_THE DISTRIBUTION OF ANIMALS AND PLANTS AS RELATED TO GEOGRAPHICAL AND
GEOLOGICAL CHANGES._
All are now agreed that to explain the extraordinary and often
apparently anomalous distribution of animals and plants over the
surface of the earth, and the occurrence of like forms in very distant
localities, and even on islands separated by vast stretches of ocean
from one another and from the continents, we must invoke the aid of
geology. We must have reference to those changes of climate and of
elevation which have occurred in the more recent periods of the earth's
history, and must carry with us the idea, at first not apparently very
reasonable, that living beings have existed much longer than many of
the lands which they inhabit, or at least than the present state of
those lands in reference to isolation or continental connection. To
what extent we may further require to call in the aid of varietal or
specific modification to explain the facts, may be more doubtful; and I
think we shall find that a larger acquaintance with geological truths
would enable us to dispense with the aid of hypotheses of evolution, at
least in so far as the local establishment of new generic and specific
types is concerned.
One of the most remarkable and startling results of geological
investigation, and one which must be accepted as an established fact,
independently of all theoretical explanations, is that the earth has
experienced enormous revolutions of climate within comparatively
late periods, and since the date of the introduction of many existing
species of animals and plants. To this great truth, in some of its
bearings, I have endeavoured to direct attention in the previous
articles. In the present case it will be necessary to consider these
vicissitudes in their more general aspects, and with some reference to
their effects on the distribution of living beings.
The modern or human period of geology, that in which man and his
contemporaries are certainly known to have inhabited the earth, was
immediately preceded by an age of climatal refrigeration known as
the Glacial or Ice age. This was further characterized not only by
a prevalence of cold, unexampled so far as known either before or
since, but by immense changes of the relative levels of sea and land,
amounting, in some cases, at least, to several thousands of feet. The
occurrence of these changes is clearly proved by the undoubted traces
of the action of ice, whether land ice or floating ice, on all parts
of our continents, half way to the equator, and by the occurrence of
sea terraces and modern marine shells at high levels on mountains and
table-lands. Perhaps we scarcely realize as we should the stupendous
character of the changes involved in the driftage of heavy ice over
our continents as far south as the latitude of 40°, in the deposit
of boulders on hills several thousands of feet in height, and in the
occurrence of shells of species still living in the sea, in beds raised
to more than twelve hundred feet above its present level. Yet such
changes must have occurred in the latest geological period immediately
preceding that in which we live. Proceeding farther back in geological
time, we find the still more extraordinary fact that in the middle and
earlier Tertiary the northern hemisphere enjoyed a climate so much more
mild than that which now prevails, that plants at present confined to
temperate latitudes could flourish in Greenland and Spitzbergen.[183]
The age in which we live is thus one of mediocrity, attaining neither
to the Arctic rigour of the later Pleistocene, nor to the universal
mildness of the preceding Miocene.
[183] As I have elsewhere shown, a warm climate in an Arctic region
seems to have afforded the necessary conditions for the great
colonizing floras of all geological periods.
The causes of these changes of climate we have discussed elsewhere.
It remains for us now to consider the actual condition of our present
continents, and the bearing of past conditions on the distribution of
their living inhabitants.
In speaking of continents and islands, it may be as well to remark at
the outset that all the land existing, or which probably has at any
time existed, consists of islands great or small. It is all surrounded
by the ocean. Two of the greater masses of land are, however,
sufficiently extensive to be regarded as continents, and from their
very extent and consequent permanence may be considered as the more
special homes of the living beings of the land. Two other portions of
land, Australia and the Antarctic polar continent, may be regarded
either as smaller continents or large islands, but partake of insular
rather than continental characters in their animals and plants. All the
other portions of land are properly islands; but while these islands,
and more especially those in mid-ocean, cannot be regarded as the
original homes of many forms of life, we shall find that they have a
special interest as the shelters and refuges of numerous very ancient
and now decaying species.
The two great continents of America and Eurasia have been the most
permanent portions of the land throughout geological time, some parts
of them having always been above water, probably from the Laurentian
age downward, though at various times they have been reduced to little
more than groups of islands. On them, and more especially in their
more northern parts, in which the long continuance of daylight in
summer seems in warm periods to have been peculiarly favourable to the
introduction of new vegetable and animal forms, and to the giving to
them that vigour necessary for active colonization, have originated the
greater number of the inhabitants of the land.
