_CAUSES OF CLIMATAL CHANGE._
The subject of this chapter is one which has been in dispute ever since
I began to read anything on geology, nearly sixty years ago. It ought
to have been settled, but up to to-day one finds in geological works
and papers--especially those relating to the Glacial age--the most
divergent views; and in the writings of men not geologists, it is not
unusual to find exploded theories gravely stated as established facts
of science. The subject is one which I cannot hope to make interesting,
but if the reader will wade through a short chapter, he will be able
to find some of the data on which statements on this subject in other
papers of this series are based.
Mr. Searles V. Wood, in an able summary of the possible causes of
the succession of cold and warm climates in the northern hemisphere,
enumerates no fewer than seven theories which have met with more or
less acceptance, and he might have added an eighth. These are:--
(1) The gradual cooling of the earth from a condition of original
incandescence.
(2) Changes in the obliquity of the ecliptic.
(3) Changes in the position of the earth's axis of rotation.
(4) The effect of the precession of the equinoxes, along with changes
of the eccentricity of the earth's orbit.
(5) Variations in the amount of heat given off by the sun.
(6) Differences in the temperature of portions of space passed through
by the earth.
(7) Differences in the distribution of land and water in connection
with the flow of oceanic currents.
(8) Variations in the properties of the atmosphere with reference to
its capacity for allowing the radiation of heat.
Something may be said in favour of all these alleged causes; but as
efficient in any important degree in producing the cold and warm
climates of the Tertiary period, the greater number of them may be
dismissed as incapable of effecting such results, or as altogether
uncertain with reference to the fact of their own occurrence.
(1) That the earth and the sun have diminished in heat during
geological time seems probable; but physical and geological facts
alike render it certain that this influence could have produced no
appreciable effect, even in the times of the earliest animals and
plants, and certainly not in the case of Tertiary floras or faunas.
(2) The obliquity of the ecliptic is not believed by astronomers to
have changed to any great degree, and its effect would be merely a
somewhat different distribution of heat in different periods of the
year.
(3) Independently of astronomical objections, there is good geological
evidence that the poles of the earth must have been nearly in their
present places from the dawn of life until now. From the Laurentian
upward, those organic limestones which mark the areas where warm and
shallow equatorial water was spreading over submerged continents, are
so disposed as to prove the permanence of the poles. In like manner
all the great foldings of the crust of the earth have followed lines
which are parts of great circles tangent to the existing polar circles.
So, also, from the Cambrian age the great drift of sediment from the
north has followed the line of the existing Arctic currents from the
north-east to the south-west, throwing itself, for example, along the
line of the Appalachian uplifts in Eastern America, and against the
ridge of the Cordilleras in the west.
(4) The effects of change of eccentricity and precession have been
so ably urged by Croll, and recently by Ball, and have so strongly
influenced the minds of those who are not working geologists, that they
deserve a more detailed notice.
(5) The heat of the sun is known to be variable, and the eleven years'
period of sun spots has recently attracted much attention as producing
appreciable effects on the seasons. There may possibly be longer cycles
of solar energy; or the sun may be liable, like some variable stars, to
paroxysms of increased energy. Such changes are possible, but we have
no evidence of their occurrence, and they could not account for periods
of refrigeration of limited duration like the Glacial age.
(6) It has been supposed that the earth may have at different times
traversed more or less heated zones of space, giving alternations of
warm and cold temperature. No such differences in space are, however,
known, nor does there seem any good ground for imagining their
existence.
(7) The differences in the form and elevation of our continents, and
in the consequent distribution of surfaces of different absorbent and
radiating power, and of the oceanic currents, are known causes of
climatal change, and have been referred to in these papers as competent
to account for many, at least, of the phenomena.
(8) Reference has already been made, in connection with the
distribution of plants, to the possibility that the primeval atmosphere
was richer in carbon than that of more modern times, and that this
might operate to produce diminution of radiation, and consequent
uniformity of temperature; but this cause could not have been efficient
in the later geological periods.
