Plant Migration and Interglacial Periods.
Among the leading propositions laid down by Arthur Renfrey, Esq., F.R.S.
etc., etc., in the able article prepared by him for "The Physical Atlas of
Natural Phenomena," by Alexander Keith Johnston, Edinburg Edition, 1856,
on "The Geographical Distribution of the most Important Plants Yielding
Food," are the following:--
1. "The primary condition of the existence of any species of plant, is its
absolute creation, of which we know nothing.
2. "But we assume each species to have been _created but once in time and
in place_, and that its present diffusion is the result of its own law of
reproduction under the favorable or restrictive influences of laws
external to it.[14]
3. "The most important of external laws are those relating to climate,
since _any species can flourish only within narrower or wider, but always
fixed limits, of temperature, humidity etc_.,
4. "The climate depends primarily on latitude, since this indicates
distance from the source of heat, and the degree of obliquity of the
heating rays."
There are other governing conditions, of course, such as the average
rain-fall, distance from the equator, the elevation above the sea level in
the various mountain systems of vegetation, etc., including the
hygrometric, thermometric, telluric, and other conditions, of the several
localities in which the different species of vegetation make their
appearance.
But why should this distinguished naturalist insist upon the specific
creation of either plants or animals? No scientific work of any paramount
value confines the creative power of the universe to such narrow and
restricted limits. Nor is there a particle of evidence to be drawn from
the Bible that either plants or animals primarily originated in pairs.
"Let the earth bring forth" is a command without limitation, or
restriction, as to time, place, or number; and there is no reason to
doubt that myriads of living forms swarmed everywhere, at first as now,
in nature.
The idea, as expressed by Mr. Renfrey, that they were specifically created
at one time and place only, whether in pairs, tens, twenties, or hundreds,
is neither a rational one, nor has it any experience-argument or
scientific authority on which to stand. Take, for instance, an
experience-argument directly in point:--When the salt wells were first
bored at Syracuse, N.Y., and the salt water was suffered to flow in waste
over the low grounds about the salt-works, the small saline plants
peculiar to salt-marshes in the warm temperate zone made their appearance,
not in pairs, tens or hundreds, but in thousands rather, and have
nourished there ever since. They came because conditions favored; because
a salt-marsh had been artificially produced hundreds of miles away from
the sea coast. This is only one of a large number of cases--more than we
have room to specify in this connection--showing that wherever man,
artificially or otherwise, produces the necessary conditions of
plant-life, nature responds to the germinal law precisely as she did
millions of years ago when the first salt-marsh favored the appearance of
these saline plants--such as grow under no other conditions or
circumstances.
But this idea of plants coming primarily from a single pair of
progenitors, and each primordial pair branching off into diversified
offspring, as in the case of the cabbage, assumed to be the original
ancestor of all the turnips and ruta-bagas, may be an article of botanical
faith, but never of experimental proof. "_Entia non sunt multiplicanda
præter necessitatem_" is an old and well-approved maxim, applicable alike
to the countless myriads of living organisms, as to the innumerable
crystalline forms to be found everywhere in nature. Nothing is produced
without the necessary conditions on which its production depends.
"Necessity," in its primitive signification, is a term of the very widest
meaning, and most universal application. It applies as well to the course
of nature as to the course of human events--to the laws of vegetable and
animal growth as to the inevitable march and order of celestial movements.
As applied to any form of life-manifestation it implies a law of
development and growth, as well as the physiological conditions without
which vital manifestations are impossible. For law, in a physiological
sense, is that mode of vital action by which effects are invariably and
inevitably produced.[15] And this law is just as dependent on necessary
vital conditions as vital manifestations are dependent on a physiological
law. There must always be this reciprocal dependence and relationship
between conditioning causes and effects. Whenever and wherever the
necessary vital conditions exist, the physiological law takes effect, and
the requisite vital manifestation is witnessed. And this is no doubt as
true of animal as of vegetable life.
The earth's surface has been divided into eight separate zones, each of
which is distinguished by its peculiar or characteristic fauna and flora.
Their order, measured from the geographical equator, is as follows;
1. The Equatorial Zone, extending from 0° to 15°.
2. " Tropical " " " 15° " 23°.
