have even carried brushwood, bones, and the nest of a land-bird, it can
hardly be doubted that they must occasionally, as suggested by Lyell,
have transported seeds from one part to another of the Arctic and
Antarctic regions, and during the Glacial period from one part of the
now temperate regions to another. In the Azores, from the large number
of plants common to Europe, in comparison with the species on the other
islands of the Atlantic, which stand nearer to the mainland, and (as
remarked by Mr. H. C. Watson) from their somewhat northern character in
comparison with the latitude, I suspected that these islands had been
partly stocked by ice-borne seeds during the Glacial epoch.
THESE MEANS OF TRANSPORT NOT ACCIDENTAL.
[Page 329.]
These means of transport are sometimes called accidental, but this is
not strictly correct; the currents of the sea are not accidental, nor
is the direction of prevalent gales of wind. It should be observed
that scarcely any means of transport would carry seeds for very great
distances: for seeds do not retain their vitality when exposed for
a great length of time to the action of sea-water; nor could they
be long carried in the crops or intestines of birds. These means,
however, would suffice for occasional transport across tracts of sea
some hundred miles in breadth, or from island to island, or from a
continent to a neighboring island, but not from one distant continent
to another. The floras of distant continents would not by such means
become mingled; but would remain as distinct as they now are. The
currents, from their course, would never bring seeds from North
America to Britain, though they might and do bring seeds from the West
Indies to our western shores, where, if not killed by their very long
immersion in salt-water, they could not endure our climate. Almost
every year, one or two land-birds are blown across the whole Atlantic
Ocean, from North America to the western shores of Ireland and England;
but seeds could be transported by these rare wanderers only by one
means, namely, by dirt adhering to their feet or beaks, which is in
itself a rare accident. Even in this case, how small would be the
chance of a seed falling on favorable soil and coming to maturity! But
it would be a great error to argue that, because a 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, a poorly-stocked island, though standing more remote
from the mainland, would not receive colonists by similar means. Out of
a hundred kinds of seeds or animals transported to an island, even if
far less well-stocked than Britain, perhaps not more than one would be
so well fitted to its new home as to become naturalized. But this is
no valid argument against what would be effected by occasional means
of transport, during the long lapse of geological time, while the
island was being upheaved, 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, if
fitted for the climate, would germinate and survive.
DISPERSAL DURING THE GLACIAL PERIOD.
[Page 434.]
The Glacial period is defined “as a period of great cold and of
enormous extension of ice upon the surface of the earth. It is believed
that glacial periods have occurred repeatedly during the geological
history of the earth, but the term is generally applied to the close of
the Tertiary epoch, when nearly the whole of Europe was subjected to an
Arctic climate.”
* * * * *
[Origin of
Species,
page 330.]
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 burned
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 bowlders, 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 bowlders 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 farther and farther 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 traveled
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, while 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.
THE THEORY OF CREATION INADEQUATE.
[Page 334.]
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 can not
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.
CAUSES OF A GLACIAL CLIMATE.
[Page 336.]
Mr. Croll, in a series of admirable memoirs, has attempted to show
that a glacial condition of climate is the result of various physical
causes, brought into operation by an increase in the eccentricity of
the earth’s orbit. All these causes tend toward the same end; but the
most powerful appears to be the indirect influence of the eccentricity
of the orbit upon oceanic currents. According to Mr. Croll, cold
periods regularly recur every ten to fifteen thousand years; and these
at long intervals are extremely severe, owing to certain contingencies,
of which the most important, as Sir C. Lyell has shown, is the relative
position of the land and water. Mr. Croll believes that the last great
Glacial period occurred about two hundred and forty thousand years ago,
and endured with slight alterations of climate for about one hundred
and sixty thousand years. With respect to more ancient Glacial periods,
several geologists are convinced from direct evidence that such
occurred during the Miocene and Eocene formations, not to mention still
more ancient formations. But the most important result for us, arrived
at by Mr. Croll, is that, whenever the northern hemisphere passes
through a cold period, the temperature of the southern hemisphere is
actually raised, with the winters rendered much milder, chiefly through
changes in the direction of the ocean-currents. So conversely it will
be with the northern hemisphere, while the southern passes through a
glacial period. This conclusion throws so much light on geographical
distribution that I am strongly inclined to trust in it.
DIFFICULTIES NOT YET REMOVED.
[Page 341.]
I am far from supposing that all the difficulties in regard to the
distribution and affinities of the identical and allied species, which
now live so widely separated in the north and south, and sometimes on
the intermediate mountain-ranges, are removed on the views above given.
The exact lines of migration can not be indicated. We can not say why
certain species and not others have migrated; why certain species have
been modified and have given rise to new forms, while others have
remained unaltered. We can not hope to explain such facts, until we can
say why one species and not another becomes naturalized by man’s agency
in a foreign land; why one species ranges twice or thrice as far, and
is twice or thrice as common, as another species within their own homes.