Regarded as portions of the earth's crust, the continents are areas
in which the lateral thrust, caused by the secular contraction of the
interior of the earth and unequal settlement of the crust, has ridged
up and folded the rocks, producing mountain chains. This process began
in the earliest geological periods, and has been repeated at long
intervals, the original lines of folding guiding those formed in each
new thrust proceeding from the broad oceanic areas. Along the ridges
thus produced, and in the narrower spaces between them, the greater
part of the sediment carried by water was laid down, thus producing
plateaus in connection with the mountain-chains, while the weight of
new sediments and the removal of matter from other areas by denudation,
have been constantly producing local depression and elevation. The
tendency of the ocean to be thrown toward the poles by the retardation
of the earth's rotation, alternating with great collapses of the
crust at the equator proceeding from the same cause, along with the
secular cooling, have produced alternate submergence and emergence
of these plateaus. This has been further complicated by the constant
tendency of the Arctic and Antarctic currents, aided by ice, to drift
solid materials, set free by the vast denuding action of frost, from
the polar to the temperate regions, and by the further tendency of
animal life to heap up calcareous accumulations under the warm waters
of the tropical regions. All these changes, as already stated, have
conspired to modify the directions of the great oceanic currents, and
to produce vicissitudes of climate under which animals and plants have
been subjected in geological time to those migrations, extinctions,
and renovations of which their fossil remains and present distribution
afford evidence.
Still, it is true that throughout the whole of these great mutations,
since the beginning of geological history, there seems never to have
been any time when the ocean so regained its dominion as to produce a
total extinction of land life; still less was there any time when the
necessary conditions of all the various forms of marine life failed to
be found; nor was there any climatal change so extreme as to banish any
of the leading forms of life from the earth. To geologists it is not
necessary to say that the conclusions sketched above are those that
have been reached as the results of long and laborious investigation,
and which have been illustrated and established by Lyell, Dana,
Wallace,[184] and many other writers.[185] Let us now place beside them
some facts as to the present distribution of life, and of the agencies
which influence it.
[184] Wallace, "Geographical Distribution of Animals" and "Island
Life." Second edition.
[185] The writer has endeavoured to popularize these great results of
geology in his work, the "Story of the Earth." Ninth Edition. London,
1887. They are often overlooked by specialists, and by compilers of
geological manuals.
Just as political geography sometimes presents boundaries not
in accordance with the physical structure of countries, so the
distribution of animals and plants shows many peculiar and unexpected
features. Hence naturalists have divided the continents into what
Sclater has called zoological regions, which are, so to speak, the
great empires of animal life, divisible often by less prominent
boundaries into provinces. In vegetable life similar boundaries may
be drawn, more or less coincident with the zoological divisions.
Zoologically, North America and Greenland may be regarded as one
great region, the Nearctic, or new Arctic, the prefix not indicating
that the animals are newer than those of the old world, which is by
no means the case. South America constitutes another region the
Neotropical. If now we turn to the greater Eurasian continent, with
its two prolongations to the south in Africa and Australia, we shall
find the whole northern portion, from the Atlantic to the Pacific,
constituting one vast region of animal life, the Palearctic, which also
includes Iceland and a strip across North Africa. Africa itself, with
Madagascar, whose allegiance is, however, only partial, constitutes
the Ethiopian region. India, Burmah, the south of China, and certain
Asiatic islands form the Oriental region. Australia, New Guinea, and
the Polynesian islands constitute the Australian region. All of these
regions may in a geological point of view be considered as portions of
old and permanent continental masses, which, though with movements of
elevation and depression, have continued to exist for vast periods.
Some of them, however, seem to have enjoyed greater immunity from
causes of change than others, and present, accordingly, animals and
plants having, geologically speaking, an antique aspect in comparison.