3. " Sub-tropical " " " 23° " 34°.
4. " Warm Temperate " " " 34° " 45°.
5. " Cold " " " 45° " 58°.
6. " Sub-arctic " " " 58° " 66°.
7. " Arctic " " " 66° " 72°.
8. " Polar " " " 72° " 82°.
These several zones become sixteen in number when considered with
reference to both the northern and southern hemispheres. And a like
division of isothermals is made in the case of all our mountain systems,
extending in both directions from the equator. In ascending our
equatorial, tropical, and sub-tropical mountains, we find, of course, at
their several bases, the temperature of the zones in which they
respectively lie; from two thousand to three thousand feet, we reach the
next higher zone, and so on, at about the same ratio of altitude, until we
ascend to the polar zone or the line of perpetual ice and snow. The peak
of Teneriffe, for instance, lies in the sub-tropical zone, but, at the
elevation named, we meet with the vegetation which characterizes the warm
temperate zone. And this holds true of all our mountain systems, in all
latitudes, and at all altitudes, in all parts of the globe.
They all present the same or strikingly similar characteristics in plant
life, with such variations and modifications only as might be accounted
for, were all the influencing conditions and surrounding circumstances,
modifying geographical distribution, known to us. From the lowest to the
highest regions in which vegetation flourishes, this rule, with slight
exceptions only, will be found to obtain, and it is in this direction that
the observations of the scientific, as well as practical botanist, should
hereafter be extended.
Humboldt noticed this characteristic feature of the earth's vegetation
quite early in his explorations, and accordingly divided the tropical
mountains, as the earth's surface was then divided, into three separate
zones, the tropical, the temperate, and the frigid. But a closer
classification now distinguishes them into the same number of zones as are
marked, in approximate isotherms, on the earth's surface. Mr. Renfrey
gives us further statistics of great value respecting these several plant
zones of the globe, all of which fit so admirably into our theory of
plant-distribution, that we can hardly see how the most prejudiced mind
can resist the force of its application. Among the most important of these
statistical facts are tables giving the comparative rain-falls in the
different plant zones of the old and new worlds, and the classes of
vegetation peculiar to each of them.
The Equatorial zone, for instance, is characterized by extreme luxuriance
in growth, owing no doubt to the great heat and abundant moisture therein,
and exhibits a vegetation which is peculiar to itself, and which could
only thrive under the hygrometric, thermometric, telluric, and other
conditions of that extensive zone.
The Tropical zones (those north and south of the equator) are
characterized by a more abundant and diversified underwood, and, while
retaining some of the equatorial forms, present fewer parasites and less
rapid and luxuriant growths. They contain many plants and trees which are
peculiar to their own limits, and these are generally the hardiest and
most abundant. All equatorial forms disappear in these zones, that is do
not pass into the sub-tropical zones. And these characteristics obtain in
both the northern and southern tropical zones, as well as in the mountain
systems within the equatorial regions.
The Sub-tropical zones, while retaining some of the more marked forms and
general features of the tropical zones, such as palms, bananas, etc.,
exhibit the most striking characteristics of their own, consisting of a
greater abundance of forest trees, especially those having broad, leathery
and shining leaves, like the magnolias, the different species of laurels,
and plants of the myrtle family. The tropical forms all disappear in these
zones, as the equatorial do in the tropical zones.
The Warm Temperate zones exhibit the same disposition to retain some of
the hardier and more abundant sub-tropical forms that characterize the
other zones, in respect to their adjoining isotherms. But the trees and
plants peculiar to this zone north, (and the same is no doubt true of the
corresponding zone south), are more numerous, and embrace a wider range of
deciduous, as well as evergreen growths. Evergreen shrubs, heaths,
cistusses, and leguminous plants are everywhere more abundant. The marked
characteristic of these zones is that the trees, plants, and arborescent
grasses differ more widely in their general character, as well as run more
extensively into varieties.
The Cold Temperate zones retain many of the deciduous trees of the warm
temperate, but with less conspicuous blossoms, while a stronger tendency
is shown toward social conifers, and the trunks of the deciduous trees are
more profusely overrun with mosses, lichens, etc. These zones are also
abundant in grasses.