There may thus be said to remain two theories of those enumerated
by Wood, to which more detailed consideration may be given, namely,
numbers four and seven, which may be named respectively those of Croll
and Lyell, or the astronomical and geographical theories.
The late Mr. Croll has, in his valuable work "Climate and Time," and
in various memoirs, brought forward an ingenious astronomical theory
to account for changes of climate. This theory, as stated by himself,
is that when the eccentricity of the earth's orbit is at a high value,
and the northern winter solstice is in perihelion, agencies are
brought into operation which make the south-east trade winds stronger
than the north-east, and compel them to blow over upon the northern
hemisphere as far as the Tropic of Cancer. The result is that all the
great equatorial currents of the ocean are impelled into the northern
hemisphere, which thus, in consequence of the immense accumulation of
warm water, has its temperature raised, so that ice and snow must, to a
great extent, disappear from the Arctic regions. In the prevalence of
the converse conditions the Arctic zone becomes clad in ice, and the
southern has its temperature raised.
At the same time, according to Croll's calculations, the accumulation
of ice on either pole would tend, by shifting the earth's centre of
gravity, to raise the level of the ocean and submerge the land on the
colder hemisphere. Thus a submergence of land would coincide with a
cold condition, and emergence with increasing warmth. Facts already
referred to, however, show that this has not always been the case, but
that in many cases submergence was accompanied with the influx of warm
equatorial waters and a raised temperature, this apparently depending
on the question of local distribution of land and water; and this, in
its turn, being regulated not always by mere shifting of the centre
of gravity, but by foldings occasioned by contraction, by equatorial
subsidences resulting from the retardation of the earth's rotation, and
by the excess of material abstracted by ice and frost from the Arctic
regions, and drifted southward along the lines of arctic currents. This
drifting must in all geological times have greatly exceeded, as it
certainly does at present, the denudation caused by atmospheric action
at the equator, and must have tended to increase the disposition to
equatorial collapse occasioned by retardation of rotation.[178]
[178] Croll, in "Climate and Time," and in a note read before the
British Association in 1876, takes an opposite view; but this is
clearly contrary to the facts of sedimentation, which show a steady
movement of _débris_ toward the south and south-west.
While such considerations as those above referred to tend to reduce
the practical importance of Mr. Croll's theory, on the other hand they
tend to remove one of the greatest objections against it--namely,
that founded on the necessity of supposing that glacial periods recur
with astronomical regularity in geological time. They cannot do so if
dependent on other causes inherent in the earth itself, and producing
important movements of its crust.
Sir Robert Ball has in a recent work very ingeniously improved this
theory by showing that Croll was mistaken in assigning equal amounts
of heat to the earth, as a whole, in the periods of greater and less
eccentricity. This would tend to augment the effect of astronomical
revolutions as causes of difference of temperature; but has no bearing
on the more serious geological objections to the theory in question.
A fatal objection, however, to Croll's theory, the force of which has
been greatly increased by recent discoveries, is that the astronomical
causes which he adduces would place the close of the last Glacial
period at least 80,000 years ago, whereas it is now certainly known
from geological facts that the close of the last Glacial period cannot
be older than about an eighth or a tenth of that time. This difficulty
seems to have caused the greater number of geologists, specially
acquainted with the later geological periods, to regard this theory as
quite inapplicable to the facts.
We are thus obliged to fall back upon the old Lyellian theory of
geographical changes, with such modifications as recent discoveries
have rendered necessary. Taking this as our guide, we reach at once the
important conclusion that the movements and distribution of animals
and plants, however dependent on climate, altitude and depth, have,
when regarded in connection with geological time, been primarily
determined by those great movements of the crust of the earth which
have established our islands, continents and ocean depths. These
geographical changes have also in connection with animal and vegetable
growth, deposition of sediments and volcanic ejections, fixed even the
stations, soils and exposures of plants and animals. Thus, subject to
those great astronomical laws which regulate the temperature of our
planet as a whole, our attention may be restricted to the factors of
physical geography itself. We must, however, carry with us the idea
that though the great continents and the ocean depths may have been
fixed throughout geological time, their relative elevations, and
consequently their limits, have varied to a great extent, and are
constantly changing.