In this sense the Australian province may be regarded as the oldest of
all in the facies of its animal forms, since creatures exist there of
genera and families which have very long ago become extinct everywhere
else. Next in age to this should rank the Neotropical or South American
region, which, like Australia, presents many low and archaic forms
of animal life. The Ethiopian region stands next to it in this, the
Oriental and Nearctic next, and last and most modern in its aspect is
the great Palearctic region, to which man himself belongs, and the
animals and plants of which vindicate their claims to youth by that
aggressive and colonizing character already referred to, and which has
enabled them to spread themselves widely over the other regions, even
independently of the influence of man. On the other hand, the animals
and plants of the Australian and South American regions show no such
colonizing tendency, and can scarcely maintain themselves against those
of other regions when introduced among them. Thus we have at once
in these continental regions a great and suggestive example of the
connection of geographical and geological distribution, the details of
which are of the deepest interest, and have not yet been fully worked
out. One great principle is, however, sufficiently established; namely,
that the northern regions have been the birthplace of new forms of
land life, whence they have extended themselves to the south, while
the comparative isolation and equable climate of the South American
and Australian regions have enabled them to shelter and retain the old
moribund tribes.
Those smaller portions of land separated from the continental masses,
the islands properly so called, present, as might be expected, many
peculiar features. Wallace divides them into two classes, though he
admits that these pass into each other. Continental islands are those
in the vicinity of continents. They consist of ancient as well as
modern rock formations, and contain animals which indicate a former
continental connection. Some of these are separated from the nearest
mainland only by shallow seas or straits, and may be assumed to have
become islands only in recent geological times. Others are divided from
the nearest continent by very deep-water, so that they have probably
been longer severed from the mainland. These contain more peculiar
assemblages of animals and plants than the islands of the former class.
Oceanic islands are more remote from the continents. They consist
mostly of rocks belonging to the modern geological periods, and contain
no animals of those classes which can migrate only by land. Such
islands may be assumed never to have been connected with any continent.
The study of the indigenous population of these various classes of
islands affords many curious and interesting results, which Wallace
has collected with vast industry and care, and which, on the whole,
he explains in a judicious manner and in accordance with the facts of
geology. When, however, he maintains that evolution of the Darwinian
type is "the key to distribution," he departs widely from any basis of
scientific fact. This becomes apparent when we consider the following
results, which appear everywhere in the discussion of the various
insular faunas and floras:--(1) None of these islands, however remote,
can be affirmed to have been peopled by the spontaneous evolution of
the higher animals or plants from lower forms. Their population is in
every case not autochthonous, but derived. (2) Even in those which are
most distant from the continents, and may be supposed to have been
colonized in very ancient times, there is no evidence of any very
important modification of their inhabitants. (3) While the facts point
to the origin of most forms of terrestrial life in the Palearctic and
Nearctic regions, they afford no information as to the manner or cause
of their origination. In short, so far is evolution from being a key
to distribution, that the whole question would become much more simple
if this element were omitted altogether. A few examples may be useful
to illustrate this, as well as the actual explanation of the phenomena
afforded by legitimate science.
The Azores are situated in a warm temperate latitude about 900 miles
west of Portugal, and separated from it by a sea 2,500 fathoms in
depth. The islands themselves are almost wholly volcanic, and the
oldest rocks known in them are of late Miocene age. There is no
probability that these islands have ever been connected with Europe
or Africa, nor is there at present any certainty that they have been
joined to one another, or have formed part of any larger insular
tract. In these islands there is only one indigenous mammal, a bat,
which is identical with a European species, and no doubt reached the
islands by flight. There is no indigenous reptile, amphibian, or
fresh-water fish. Of birds there are, exclusive of waterfowl, which
may be regarded as visitors, twenty-two land birds; but of these, four
are regarded as merely accidental stragglers, so that only eighteen
are permanent residents. Of these birds fifteen are common European or
African species, which must have flown to the islands, or have been
drifted thither in storms. Of the remaining three, two are found also
in Madeira and the Canaries, and therefore may reasonably be supposed
to have been derived from Africa. One only is regarded as peculiar
to the Azores, and this is a bullfinch, so nearly related to the
European bullfinch that it may be regarded as merely a local variety.
Wallace accounts for these facts by supposing that the Azores were
depopulated by the cold of the Glacial age, and that all these birds
have arrived since that time. There is, however, little probability in
such a supposition. He further supposes that fresh supplies of stray
birds from the mainland, arriving from time to time, have kept up
the identity of the species. Instead of evolution assisting him, he
has thus somewhat to strain the facts to agree with that hypothesis.