The Sub-arctic zone north largely retains its hold upon the social
conifers, giving place, northward, on this continent, as well as in Europe
and Asia, to birch and alder, alternating with willows where the soil is
sufficiently moist. Green pastures are still abundant, and showy flowering
herbs abound during the brief spring, summer, and autumn months.
The Arctic zone retains few of the sub-arctic forms and its vegetation
generally corresponds to what we call alpine shrubs, grasses, etc.
The North Polar zone shows few signs of vegetation and is thought to be
entirely devoid of shrubs. A few small herbacious perennials of the most
extreme dwarf habit, with a few lichens and mosses, constitute its entire
vegetation.
There are some seeming exceptions to these general statements respecting
plant-distribution, but they are hardly exceptions when we consider the
elevation at which any one species, as the birches for instance, may
appear, as they frequently do, in three several zones.
From these facts, gathered from the highest authorities, and well-attested
on all hands, what general conclusions, if any, are to be drawn? Before
answering this inquiry, let us proceed to state what conclusions _have_
been drawn. According to all the authorities we have examined on the
distribution of plant life; on the migration of plants and animals; on
climate and time as affecting the transference of isothermal and
isochimenal lines; on glacial and inter-glacial periods (with one
important exception only), the assumption maintained is substantially that
of Mr. Renfrey, that "each species of plant and animal was created but
once in time and place," and that its present diffusion is the result of
its "own law of reproduction under the favorable or restrictive influences
of laws external to it." In other words, they insist upon original
plant-centres, without definitely stating when or where they occurred, and
that from these centres both plants and animals have migrated to all parts
of the globe where they now appear, even crossing the equatorial zones
where they could not live for a single day. This migration theory they
attempt to explain in a way that is altogether more ingenious than
satisfactory.
The important exception to which we refer is that of Professor Agassiz, as
reported by his associate professor of Harvard University, Mr. Asa Gray,
in his "Essays and Reviews Pertaining to Darwinism." In this work
Professor Gray says of his late distinguished associate, that so far as he
was aware, Professor Agassiz was the only leading naturalist "who did not
take into his very conception of a species, explicitly or by implication,
the notion of a material connection resulting from the descent of the
individuals composing it from a common stock, of a local origin."
And Professor Gray adds this further testimony to the closeness of his
associate's observations, in considering the very point here under
consideration: "Agassiz wholly eliminates community of descent from his
idea of species, and even conceives a species to have been as numerous in
individuals, and as widely spread over space, or as segregated in
discontinuous spaces, from the first to the later periods." And this view
is undoubtedly the correct one. At all events, it entirely harmonizes with
the facts of the biblical genesis, and obviates the necessity of
accounting for the appearance of the same genera and species of plants or
animals in the southern as in the northern hemispheres; in fact, their
appearance in all parts of the globe, in corresponding isotherms, and
under similar conditions of moisture and soil-constituents.
Wherever the hygrometric, thermometric, telluric, and other conditions
favor, the class of vegetation indicated by the presence of these
conditions makes its appearance, just as the fire-weed makes its
appearance in our warm temperate zone, not from the presence of seed, but
simply the presence of "conditions"--the _pro_vision of man harmonizing
with the _pre_vision of nature. In the same way the "Japan clover" made
its appearance, as Professor Thurber states, "all over the southern
states" during the late civil war, not from the migration of plants, but
the presence of natural conditions.[16]
The numerous facts we have already given, and many others that might be
arrayed in advocacy of our position, taken in connection with the general
facts here presented in regard to plant-distribution, all point directly
to climatal and soil conditions as the real cause of dissemination, and
not to their migration from continent to continent, and across vast
intervening seas and oceans, as the theory of Professor Gray and others
would require us to believe. Take the case of the _Schizoea pusilla_ of
the New Jersey pine barrens, to which we have already referred, growing in
similar barrens in New Zealand, and how are we to account for their
antipodal appearance upon the globe? Professor Thurber refers to this
plant as a "purely local fern" of New Jersey, and says it was for a long
time supposed to be peculiar to that state until it was ascertained that
it grew in New Zealand. Whether this plant "travelled" from New Zealand to
New Jersey, or journeyed in the opposite direction, none of these
"specific-centre" gentlemen can well inform us. Professor Agassiz would
have said that it might have appeared, in numerous individuals, in both
localities at the same time, or at different times, as conditions favored;
and this would have been an exact scientific statement, no doubt, of the
fact. Mr. Arthur Renfrey, and those who accept his scientific formulæ,
must insist that this most beautiful of all our ferns was such a "favorite
child of nature" that she condescended to create it _twice_ "in time and
place," instead of only _once_. It is a poor rule, they may say, that has
no exceptions in phenomenal manifestation.
