well-stocked island, like Great Britain, has not, as far as is known (and
it would be very difficult to prove this), received within the last few
centuries, through occasional means of transport, immigrants from Europe or
any other continent, that a poorly-stocked island, though standing more
remote from the mainland, would not receive colonists by similar means. I
do not doubt that out of twenty seeds or animals transported to an island,
even if far less well-stocked than Britain, scarcely more than one would be
so well fitted to its new home, as to become naturalised. But this, as it
seems to me, is no valid argument against what would be effected by
occasional means of transport, during the long lapse of geological time,
whilst an island was being upheaved and formed, and before it had become
fully stocked with inhabitants. On almost bare land, with few or no
destructive insects or birds living there, nearly every seed, which chanced
to arrive, would be sure to germinate and survive.
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 the 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
to the other. It is indeed a remarkable fact to see so many of the same
plants 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 several 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 rocks scored by drifted
icebergs and coast-ice, plainly reveal a former cold period.
The former influence of the glacial climate on the distribution of the
inhabitants of Europe, as explained with remarkable clearness by Edward
Forbes, is substantially as follows. But we shall follow the changes more
readily, by supposing a new glacial period to come slowly on, and then pass
away, as formerly occurred. As the cold came on, and as each more southern
zone became fitted for arctic beings and ill-fitted for their former more
temperate inhabitants, the latter would be supplanted and arctic
productions would take their places. The inhabitants of the more temperate
regions would at the same time travel 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 a uniform 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. We may suppose that the Glacial period came on a little
earlier or later in North America than in Europe, so will the southern
migration there have been a little earlier or later; but this will make no
difference in the final result.
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
higher and higher, as the warmth increased, whilst their brethren were
pursuing their northern journey. Hence, when the warmth had fully
returned, the same arctic species, which had lately lived in a body
together on the lowlands of the Old and New Worlds, would be left isolated
on distant mountain-summits (having been exterminated on all lesser
heights) and in the arctic regions of both hemispheres.
Thus we can understand the identity of many plants at points so immensely
remote as on the mountains of the United States and 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 migration as the cold came on, and the re-migration
on the returning warmth, will 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 permitted their former
migration across the low intervening tracts, since become too warm for
their existence.
If the climate, since the Glacial period, has ever been in any degree
warmer than at present (as some geologists in the United States believe to
have been the case, chiefly from the distribution of the fossil Gnathodon),
then the arctic and temperate productions will at a very late period have
marched a little further north, and subsequently have retreated to their
present homes; but I have met with no satisfactory evidence with respect to
this intercalated slightly warmer period, since the Glacial period.
The arctic forms, during their long southern migration and re-migration
northward, will have been exposed to nearly the same climate, and, as is
especially to be noticed, they will have kept in a body
together;
consequently their mutual relations will not have been much disturbed, and,
in accordance with the principles inculcated in this volume, they will not
have been liable to much modification. But with our 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 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
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 commencement 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 degree of uniformity of the sub-arctic and northern temperate
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 Atlantic Ocean and by
the extreme northern part of the Pacific. During the Glacial period, when
the inhabitants of the Old and New Worlds lived further southwards than at
present, they must have been still more completely separated by wider
spaces of ocean. I believe the above difficulty may be surmounted by
looking to still earlier changes of climate of an opposite nature. We have
good reason to believe that during the newer Pliocene period, before the
Glacial epoch, and whilst the majority of the inhabitants of the world were
specifically the same as now, the climate was warmer than at the present
day. Hence we may suppose that the organisms now living under the climate
of latitude 60 deg, during the Pliocene period lived further north under
the Polar Circle, in latitude 66 deg-67 deg; and that the strictly arctic
productions then lived on the broken land still nearer to the pole. Now if
we look at a globe, we shall see that under the Polar Circle there is
almost continuous land from western Europe, through Siberia, to eastern
America. And to this continuity of the circumpolar land, and to the
consequent freedom for intermigration under a more favourable climate, I
attribute the necessary amount of uniformity in the sub-arctic and northern
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 large,
but partial oscillations of level, I am strongly inclined to extend the
above view, and to infer that during some 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 most remarkable, considering the distance of the two areas, and their
separation by the 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 inter-migration 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 must have been completely cut off from each
other. This separation, as far as the more temperate productions are
concerned, took place long ages ago. And as the plants and animals
migrated southward, they will have become mingled in the one great region
with the native American productions, and 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 the two Worlds. 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 areas completely sundered. Thus, I think, we can understand the
presence of many existing and tertiary representative forms on the eastern
and western shores of temperate North America; and the still more striking
case of many closely allied crustaceans (as described in Dana's admirable
work), of some fish and other marine animals, in the Mediterranean and in
the seas of Japan,--areas now separated by a continent and by nearly a
hemisphere of equatorial ocean.