Similar explanations are given for the still more remarkable fact that
the land plants of the Azores are almost wholly identical with European
and African forms. The insects and the land snails are, however, held
to indicate the evolution of a certain number of new specific forms on
the islands. The beetles number no less than 212 species, though nearly
half of them are supposed to have been introduced by man. Of the whole
number 175 are European, 19 are found in Madeira and the Canaries, 3
are American. Fourteen remain to be accounted for, though most of these
are closely allied to European and other species; but a few are quite
distinct from any elsewhere known. Wallace, however, very truly remarks
that our knowledge of the continental beetles is not complete; that the
species in question are small and obscure; that they may be survivors
of the Glacial period, and may thus represent species now extinct on
the mainland; and that for these reasons it may not be irrational to
suppose that these peculiar insects either still inhabit, or did once
inhabit, some part of the continents, and may be portions of "ancient
and widespread groups," once widely diffused, but now restricted to
a few insular spots. Among the land snails, if anywhere, we should
find evidence either of autochthonous evolution or of specific change.
These animals have existed on the earth since the Carboniferous period,
and, notwithstanding their proverbial slowness and sedentary habits,
they have contrived to colonize every habitable spot of land on the
globe--that is, unless in some of these places they have originated _de
novo_. In the Azores there are sixty-nine species of land snails, of
which no less than thirty-two, or nearly one-half, are peculiar, though
nearly all are closely allied to European types. What, then, is the
origin of these thirty-two species, admitting for the sake of argument
that they are really distinct, and not merely varietal forms, though it
is well known that in this group species are often unduly multiplied.
Three suppositions are possible, (1) These snails may have originated
in the islands themselves, either by creation or evolution from lower
forms; say, from sea snails. (2) They may have been modified from
modern continental species. (3) They may be unmodified descendants of
species of Miocene or Pliocene age now existing on the continents only
as fossils. As the islands appear to have existed since Miocene times,
it is no more improbable that species of that or the Pliocene age
should have found their way to them than that modern species should;
and as we know only a fraction of the Tertiary species of Europe or
Africa, it is not likely that we shall be able to identify all of these
early visitors. Unfortunately no Miocene or Pliocene deposits holding
remains of land snails are known in the Azores themselves, so that
this kind of evidence fails us. In Madeira and Porto Santo, however,
where there are numerous modern snails, there are Pliocene beds holding
remains of these animals. In Madeira there are, according to Lyell, 36
Pliocene species, and in Porto Santo 35, and of these only eight are
extinct. Thus we can prove that many of the peculiar species of these
islands have remained unchanged since Pliocene times. While differing
from modern European shells, several of these species are very near to
European Miocene species. Thus we seem to have evidence in the Madeira
group, not of modification, but of unchanged survival of Tertiary
species long since extinct in Europe. May we not infer that the same
was the case in the Azores? These results are certainly very striking
when we consider how long the Azores must have existed as islands, how
very rarely animals, and especially pairs of animals, must have reached
them, and how complete has been the isolation of these animals, and
how peculiar the conditions to which they have been subjected in their
island retreat.
Other oceanic islands present great varieties of conditions, but
leading to similar conclusions. Some, as the Bermudas, seem to have
been settled in very modern times with animals and plants nearly all
identical with those of neighbouring countries, though even here
it would appear that there are some indigenous species which would
indicate a greater age or more extended lands, now submerged.[186]
Others, like St. Helena, are occupied apparently with old settlers,
which may have come to them in early Tertiary, or even in Secondary
periods, which have long since become extinct on the continents, and
whose nearest analogues are now widely scattered over the world.
Islands are therefore places of survival of old species--special
preserves for forms of life lost to the continents. One of the most
curious of these is Celebes, which seems to be a surviving fragment
of Miocene Asia, which, though so near to that continent, has been
sufficiently isolated to preserve its old population during all
the vast lapse of time between the middle Tertiary and the present
period. This is a fact which gives to the oceanic islands the greatest
geological interest, and induces us to look into their actual fauna and
flora for the representatives of species known on the mainland only as
fossils. It is thus that we look to the marsupials of Australia as the
nearest analogues of those of the Jurassic of Europe, and that we find
in the strange Barramunda (_ceratodus_) of its rivers the only survivor
of a group of fishes once widely distributed, but which has long since
perished elsewhere.