Professor Gray may insist that such a phenomenon as this requires belief
in the supernatural, and that migration by ocean-currents is the more
rational theory of the two. But M. Alphonse de Candolle--quite as high
authority as we can quote--has come to the conclusion that marine
currents, and all other suggested means of distant transportation, "have
played only a very small part in the actual dispersion of species," even
across narrow channels and the near arms of seas. But why should the
appearance of this fern at opposite points of the globe, with thousands of
miles of ocean and continent intervening, be any more supernatural than
the presence of _Bacteria_ or _Torulæ_[17] in different organic
infusions? If the vital units of these _infusoriæ_, are present in
experimental infusion, as Professor Bastian virtually admits, why may not
the vital germs or units of this _Schizoea pusilla_ have made their
appearance, in developmental forms, both in New Zealand and New Jersey, at
the same or different periods of time? If Professor Gray regards the
microscopical forms in organic infusions, or the statical forms in
inorganic solutions, as supernatural, or as above the powers of nature,
then we have no exceptions to make to his position. First, prove that
these vital manifestations of nature are above the powers with which she
has been endowed, or was originally endowed and we will concede the
question of supernaturalness, and drop all exceptions to his line of
argument. Whenever a dynamic law, or a statical, is found to be uniformly
operative under a given set of conditions, we had supposed the operation
not to be above the powers of nature, but in entire accord with them, and
hence not supernatural.
But let us see into what an inextricable labyrinth of difficulty we are
led by this theory of plant-migration from the equatorial to the
sub-arctic zone, and _vice-versa,_ and even beyond the equator to the
sub-antarctic zone, and still _vice versa_. Before proceeding to consider
the probable duration of the several geographical epochs, called glacial
periods, on which their theory of plant-migration depends, or considering
the evidence touching these glacial periods, we will state their position
in regard to these possible migrations as briefly and concisely as we know
how. Mr. Darwin's solution of this problem is the generally accepted one
of the evolutionists, as well as most of the present scientific world. As
the truth, or rather the falsity, of his pet theory of evolution depended
on the satisfactory solution of this vexed problem, it became necessary
for him to give his best and entire mental energies to the gigantic task
which was, by universal consent, assigned him. The reader shall see how
admirably the thermal equator is crossed by Mr. Darwin, with his vast
swarms of flies, mosquitoes, insectivorous and other plants, forest trees,
anthropoid apes, and general menagerie of wild animals, such as would
gladden the heart of the "great American showman" beyond the most
extravagant comparison.
The question, bear in mind, which he was specially called upon to solve,
was how the temperate forms north--those, for instance, of the warm and
cold temperate zones--managed to cross the thermal equator, and invade the
corresponding zones in the southern hemisphere; just as though there was
any more necessity of determining this question than the opposite one, of
how the southern forms came to invade the northern hemisphere. We will
give his solution of this problem in his own language, that we may not be
charged with misrepresentation.
He says, in speaking of the glacial periods: "As the cold became more and
more intense, we know that arctic forms invaded the temperate regions;
and, from the facts just given, there can hardly be a doubt that some of
the more vigorous, dominant, and widest-spread temperate forms invaded the
equatorial lowlands. The inhabitants (flora and fauna) of these hot
lowlands would at the same time have migrated to the tropical and
sub-tropical regions of the south; for the southern hemisphere was at this
period warmer. On the decline of the glacial period, as both hemispheres
gradually recovered their former temperatures, the northern forms living
on the lowlands under the equator would have been driven to their former
homes or have been destroyed, being replaced by the equatorial forms
returning from the south. Some, however, of the northern temperate forms
would almost certainly have ascended any adjoining highland, where, if
sufficiently lofty, they would have long survived, like the arctic forms
on the mountains of Europe.