Various special difficulties also remain to be solved; for instance,
the occurrence, as shown by Dr. Hooker, of the same plants at points
so enormously remote as Kerguelen Land, New Zealand, and Fuegia; but
icebergs, as suggested by Lyell, may have been concerned in their
dispersal. The existence at these and other distant points of the
southern hemisphere of species which, though distinct, belong to genera
exclusively confined to the south, is a more remarkable case. Some of
these species are so distinct that we can not suppose that there has
been time since the commencement of the last Glacial period for their
migration and subsequent modification to the necessary degree. The
facts seem to indicate that distinct species belonging to the same
genera have migrated in radiating lines from a common center; and I am
inclined to look in the southern, as in the northern hemisphere, to a
former and warmer period, before the commencement of the last Glacial
period, when the Antarctic lands, now covered with ice, supported a
highly peculiar and isolated flora. It may be suspected that, before
this flora was exterminated during the last Glacial epoch, a few forms
had been already widely dispersed to various points of the southern
hemisphere by occasional means of transport, and by the aid, as
halting-places, of now sunken islands. Thus the southern shores of
America, Australia, and New Zealand, may have become slightly tinted by
the same peculiar forms of life.
IDENTITY OF THE SPECIES OF ISLANDS WITH THOSE OF THE MAINLAND EXPLAINED
ONLY BY THIS THEORY.
[Origin of
Species,
page 353.]
The most striking and important fact for us is the affinity of the
species which inhabit islands to those of the nearest mainland,
without being actually the same. Numerous instances could be given.
The Galapagos Archipelago, situated under the equator, lies at the
distance of between five hundred and six hundred miles from the shores
of South America. Here almost every product of the land and of the
water bears the unmistakable stamp of the American Continent. There are
twenty-six land-birds; of these, twenty-one or perhaps twenty-three
are ranked as distinct species, and would commonly be assumed to have
been here created; yet the close affinity of most of these birds to
American species is manifest in every character, in their habits,
gestures, and tones of voice. So it is with the other animals, and
with a large proportion of the plants, as shown by Dr. Hooker in
his admirable Flora of this archipelago. The naturalist, looking at
the inhabitants of these volcanic islands in the Pacific, distant
several hundred miles from the continent, feels that he is standing
on American land. Why should this be so? why should the species
which are supposed to have been created in the Galapagos Archipelago,
and nowhere else, bear so plainly the stamp of affinity to those
created in America? There is nothing in the conditions of life, in the
geological nature of the islands, in their height or climate, or in
the proportions in which the several classes are associated together,
which closely resembles the conditions of the South American coast;
in fact, there is a considerable dissimilarity in all these respects.
On the other hand, there is a considerable degree of resemblance in
the volcanic nature of the soil, in the climate, height, and size of
the islands, between the Galapagos and Cape de Verd Archipelagos;
but what an entire and absolute difference in their inhabitants! The
inhabitants of the Cape de Verd Islands are related to those of Africa,
like those of the Galapagos to America. Facts such as these admit of
no sort of explanation on the ordinary view of independent creation;
whereas, on the view here maintained, it is obvious that the Galapagos
Islands would be likely to receive colonists from America, whether
by occasional means of transport or (though I do not believe in this
doctrine) by formerly continuous land, and the Cape de Verd Islands
from Africa; such colonists would be liable to modification, the
principle of inheritance still betraying their original birthplace.
Many analogous facts could be given: indeed, it is an almost universal
rule that the endemic productions of islands are related to those of
the nearest continent, or of the nearest large island. The exceptions
are few, and most of them can be explained. Thus, although Kerguelen
Land stands nearer to Africa than to America, the plants are related,
and that very closely, as we know from Dr. Hooker’s account, to those
of America: but, on the view that this island has been mainly stocked
by seeds brought with earth and stones on icebergs, drifted by the
prevailing currents, this anomaly disappears. New Zealand in its
endemic plants is much more closely related to Australia, the nearest
mainland, than to any other region: and this is what might have been
expected; but it is also plainly related to South America, which,
although the next nearest continent, is so enormously remote that the
fact becomes an anomaly. But this difficulty partially disappears on
the view that New Zealand, South America, and the other southern lands
have been stocked in part from a nearly intermediate though distant
point, namely, from the Antarctic islands, when they were clothed with
vegetation, during a warmer tertiary period, before the commencement
of the last Glacial period. The affinity, which, though feeble, I am
assured by Dr. Hooker is real, between the flora of the southwestern
corner of Australia and of the Cape of Good Hope, is a far more
remarkable case; but this affinity is confined to the plants, and will,
no doubt, some day be explained.
VII.
EVIDENCE OF THE DESCENT OF MAN FROM SOME LOWER FORM.
[The Descent
of Man,
page 5.]