[186] Heilprin mentions eleven marine mollusks supposed to be peculiar
to the islands, and eight species of land shells, as well as a few
Crustaceans hitherto found only in the Pacific. The comparisons are,
however, admitted to be incomplete.
Perhaps one of the most interesting examples of this is furnished by
the Galapagos Islands, an example the more remarkable that no one who
has read in Darwin's fascinating "Journal" the description of these
islands, can have failed to perceive that the peculiarities of this
strange Archipelago must have been prominent among the facts which
first planted in his mind the germ of that theory of the origin of
species which has since grown to such gigantic dimensions. It is
curious also to reflect that had the bearing of geological history on
the facts of distribution been as well known forty years ago as it is
now, the reasoning of the great naturalist on this and similar cases
might have taken an entirely different direction.
The Galapagos are placed exactly on the equator, and therefore out
of reach of even the suspicion of having been visited by the glacial
cold, though from their isolation in the ocean, and the effects of
the currents flowing along the American coast, their climate is not
extremely hot. They are 600 miles west of South America, and the
separating ocean is in some parts 3,000 fathoms deep. The largest of
the islands is 75 miles in length, and some of the hills attain an
elevation of about 4,000 feet, so that there are considerable varieties
of station and climate. So far as is known they are wholly volcanic,
and they may be regarded as the summits of submerged mountains not
unlike in structure to the Andes of the mainland. Their exact
geological age is unknown, but there is no improbability in supposing
that they may have existed with more or less of extension since the
Secondary or Mesozoic period. In any case their fauna is in some
respects a survival of that age. Lyell has truly remarked, "In the
fauna of the Galapagos Islands we have a state of things very analogous
to that of the Secondary period."
Like other oceanic islands, the Galapagos have no indigenous mammals,
with the doubtful exception of a South American mouse; but, unlike
most others, they are rich in reptiles. At the head of these stand
several species of gigantic tortoises. This group of animals, so far
as known, commenced its existence in the Eocene Tertiary; and in this
and the Miocene period still more gigantic species existed on the
continents. It has been supposed that at some such early date they
reached the Galapagos from South America. Another group of Galapagan
reptiles, perhaps still more remarkable, is that of iguana-like lizards
of the genus _Amblyrhyncus_, which are vegetable feeders,--one of them
browsing on marine weeds. They recall the great iguana-like reptiles
of the European Wealden, and stand remote from all modern types. There
are also snakes of two species, but these are South American forms,
and may have drifted to the islands in comparatively recent times on
floating trees. The birds are a curious assemblage. A few are common
American species, like the rice bird. Others are quaint and peculiar
creatures, allied to South American birds, but probably representing
forms long since extinct on the continent. The bird fauna, as Wallace
remarks, indicates that some of these animals are old residents, others
more recent arrivals; and it is probable that they have arrived at
various times since the early Tertiary. He assumes that the earlier
arrivals have been modified in the islands "into a variety of distinct
types"; but the only evidence of this is that some of the species are
closely related to each other. It is more likely that they represent
to our modern eyes the unmodified descendants of continental birds of
the early Tertiary. Darwin remarks that they are remarkably sombre
in colouring for equatorial birds; but perhaps their ancestors came
from a cooler climate, and have not been able to don a tropical garb;
or perhaps they belong to a far-back age, when the vegetable kingdom
also was less rich in colouring than it is at present, and the birds
were in harmony with it. This, indeed, seems still to be the character
of the Galapagos plants, which Darwin says have "a wretched, weedy
appearance," without gay flowers, though later visitors have expressed
a more favourable opinion.
These plants are in themselves very remarkable, for they are largely
peculiar species, and are in many cases confined to particular islands,
having apparently been unable to cross from one island to another,
though in some way able to reach the group. The explanation is that
they resemble North American plants, and came to the Galapagos at a
time when a wide strait separated North and South America, allowing
the equatorial current to pass through, and drift plants to the
Galapagos, where they have been imprisoned ever since. This was
probably in Miocene times, and when we know more of the Miocene flora
of the southern part of North America we may hope to recover some of
the ancestors of the Galapagos plants. In the meantime their probable
origin and antiquity, as stated by Wallace, render unnecessary any
hypothesis of modification.