"In the regular course of events the southern hemisphere would, in its
turn, be subject to a severe glacial period, with the northern hemisphere
rendered warmer; and then the southern temperate forms would invade the
equatorial lowlands. The northern forms which had before been left on the
mountains would now descend and mingle with the southern forms. These
latter, when the warmth returned, would return to their former homes,
leaving some few species on the mountains, and carrying southward with
them some of the northern temperate forms, which had descended from their
mountain fastnesses. Thus we should have some few species identically the
same in the northern and southern temperate zones, and on the mountains of
the intermediate tropical regions."
We are sorry to spoil so ingenious a theory as this to account for
plant-migration from the temperate zones north to the corresponding zones
south. But in spite of all the great names which will frown down upon us
in the attempt, we are obliged to demolish this altitudiness structure,
even at the risk of its tumbling about our own ears.
But first let us lay down a few undeniable propositions, on the
strength of which this ingenious and purely speculative theory of Mr.
Darwin must rest:--
1. It is universally conceded by the scientific world that these glacial
epochs, however many of them there may have been in the past and however
few there may be in the future, depend, for their occurrence, upon the
maxima of eccentricity in the earth's orbit about the sun.
2. The actual amount of heat which the earth annually receives from the
sun is in no way affected by the eccentricity of its orbit. It is a
constant quantity, and only unequally distributed on the earth's surface,
being neither increased nor diminished, as our winters occur in aphelion
or perihelion.
3. The actual amount of ice-cap accumulated about the two poles of the
earth, is also a constant quantity. And to measure the severity of any
glacial epoch, we have only to determine the exact amount of ice (not
altogether an impossible problem) about the two poles at any given time,
and then determine the effect of its entire transference from one pole to
the other.
4. It is not probable that the present ice-cap of the south pole extends
continuously and permanently much farther north than 80° or 81°. Mt.
Erebus, in Victoria Land, lies in about this latitude, and it was only a
few years since that the coast line of that island or continent was
traversed, by English exploring vessels, from Mt. Erebus to a point some
ten or twelve degrees further north. [18]
5. But if we estimate the southern cap as extending continuously to 75°,
what would be the effect of its transference at once to the ice-cap of the
north pole? Would it extend it, after assuming its proper glacial slope,
below 60°, a point falling within the present subarctic zone? The utmost
limit to which Mr. Croll, in his great work on "Climate and Time,"
conceives it possible that it should extend, in any glacial epoch, is to
55°, or about the northern boundary of England.
Now unless the astronomers and physicists are all at sea about the causes
of glaciation, the warm temperate zone can never be pushed any further
south than the tropical zone, nor the cold temperate any further than the
sub-tropical. This would be the extreme limit. Mr. Croll says, in speaking
of these glacial periods; "It is, of course, absurd to suppose that an
ice-cap could ever actually reach down to the equator. It is probable that
the last great ice-cap of the glacial epoch nowhere reached half way to
the equator. Our cap (that of Europe) must therefore, terminate at a
moderately high latitude." And if the gulf stream flows southward during
the glacial period north, as he supposes probable, the cap on this
continent would probably terminate at the same moderately high latitude.
Assuming that Mr. Croll's estimate is the more probable one, it would only
push the cold temperate zone down to the line of the Gulf States; the warm
temperate, to the southern line of Mexico; the sub-tropical, to the
Central American States, and the tropical to the United States of
Columbia, Venezuela, and Guiana.
Suppose, then, that some seven hundred thousand years ago, more or less,
when the North Pole had fully donned the earth's ice-cap, with all the
isothermal and isochimenal changes thereby effected, what must have been
the line of march taken by our northern vegetal and animal forms to escape
the cataclysm of ice and snow then impending? Manifestly, they would have
flocked, first to the Gulf states, then to Mexico, and afterwards to the
Central American states; but none of them could ever have been crowded
through the Isthmus of Panama, since at the height of the last glaciation,
that portion of the continent must have been the tropical barrier to our
northern forms, as it is now the equatorial barrier.