We must also remember that something more than mere cold is necessary
to produce a glacial period. It has sometimes been assumed that the
tendency of an exceptionally cold winter would necessarily be to
accumulate so great a quantity of snow and ice, that these could
not be removed in the short though warm summer, and so would go on
accumulating from year to year. Actual experience and observation do
not confirm this supposition. In those parts of North America which
have a long and severe winter, the amount of snow deposited is not in
proportion to the lowness of the temperature, but, on the contrary, the
greatest precipitation of snow takes place near the southern margin
of a cold area, and the snow disappears with great rapidity when
the spring warmth sets in. Nor is there, as has been imagined, any
tendency to the production of fogs and mists which have been invoked
as agencies to shield the snow from the sun. In North America the
melting snow is ordinarily carried off as liquid water, or as invisible
vapour, and the sky is usually clear when the snow is melting in
spring. It is only when warm and moist winds are exceptionally thrown
upon the snow-covered land that clouds are produced; and when this is
the case, the warm rain that ensues promotes the melting of the snow.
Thus there is no possibility of continued accumulations of snow on
the lower parts of our continents, under any imaginable conditions of
climate. It is only on elevated lands in high latitudes and near the
ocean, like Greenland and the Antarctic continent, that such permanent
snow-clad conditions can occur, except on mountain tops. Wallace and
Wœickoff[179] very properly maintain, in connection with these facts,
that permanent ice and snow cannot under any ordinary circumstances
exist in low lands, and that high land and great precipitation are
necessary conditions of glaciers. The former, however, attaches rather
too much importance to snow and ice as cooling agents; for though it is
true that they absorb a large amount of heat in passing from the solid
to the liquid state, yet the quantity of snow or ice to be melted in
spring is so small in comparison with the vast and continuous pouring
of solar heat on the surface, that a very short time suffices for the
liquefaction of a deep covering of snow. The testimony of Siberian
travellers proves this, and the same fact is a matter of ordinary
observation in North America.
[179] Von Wœickoff has very strongly put these principles in a Review
of Croll's recent book, "Climate and Cosmology"; _American Journal of
Science_, March, 1886.
Setting aside, then, these assumptions, which proceed from incorrect or
insufficient information, we may now refer to a consideration of the
utmost importance, and which Mr. Croll himself, though he adduces it
only in aid of the astronomical theory of glacial periods, has treated
in so masterly a manner, as really to give it the first place as an
efficient cause. This is the varying distribution of ocean currents,
in connection with the differences in the elevation and distribution
of land. The great equatorial current, produced by the action of the
solar heat on the atmosphere and the water, along with the earth's
rotation, is thrown, by opposing continental shores, northward into
the Atlantic and Pacific in the Gulf Stream and Japan current, giving
us a hot-water apparatus which effectually raises the temperature of
the whole northern hemisphere, and especially of the western sides of
the continents. Mr. Croll imagined that if his astronomical causes
could, to ever so small an extent, intensify the action of these
currents, or their determination to the north, we should have a period
of warmth, while a similar advantage given to the southern hemisphere
would produce a glacial age in the north. But this requires us to
assume what ought to be proved; namely, that the position of aphelion,
and the increase or decrease of eccentricity, would actually so swing
the equatorial current to the north or south. It further requires us
to assume--and this is the most important defect of the theory--that
no change occurs in the distribution of land and water; because
any important change of this kind might obviously exert a dominant
influence on the currents. Let us take two examples in illustration of
this.
