas
completely sundered. Thus, I think, we can understand the presence
of some closely allied, still existing and extinct tertiary forms,
on the eastern and western shores of temperate North America; and
the still more striking fact of many closely allied crustaceans (as
described in Dana's admirable work), some fish and other marine
animals, inhabiting the Mediterranean and the seas of Japan,- these
two areas being now completely separated by the breadth of a whole
continent and by wide spaces of ocean.
These cases of close relationship in species either now or
formerly inhabiting the seas on the eastern and western shores of
North America, the Mediterranean and Japan, and the temperate lands of
North America and Europe, are inexplicable on the theory of
creation. We cannot maintain that such species have been created
alike, in correspondence with the nearly similar physical conditions
of the areas; for if we compare, for instance, certain parts of
South America with parts of South Africa or Australia, we see
countries closely similar in all their physical conditions, with their
inhabitants utterly dissimilar.
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