For the sake of the argument, however, we will suppose the northern
ice-cap to have been even more imperative in its demands than Mr. Croll
has deemed possible, driving some of our warm and cold temperate forms
down into the lowlands of Columbia, Venezuela, etc., in the extreme
northern portions of South America. But how would these forms have
managed, even then, to cross the thermal equator and secure a permanent
habitat in the present warm and cold temperate zones of that continent?
Manifestly, this question has never been practically solved, nor is it
ever likely to be in our day or generation. It is nevertheless susceptible
of solution, as Mr. Darwin thinks, by easy mental processes. We have only
to take a bird's eye view of the situation, and mentally follow these
forms in their long geographical tramp from the northern to the southern
hemisphere.
They must have started, of course, some twenty thousand years or more
before the earth reached its last superior limit of eccentricity. At that
distant epoch the sub-arctic breezes must have been blowing pretty stiffly
in our present temperate latitudes, and these forms would have been
constrained, in due time, to seek a more congenial isotherm. They must
accordingly have set out on their expedition, at about the period
indicated, with the prospect of a long and tedious journey before them.
Some twenty thousand years must have transpired before they reached the
line of the present Gulf states, and it would have taken as many more
years for them to deploy to the right and successfully enter the Mexican
states. In another twenty thousand years or so they might have doubled
Vera Cruz, and headed, in a southeasterly direction, for the Central
American states. The thermal equator would by this time have reached a
point some thirty degrees south of the geographical equator, while the
northern ice-cap would have swept down upon the traditional "hub of the
universe," or some ten or twelve degrees in excess of Mr. Croll's
calculations.
To have accomplished this grand glaciatorial feat the North Pole must have
donned some twenty times the amount of ice now about both poles of the
earth, and so changed the earth's centre of gravity as to have inundated
every foot of land on its habitable surface. But if this terrible
catastrophy had been avoided, and some of our extreme northern forms had
forced their way through the Isthmus into the lowlands of Columbia, they
must have done so at their greatest possible peril, even if they had
reached the base of Old Mt. Tolima in advance of the thermal equator, now
fleeing in dismay before the southern Ice-monarch, with all his
isochimenal hosts in mad pursuit of their invaders. And if these
adventurous northern forms had succeeded in ascending Mt. Tolima, they
could never have got down again, with the assistance of forty glaciations.
But we can imagine Mr. Darwin promptly snatching his pen to show the
stupidity of these northern forms in not climbing Popocatepetl or some
other lofty mountain in Central America or Mexico, on their retreat before
the still advancing thermal equator. But how this would have helped them
to cross the geographical equator, we fail to see. When Mr. Darwin, and
the eminent corps of geologists and physicists accepting his solution of
this "vexed question," can make a "warm term" south _succeed_ a "cold
term" north, we shall have no difficulty in solving the problem ourself.
But, unfortunately, the two terms--the cold one north and the warm one
south--are simultaneous in occurrence, and the same causes which forced
these northern invaders into the tropics, when they followed _after_ the
thermal equator, would have driven them ignominously back again _before_
it. The climbing of mountains would only have prolonged their disaster.
For after the glaciation north comes the glaciation south, and unless our
cold temperate zone were pushed down beyond the geographical equator, none
of its living forms could ever have reached the corresponding zone in the
southern hemisphere.
But as this "migration theory" is one of paramount importance to modern
science, and especially to "Darwinism," [19] distinctively so called, let
us, at the risk of repetition and tediousness, propose a scientific
expedition for the better solution of this problem. To do this, we propose
to cut loose from our stupid predecessors, the plants and animals, and
invite Mr. Darwin and some of his more distinguished European
contemporaries, not omitting Professors Gray, Winchell, Yeomans, and some
few other American admirers of his, to accompany us on a fresh expedition
from the warm and cold temperate zones north to the corresponding zones
south, _purely in the interest of science_. To make it certain that the
time fixed upon for this "expedition" to start, will not escape their
attention, we will state what many of them already well know, that the
present eccentricity of the earth's orbit is very low, being only 0.0168,
and that, in the year of our Lord 851,800, it will reach its next superior
limit, with a few intervening oscillations of such minimum value as to
render it hardly worth our while to start before that time.