At the present time the warm water thrown into the North Atlantic,
co-operating with the prevalent westerly winds, not only increases
the temperature of its whole waters, but gives an exceptionally mild
climate to western Europe. Still the countervailing influence of the
Arctic currents and the Greenland ice, is sufficient to permit numerous
icebergs to remain unmelted on the coast of Labrador and Newfoundland
throughout the summer. Some of the bergs which creep down to the mouth
of the Strait of Belle-Isle, in the latitude of the south of England,
actually remain unmelted till the snows of a succeeding winter fall
upon them. Now let us suppose that a subsidence of land in tropical
America were to allow the equatorial current to pass through into the
Pacific. The effect would at once be to reduce the temperature of
Norway and Britain to that of Greenland and Labrador at present, while
the latter countries would themselves become colder. The northern
ice, drifting down into the Atlantic, would not, as now, be melted
rapidly by the warm water which it meets in the Gulf Stream. Much
larger quantities of it would remain undissolved in summer, and thus
an accumulation of permanent ice would take place, along the American
coast at first, but probably at length even on the European side.
This would still further chill the atmosphere, glaciers would be
established on all the mountains of temperate Europe and America, the
summer would be kept cold by melting ice and snow, and at length all
eastern America and Europe might become uninhabitable, except by Arctic
animals and plants, as far south as perhaps 40° of north latitude. This
would be simply a return of the glacial age. I have assumed only one
geographical change; but other and more complex changes of subsidence
and elevation might take place, with effects on climate still more
decisive.[180]
[180] According to Bonney, the west coast of Wales is about 12° above
the average for its latitude, and if reduced to 12° below the average,
its mountains would have large glaciers. So near is England even now to
a glacial age.
We may suppose an opposite case. The high plateau of Greenland might
subside, or be reduced in height, and the opening of Baffin's Bay might
be closed. At the same time the interior plain of America might be
depressed, so that, as we know to have been the case in the Cretaceous
period, the warm waters of the Mexican gulf might circulate as far
north as the basins of the present great American lakes. In these
circumstances there would be an immense diminution of the sources
of floating ice, and a correspondingly vast increase in the surface
of warm water. The effects would be to enable a temperate flora to
subsist in Greenland, and to bring all the present temperate regions of
Europe and America into a condition of subtropical verdure.
It is only necessary to add that we actually know that changes
not dissimilar from those above sketched have really occurred in
comparatively recent geological times, to enable us to perceive that
we can dispense with all other causes of change of climate, though
admitting that some of them may have occupied a secondary place. This
will give us, in dealing with the distribution of life, the great
advantage of not being tied up to definite astronomical cycles of
glaciation, which do not well agree with the geological facts, and
of correlating elevation and subsidence of the land with changes of
climate affecting living beings. It will, however, be necessary, as
Wallace well insists, that we shall hold to a certain fixity of the
continents in their position, notwithstanding the submergences and
emergences which they have experienced.
Sir Charles Lyell, more than forty years ago, published in his
"Principles of Geology" two imaginary maps, which illustrate the
extreme effects of various distribution of land and water. In one, all
the continental masses are grouped around the equator. In the other
they are all placed around the poles, leaving an open equatorial ocean.
In the one case the whole of the land and its inhabitants would enjoy a
perpetual summer, and scarcely any ice could exist in the sea. In the
other, the whole of the land would be subjected to an Arctic climate,
and it would give off immense quantities of ice to cool the ocean. Sir
Charles remarks on the present apparently capricious distribution of
land and water, the greater part being in the northern hemisphere, and,
in this, placed in a very unequal manner. But Lyell did not suppose
that any such distribution as that represented in his maps had actually
occurred, though this supposition has been sometimes attributed to him.
He merely put what he regarded as an extreme case to illustrate what
might occur under conditions less exaggerated. Sir Charles, like all
other thoughtful geologists, was well aware of the general fixity of
the areas of the continents, though with great modifications in the
matter of submergences and of land conditions. The union, indeed, of
these two great principles of fixity and diversity of the continents
lies at the foundation of theoretical geology.
We can now more precisely indicate this than was possible when Lyell
produced his "Principles," and can reproduce the conditions of our
continents in even the more ancient periods of their history. An
example of this may be given from the American continent, which is
more simple in its arrangements than the double continent of Eurasia.
Take, for instance, the early Devonian or Erian period, in which the
magnificent flora of that age, the earliest certainly known to us,
made its appearance. Imagine the whole interior plain of North America
submerged, so that the continent is reduced to two strips on the
east and west, connected by a belt of Laurentian land on the north.