He who wishes to decide whether man is the modified descendant of some
pre-existing form would probably first inquire whether man varies,
however slightly, in bodily structure and in mental faculties; and,
if so, whether the variations are transmitted to his offspring in
accordance with the laws which prevail with the lower animals. Again,
are the variations the result, as far as our ignorance permits us
to judge, of the same general causes, and are they governed by the
same general laws, as in the case of other organisms; for instance,
by correlation, the inherited effects of use and disuse, etc.? Is
man subject to similar malconformations, the result of arrested
development, of reduplication of parts, etc., and does he display
in any of his anomalies reversion to some former and ancient type
of structure? It might also naturally be inquired whether man, like
so many other animals, has given rise to varieties and sub-races,
differing but slightly from each other, or to races differing so much
that they must be classed as doubtful species. How are such races
distributed over the world; and how, when crossed, do they react on
each other in the first and succeeding generations? And so with many
other points.
The inquirer would next come to the important point whether man
tends to increase at so rapid a rate as to lead to occasional severe
struggles for existence; and consequently to beneficial variations,
whether in body or mind, being preserved, and injurious ones
eliminated. Do the races or species of men, whichever term may be
applied, encroach on and replace one another, so that some finally
become extinct? We shall see that all these questions, as indeed
is obvious in respect to most of them, must be answered in the
affirmative, in the same manner as with the lower animals.
POINTS OF CORRESPONDENCE BETWEEN MAN AND THE OTHER ANIMALS.
[The Descent
of Man,
page 6.]
It is notorious that man is constructed on the same general type or
model as other mammals. All the bones in his skeleton can be compared
with corresponding bones in a monkey, bat, or seal. So it is with his
muscles, nerves, blood-vessels, and internal viscera. The brain, the
most important of all the organs, follows the same law, as shown by
Huxley and other anatomists. Bischoff, who is a hostile witness, admits
that every chief fissure and fold in the brain of man has its analogy
in that of the orang; but he adds that at no period of development do
their brains perfectly agree; nor could perfect agreement be expected,
for otherwise their mental powers would have been the same.
* * * * *
Man is liable to receive from the lower animals, and to communicate
to them, certain diseases, as hydrophobia, variola, the glanders,
syphilis, cholera, herpes, etc.; and this fact proves the close
similarity of their tissues and blood, both in minute structure and
composition, far more plainly than does their comparison under the
best microscope, or by the aid of the best chemical analysis. Monkeys
are liable to many of the same non-contagious diseases as we are; thus
Rengger, who carefully observed for a long time the _Cebus Azaræ_ in
its native land, found it liable to catarrh, with the usual symptoms,
and which, when often recurrent, led to consumption. These monkeys
suffered also from apoplexy, inflammation of the bowels, and cataract
in the eye. The younger ones when shedding their milk-teeth often died
from fever. Medicines produced the same effect on them as on us. Many
kinds of monkeys have a strong taste for tea, coffee, and spirituous
liquors: they will also, as I have myself seen, smoke tobacco with
pleasure. Brehm asserts that the natives of Northeastern Africa catch
the wild baboons by exposing vessels with strong beer, by which they
are made drunk. He has seen some of these animals, which he kept in
confinement, in this state; and he gives a laughable account of their
behavior and strange grimaces. On the following morning they were very
cross and dismal; they held their aching heads with both hands, and
wore a most pitiable expression: when beer or wine was offered them,
they turned away with disgust, but relished the juice of lemons. An
American monkey, an Ateles, after getting drunk on brandy, would never
touch it again, and thus was wiser than many men. These trifling facts
prove how similar the nerves of taste must be in monkeys and man, and
how similarly their whole nervous system is affected.
Man is infested with internal parasites, sometimes causing fatal
effects; and is plagued by external parasites, all of which belong to
the same genera or families as those infesting other mammals, and in
the case of scabies to the same species. Man is subject, like other
mammals, birds, and even insects, to that mysterious law which causes
certain normal processes, such as gestation, as well as the maturation
and duration of various diseases, to follow lunar periods. His wounds
are repaired by the same process of healing; and the stumps left after
the amputation of his limbs, especially during an early embryonic
period, occasionally possess some power of regeneration, as in the
lowest animals.
* * * * *
[Page 9.]
Man is developed from an ovule, about the 125th of an inch in diameter,
which differs in no respect from the ovules of other animals. The
embryo itself at a very early period can hardly be distinguished
from that of other members of the vertebrate kingdom. At this period
the arteries run in arch-like branches, as if to carry the blood to
branchiæ which are not present in the higher vertebrata, though the
slits on the side of the neck still remain, marking their former
position. At a somewhat later period, when the extremities are
developed, “the feet of lizards and mammals,” as the illustrious Von
Baer remarks, “the wings and feet of birds, no less than the hands
and feet of man, all arise from the same fundamental form.” It is,
says Professor Huxley, “quite in the later stages of development that
the young human being presents marked differences from the young ape,
while the latter departs as much from the dog in its developments as
the man does. Startling as this last assertion may appear to be, it is
demonstrably true.”
THE FACTS OF EMBRYOLOGY AND THE THEORY OF DEVELOPMENT.
[Origin of
Species,
page 386.]