We shall be obliged, of course to invite our distinguished European party
to join us on this side of the Atlantic, as their own narrow and
contracted continent furnishes no proper field for determining the problem
in question. We shall insist upon one condition only: "_That they shall
never leave the warm temperate zone in which we shall set out on our
expedition, except to pass halfway into an adjoining zone as is the habit,
at times, with plants and animals_." This condition will have to be
rigidly observed, otherwise our expedition would be of no scientific value
to future generations. As we shall have plenty of time to provide the
necessary outfit, we will appoint Mr. Darwin purveyor-general of the
party, and hold him responsible for any misadventure.
We will arrange for the expedition to start in the early autumn of the
year of our Lord 831,800, or about twenty thousand years before the earth
shall reach its next superior limit of eccentricity,--all of us eager, of
course, to brave the climatic vicissitudes of the journey, and to solve
the "great problem of the ages," which is, to determine how the gigantic
elephantoids of the Eocene period managed to cross the thermal equator,
and pass into the present arctic regions of our globe.
As "the king never dies," so the old southern Ice-monarch will be
succeeded by the young northern one, at about the period named. We shall
then have a decided advantage over our predecessors, the plants and
animals, in their journey southward, since we shall know the exact route
they took, and need only follow it. Presumably they had no such
information, nor had they either chart or compass to guide them,--a
circumstance which Mr. Darwin has not sufficiently taken into account in
predicating intelligence of his favorite pedestrians. Besides, these
vegetal and animal forms had one difficulty to encounter which we shall
not experience. With all the northern forms driven down into the Central
American states, they must have been sadly crowded for room, especially
near the Isthmus. The social conifers must have monopolized all the more
favored sites on the mountain sides and tops, while the humbler denizens
of the forest must have contented themselves with still more limited
quarters. The more impatient animals, for lack of necessary forage, must
have crowded through the Isthmus only to be driven back by the tropical
heats to their proper isotherms.
But our warm temperate zone is now moving southward, and our scientific
expedition is moving with it. The northern Ice-monarch has resumed
absolute sway, and our aphelion distance from the sun has increased some
tens millions of miles. We have, in the mean time, moved down to the line
of the Gulf states, and are deploying to the right in order to make a
triumphant entry into Mexico. Mr. Darwin is daily consulting the
isochimenals, and is confident that our northern ice-cap will equal Mr.
Croll's highest expectations. The news finally reaches us that the Gulf
stream has turned its course southward, and is now pouring its immense
treasures of heat into the South Atlantic, if not turning the African
"horn" and washing the far-off Australian coast. This fact greatly
increases the enthusiasm of our European party, and they hasten forward
into the sub-tropical zone, almost "violating conditions" in their haste
to enter the tropics.
At length, we crowd the narrow passages of the Isthmus, and the glory of a
warm temperate climate bursts upon our view in the Columbian states, of
South America. _The expedition promises to be an entire success_. At
least, Mr. Darwin thinks so, and he is now the Sir Oracle of our party. We
deliberately enter the lowlands of Columbia, and make ready to ascend the
sub-tropical mountains--those formerly equatorial--where the "great
scientific problem of the ages" is to be demonstrated. But we are
measuring time by almost _Sirius_ distances, and vast geologic periods
sweep by without apparent record. The northern ice-cap has been a
prodigious one, crowding us nearly down to the geographical equator, with
the advantage we have of appropriating some five and half degrees of the
sub-tropical zone.
But the year Anno Domini 851,800 finally rolls round, and the maximum of
the earth's ice-cap is reached. Old Mt. Tolima looms up in the distance,
and we soon ascertain that its height is sufficient for all scientific
purposes. Its summit displays a glittering ice-cap, and we are certain to
find the proper isotherm by climbing its umbrageous sides. We accordingly
make haste to reach its base, and get there not a minute too soon; for the
young southern Ice-monarch has stolen a march on the thermal equator, and
is driving it irresistibly back to its old quarters. His march northward
is a continuous triumph and ovation up to 55°, and the heart of Patagonia
is made glad by his near approach. True, the white gates of commerce are
closed about the Horn; but that is no concern of these wild Patagonians.