Before leaving this subject, it is proper to observe that on the
continents themselves there are many remarkable cases of isolation of
species, which help us better to understand the conditions of insular
areas. The "variable hare" of the Scottish highlands, and of the
extreme north of Europe, appears again in the Alps, the Pyrenees, and
the Caucasus, being in these mountains separated by a thousand miles
of apparently impassable country from its northern haunts. It no doubt
extended itself over the intervening plains at a time when Europe was
colder than at present. Another curious case is that of the marsh-tit
of Europe. This little bird is found throughout south-western Europe.
It reappears in China, but is not known anywhere between. In Siberia
and northern Europe there is, however, a species or distinct race which
connects these isolated patches. In this case, if the Siberian species
is truly distinct, we have a remarkable case of isolation and of the
permanence of identical characters for a long time; for in that case
this bird must be a survivor of the Pliocene or Miocene time. On the
other hand, if, as is perhaps more likely, the marsh-tit is only a
local variety of the Siberian species, we have an illustration of the
local recurrence of this form when the conditions are favourable, even
though separated by a great space and long time.
The study of fossils gives us the true meaning of such facts, and
causes us to cease to wonder at any case of local repetition of
species, however widely separated. The "big trees" of California
constitute a remarkable example. There are at present two very distinct
species of these trees, both found only in limited areas of the western
part of North America. Fossil trees of the same genus (_Sequoia_)
occur as far back as the Cretaceous age; but in this age ten or more
species are known. Nor are they confined to America, but occur in
various parts of the Eurasian continent as well. Two of the Lower
Cretaceous species are so near to the two modern ones that even an
unbeliever in evolution may suppose them to be possible ancestors; the
remaining eight are distinct, but some of them intermediate in their
characters. In the Tertiary period, intervening between the Cretaceous
and the modern, fourteen species of _Sequoia_ are believed to have been
recognised, and they appear to have existed abundantly all over the
northern hemisphere. Thus we know that these remarkable Californian
giants are the last remnant of a once widely distributed genus,
originating, as far as known, in the Cretaceous age. Now had a grove
of _Sequoias_, however small, survived anywhere in Europe or Asia, and
had we no knowledge of the fossil forms, we might have been quite at
a loss to account for their peculiar distribution. The fossil remains
of the Tertiary rocks, both animal and vegetable, present us with many
instances of this kind.
The discussion of the distribution of animals and plants, when carried
on in the light of geology, raises many interesting questions as to
time, which we have already glanced at, but which deserve a little more
attention. As to the vast duration of geological time all geologists
are agreed. It is, however, now well understood that science sets
certain limits to the time at our disposal. Edward Forbes humorously
defined a geologist to be "an amiable enthusiast who is content if
allowed to appropriate as much as he pleases of that which other men
value least, namely, past time "; but now even the geologist is obliged
to be content with a limited quantity of this commodity.
The well-known estimate of Lord Kelvin gave one hundred millions of
years as the probable time necessary for the change of the earth from
the condition of a molten mass to that which we now see. On this
estimate we might fairly have assumed fifty millions of years as
covering the time from the Laurentian age to the modern period. The
great physicist has, however, after allowing us thus much credit in the
bank of time, "suddenly put up the shutters and declared a dividend of
less than four shillings in the pound."[187] In other words, he has
reduced the time at our disposal to twenty millions of years. Other
physicists, reasoning on the constitution of the sun, agree with this
latter estimate, and affirm that "twenty millions of years ago the
earth was enveloped in the fiery atmosphere of the sun."[188] Geology
itself has attempted an independent calculation based on the wearing
down of our continents, which appears to proceed at the rate of about
a foot in four or five thousand years, and on the time required to
deposit the sediments of the several geological formations, estimated
at about 70,000 feet in thickness. These calculations would give us,
say, eighty-six millions of years since the earth began to have a solid
crust, which would, like Lord Kelvin's earlier estimate, give us nearly
fifty millions of years for the geological time since the introduction
of life. The details of the several estimates made it would be tedious
and unprofitable to enter into, but I may state as my own conclusion,
that the modern rates of denudation and deposit must be taken as far
below the average, and that perhaps the estimate stated by Wallace on
data supplied by Houghton, namely, twenty-eight millions, may be not
far from the truth, though perhaps admitting of considerable abatement.
[187] Bonney, Address before British Association, 1888.
[188] Newcomb, Helmholtz, Tait, etc.