These cases of relationship, without identity, o
f the inhabitants of seas
now disjoined, and likewise of the past and present inhabitants of the
temperate lands of North America and Europe, are inexplicable on the theory
of creation. We cannot say that they 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 the
southern continents of the Old World, we see countries closely
corresponding in all their physical conditions, but with their inhabitants
utterly dissimilar.
But we must return to our more immediate subject, the Glacial period. I am
convinced that Forbes's view may be largely extended. In Europe we have
the plainest evidence of the cold period, from the western shores of
Britain to the Oural range, and southward to the Pyrenees. We may infer,
from the frozen mammals and nature of the mountain vegetation, that Siberia
was similarly affected. Along the Himalaya, at points 900 miles apart,
glaciers have left the marks of their former low descent; and in Sikkim,
Dr. Hooker saw maize growing on gigantic ancient moraines. South of the
equator, we have some direct evidence of former glacial action in New
Zealand; and the same plants, found on widely separated mountains in this
island, tell the same story. If one account which has been published can
be trusted, we have direct evidence of glacial action in the south-eastern
corner of Australia.
Looking to America; in the northern half, ice-borne fragments of rock have
been observed on the eastern side as far south as lat. 36 deg-37 deg, and
on the shores of the Pacific, where the climate is now so different, as far
south as lat. 46 deg; erratic boulders have, also, been noticed on the
Rocky Mountains. In the Cordillera of Equatorial South America, glaciers
once extended far below their present level. In central Chile I was
astonished at the structure of a vast mound of detritus, about 800 feet in
height, crossing a valley of the Andes; and this I now feel convinced was a
gigantic moraine, left far below any existing glacier. Further south on
both sides of the continent, from lat. 41 deg to the southernmost
extremity, we have the clearest evidence of former glacial action, in huge
boulders transported far from their parent source.
We do not know that the Glacial epoch was strictly simultaneous at these
several far distant points on opposite sides of the world. But we have
good evidence in almost every case, that the epoch was included within the
latest geological period. We have, also, excellent evidence, that it
endured for an enormous time, as measured by years, at each point. The
cold may have come on, or have ceased, earlier at one point of the globe
than at another, but seeing that it endured for long at each, and that it
was contemporaneous in a geological sense, it seems to me probable that it
was, during a part at least of the period, actually simultaneous throughout
the world. Without some distinct evidence to the contrary, we may at least
admit as probable that the glacial action was simultaneous on the eastern
and western sides of North America, in the Cordillera under the equator and
under the warmer temperate zones, and on both sides of the southern
extremity of the continent. If this be admitted, it is difficult to avoid
believing that the temperature of the whole world was at this period
simultaneously cooler. But it would suffice for my purpose, if the
temperature was at the same time lower along certain broad belts of
longitude.
On this view of the whole world, or at least of broad longitudinal belts,
having been simultaneously colder from pole to pole, much light can be
thrown on the present distribution of identical and allied species. In
America, Dr. Hooker has shown that between forty and fifty of the flowering
plants of Tierra del Fuego, forming no inconsiderable part of its scanty