The aggressive Britton is driven out of New Zealand, and that is another
source of joy to the savage breast. Tasmania would extend a gladder
welcome than all to the Ice-crowned monarch, but alas, not a drop of
Tasmanian blood runs in human veins! Cape Good Hope has now a sub-arctic
climate, and the heart of the wild Kaffir and Zulu rejoices that the
sceptre of "perfidious Albion" is broken.
The thermal equator at length reaches the base of Mt. Tolima, and hastens
northward to the Isthmus, and thence to Hondurus and New Guatemala, where,
by sheer force of exhaustion, it comes to a halt.
But, as the equatorial zone extends fifteen degrees both ways from the
thermal equator, its southern limit now rests on the geographical equator,
and accordingly encircles the base of our "mount of refuge." We are now up
this mountain some sixteen thousand feet above the equatorial lowlands,
with the sub-tropical, tropical, and equatorial zones between us and the
possibility of our further migration southward, without violating the
express conditions imposed at the outset of our expedition.
The fact soon stares us in the face that we have been no more successful,
in our efforts to cross the thermal equator and pass into high southern
latitudes, than the stupid plants and animals before us; and Mr Darwin's
faith in high mountains springing from equatorial lowlands, disappears in
jest and derision as we all good-humoredly agree "to break conditions,"
and find our way back to the centres of activity and trade in the Old and
New Worlds, leaving the great scientific problem of the ages to solve
itself as best it may. We accordingly descend from our mountain fastness,
hasten to the coast, and take passage by steamer to Manhattan, the great
commercial metropolis of the world. Here we find that the barometer of
exchange was long ago taken down in London and hung up in New York. The
Old Antiquarian Society rooms are the first object of interest sought by
us. On making our way thither we look for a copy of the _Herald_, of the
date of our departure, in which we find an account of the scientific
expedition fitted out by us, facetiously termed "_The Great Wild-Goose
Chase after the Thermal Equator_"--presenting one of the most humorous
bits of sensational pleasantry ever given to the American public.
But an apology is due the staider reader for the seeming levity of this
narrative adventure. The exposition of Mr. Darwin, though widely accepted
on both sides of the Atlantic by the scientific world, has seemed to us
too trivial for serious reply. If we have leaped over vast periods of
time, it makes no difference with the argument. So long as the thermal
equator, or more properly the equatorial zone, or any part of it, lies
between the warm or cold temperate forms, whether plants or animals, and
their point of destination in the southern hemisphere, they can never
migrate thither, any more than the right whale of the arctic seas can swim
the equatorial oceans. Nothing is gained by going out of the way to climb
mountains, except to hopelessly retard the return of both plants and
animals to their native zones. If we have not demonstrated this fact to
the reader's fullest comprehension, it will be useless for him ever to
write a Q.E.D. at the end of any proposition.
It is true that some eminent astronomers and physicists hesitate to
accept the theory that these glacial epochs are due to the eccentricity
of the earth's orbit. But the argument favoring it is well fortified and
ably advanced, and if we add to the astronomical considerations involved,
the physical proofs of a change in the earth's centre of gravity, caused
by the excessive accumulation of ice about either pole, and the probable
shifting of the Gulf stream to a southerly direction during the glacial
period north, it is difficult to resist the conviction that the real
cause of glaciation has been suggested in this theory. With all the ice
now accumulated about the south pole transferred to the north pole, it
would make an ice-cap of over thirty miles in thickness at the pole, and
one sloping in all directions southward to about 60°. This accumulation,
it is claimed, would so change the earth's centre of gravity as to cause
all the equatorial warm waters to flow southward instead of northward, as
they now do.
This would certainly seem to be a most wonderful provision of nature, as
well as one strongly calculated to impress the human mind with the belief
that an Infinite _Pre_vision lies behind all possible _pro_vision, whether
witnessed in the heavens or in the earth, in astronomical or physical
phenomena. Everywhere we see infinite perfection, combined with infinite
beneficence, in the adaptation of means to ends. Nothing runs to
waste--all things are conserved for use.
But in all the outspoken grandeur of the universe, there is nothing so
grand, in exhibition at least, as the simple faith of a child, that "He
who watereth the hills from his chambers," and "causeth the day-spring to
know his place," will watch over the trustful little sleeper during the
darkness and silence of the night.