This reduced estimate of geological time would still give scope enough
for the distribution of animals and plants, but it will scarcely
give that required by certain prevalent theories of evolution. When
Darwin says, "If the theory (of natural selection) be true, it is
indisputable that before the lowest Cambrian stratum was deposited
long periods elapsed, as long as, or probably far longer than, the
whole interval from the Cambrian to the present day," he makes a demand
which geology cannot supply; for independently of our ignorance of
any formations or fossils, except those included in the Archæan, to
represent this vast succession of life, the time required would push
us back into a molten state of the planet. This difficulty is akin to
that which meets us with reference to the introduction of many and
highly specialized mammals in the Eocene, or of the forests of modern
type in the Cretaceous. To account for the origin of these by slow and
gradual evolution requires us to push these forms of life so far back
into formations which afford no trace of them, but, on the contrary,
contain other creatures that appear to be exclusive of them, that our
faith in the theory fails. The only theory of evolution which seems to
meet this difficulty is that advanced by Mivart, Leconte, and Saporta,
of "critical periods," or periods of rapid introduction of new species
alternating with others of comparative inaction. This would much better
accord with the apparently rapid introduction of many new forms of life
over wide regions at the same period. It would also approach somewhat
near, in its manner of stating the problem to be solved, to the theory
of "creation by law" as held by the Duke of Argyll, or to what may be
regarded as "mediate creation," proceeding in a regular and definite
manner, but under laws and forces as yet very imperfectly known,
throughout geological time.
It seems singular, in view of the facts of palæontology, that
evolutionists of the Darwinian school are so wedded to the idea of one
introduction only of each form of life, and its subsequent division by
variation into different species, as it progressively spreads itself
over the globe, or is subjected to different external conditions. It
is evident that a little further and very natural extension of their
hypothesis would enable them to get rid of many difficulties of time
and space. For example, certain Millipedes and Batrachians are first
known in the coal formation, and this not in one locality only, but
in different and widely separated regions. If they took beginning in
one place, and spread themselves gradually over the world, this must
have required a vast lapse of time--more than we can suppose probable.
But if, in the coal-formation age, a worm could anywhere change into
a Millipede, or a fish into a Batrachian, why might this not have
occurred in many places at once? Again, if the oldest known land snails
occur in the coal formation, and we find no more specimens till a much
later period, why is it necessary to suppose that these creatures
existed in the intervening time, and that the later species are the
descendants of the earlier? Might not the process have been repeated
again and again, so as to give animals of this kind to widely separated
areas and successive periods without the slow and precarious methods
of continuous evolution and migration? This apparent inconsistency
strikes one constantly in the study of discussions of the theory of
derivation in connection with geographical and geological distribution.
We constantly find the believers in derivation laboriously devising
expedients for the migration of animals and plants to the most unlikely
places, when it would seem that they might just as well have originated
in those places by direct evolution from lower forms. Those who believe
in a separate centre of creation for each species must of course invoke
all geological and geographical possibilities for the dispersion of
animals and plants; but surely the evolutionist, if he has faith in his
theory, might take a more easy and obvious method, especially when in
any case he is under the necessity of demanding a great lapse of time.
That he does not adopt this method perhaps implies a latent suspicion
that he must not repeat his miracle too often. He also perceives that
if repeated and unlimited evolution of similar forms had actually
occurred, there could have remained little specific distinctness,
and the present rarity of connecting links would not have occurred.
Further, a new difficulty would have sprung up in the geographical
and geological relations of species and genera, which would then have
assumed too much of the aspect of a preconceived plan. It is only fair
to a well-known and somewhat extreme European evolutionist, Karl Vogt,
to state that he launches boldly into the ocean of multiple evolution,
not fearing to hold that identical species of mollusks have been
separately evolved in separate Swiss lakes, and that the horse has been
separately evolved in America and in Europe, in the former along a line
beginning with Eohippus, and in the latter along an entirely separate
line, commencing with _Palæotherium_. The serious complications
resulting from such admissions are evident, but Vogt deserves credit
for faith and consistency beyond those of his teachers.
With reference to the actual distribution of species, the question
of time becomes most important when applied to the Glacial period,
since it is obvious that much of the present distribution must have
been caused, or greatly modified, by that event. The astronomical
theory would place the close of the Glacial age as far back as 70,000
or 80,000 years ago. But we have already seen in the chapter on that
period that geological facts bring its close to only from 10,000 to
7,000 years before our time. If we adopt the shorter estimates afforded
by these facts, it will follow that the submergences and emergences
of land in the Glacial ages were more rapid than has hitherto been
supposed, and that this would react on our estimate of time by giving
facilities for more rapid denudation and deposition. Such results would
greatly shorten the duration assignable to the human period. They
would render it less remarkable that no new species of animals seem to
have been introduced since the Glacial age, that many insular faunas
belong to far earlier times, and that no changes even leading to the
production of well-marked varieties have occurred in the post-glacial
or modern age.
In conclusion, does all this array of fact and reasoning bring us any
nearer to the comprehension of that "mystery of mysteries," the origin
and succession of life? It certainly does not enable us to point to any
species, and to say precisely here, at this time and thus it originated.
If we adopt the theory of evolution, the facts seem to restrict us to
that form of it which admits paroxysmal or intermittent introduction of
species, depending on the concurrence of conditions favourable to the
action of the power, whatever it may be, which produces new organisms.
Nor is there anything in the facts of distribution to invalidate the
belief in creation, according to definite laws, if that really differs
in its nature from certain forms of the hypothesis of evolution. We
have also learned that, time being given, animals and plants manifest
wonderful powers of migration, that they can vary within considerable
limits without ceasing to be practically the same species, and that
under certain conditions they can endure far longer in some places than
in others. We also see evidence that it is not on limited islands,
but on the continents, that land animals and plants have originated,
and that swarms of new and vigorous species have issued from the more
northern regions in successive periods of favourable Arctic climate.
The last of these new swarms or "centres of creation," that with which
man himself is more closely connected, belongs to the Palearctic
region. We have already seen that in every geological period, when the
submerged continental plateaus were pervaded by the warm equatorial
waters, multitudes of new marine species appear. In times when, on
the contrary, the colder Arctic currents poured over these submerged
surfaces, carrying mud and stones, great extinction took place, but
certain northern forms of life swarmed abundantly, and when elevation
took place, marine species became extinct or were forced to migrate.
Everywhere and at all times multiplication of species was promoted
by facilities for expansion. The great limestones of our continents,
full of corals and shells of new species, belong to times when the
ocean spread itself over the continental plateaus, affording wide,
untenanted areas of warm and shallow water. The introduction of new
faunas and floras on the land belongs to times when vast supplies of
food for plants and animals and favourable conditions of existence were
afforded by the emergence of new lands possessing fertile soils and
abundantly supplied with light, heat, and moisture. Thus geological and
geographical facts concur with ordinary observation and experience in
reference to varietal forms, in testifying that it is not mere struggle
for existence, but facilities for easy existence and rapid extension,
that afford the conditions necessary for new and advanced forms of
life. These considerations do not, of course, reach to the first cause
of the introduction of species, nor even to the precise mode in which
this may have acted in any particular case: but perhaps we cannot
fully attain to this by any process of inductive inquiry. The study of
geographical distribution,' therefore, does not enable us to solve the
question of the origin of specific types, but, on the contrary, points
to marvellous capacities for migration and a wonderful tenacity of life
in species. In these respects, however, it is a study full of interest,
and in nothing more so than in the evidence which it affords of the
practically infinite provisions made for the peopling of every spot of
land or sea with creatures fitted to flourish and enjoy life therein,
and to carry on the great and progressive plan of the Creator.
References:--Continental and Island Life, _Princeton Review_, July,
1881. Address to American Association, 1883. Papers and Addresses
to Natural History Society, _Canadian Naturalist_, Montreal. "The
Story of the Earth and Man," 1st ed., 1873, 9th ed., London, 1887.
_ALPINE AND ARCTIC PLANTS IN CONNECTION WITH GEOLOGICAL HISTORY_
DEDICATED TO THE MEMORY OF
DR. ASA GRAY,
THE GREATEST AND MOST PHILOSOPHICAL EXPONENT
OF AMERICAN BOTANY.
A Botanico-Geological Excursion in the White Mountains--Distribution
and Migrations of Alpine Plants--Relations to the Later Geological
Changes--Bearing on the Vegetation of Earlier Times
[Illustration: Mount Washington, from Tuckerman's Ravine. (p. 426.)
(After Filmer, in King's "White Hills.")]