In the great mediterranean sea thus produced, the tepid water of the
equatorial current was circulated, and it swarmed with corals, of
which we know no less than 150 species, and with other forms of life
appropriate to warm seas. On the islands and coasts of this sea was
introduced the Erian flora, appearing first in the north, and with that
vitality and colonizing power of which, as Hooker has well shown, the
Scandinavian flora is the best modern type, spreading itself to the
south. A very similar distribution of land and water in the Cretaceous
age gave a warm and equable climate in those portions of North America
not submerged, and coincided with the appearance of the multitude
of broad-leaved trees of modern types which appeared in the middle
Cretaceous, and prepared the way for the mammalian life of the Eocene.
We have in America ancient periods of cold as well as of warmth. I
have elsewhere referred to the boulder conglomerates of the Huronian,
of the early Lower Silurian, and of the Millstone grit period of the
Carboniferous; but I have not ventured to affirm that either of these
periods was comparable in its cold with the later glacial age, still
less with that imaginary age of continental glaciation, assumed by
the more extreme theorists. We know that these ancient conglomerates
were produced by floating ice, and this at periods when in areas not
very remote, temperate floras and faunas could flourish. The glacial
periods of our old continent occurred in times when the surface of the
submerged land was opened up to the northern currents drifting over it
mud and sand and stones, and rendering nugatory, in so far, at least,
as the bottom of the sea was concerned, the effects of the superficial
warm streams. Some of these beds are also peculiar to the eastern
margin of the continent, and indicate ice drift along the Atlantic
coast much as at present, while conditions of greater warmth existed in
the interior. Even in the more recent glacial age, while the mountains
were covered with snow, and the low lands submerged under a sea laden
with ice, there were interior tracts in somewhat high latitudes of
America in which hardy forest trees and herbaceous plants flourished
abundantly, and these were by no means exceptional "interglacial"
periods. Thus we can prove that from the remote Huronian period to the
Tertiary, the American land occupied the same position as at present,
and that its changes were merely changes of relative level, as compared
with the sea; but which so influenced the ocean currents as to cause
great vicissitudes of climate.
Uniformitarian geologists have recently been taunted with a willingness
to assume great and frequent elevations and submergences of
continents, as if this were contrary to their principle. But rational
uniformitarianism allows us to use any cause of whose operation in the
past there is good geological evidence, and Lyell himself was perfectly
aware of this.
While no geologists can fail to appreciate the evidence of the power
of geographical change in affecting climatal change, and the fact that
such change has occurred at various geological periods, there are some,
and especially those who take extreme views as to the latest period
of cold climate, who doubt its sufficiency to account for all the
phenomena observed. It is instructive, however, to notice that some of
the ablest of these, in default of other probable causes, are driven to
fall back either on agencies of a wholly improbable character, or to
give up the problem as insoluble. Two recent examples of this deserve
citation.
The late Dr. Newmayr, of Vienna, a veteran physical geographer, in
an able discussion of the climates of past ages, one of his last
scientific papers, has fallen back on the hypothesis of a change in
the position of the poles.[181] His failure to account for ancient
climates by other causes evidently, however, depends on an inadequate
conception of the effects of geographical changes, along with serious
misconceptions as to the distribution of plants and the characters of
vegetation at different periods. These points we shall have to discuss
in subsequent pages.
[181] _Society for Dissemination of Natural Science._ Vienna, January,
1889.
In an address before the American Association, in 1886, Dr.
Chamberlain, one of the ablest American authorities on the Glacial
period, makes the following remarks as to the causes of the Pleistocene
cold:--
"If we turn to the broader speculations respecting the origin of the
Glacial epoch, we find our wealth little increased. We have on hand
practically the same old stock of hypotheses, all badly damaged by
the deluge of recent facts. The earlier theory of northern elevation
has been rendered practically valueless; and the various astronomical
hypotheses seem to be the worse for the increased knowledge of the
distribution of the ancient ice sheet. Even the ingenious theory
of Croll becomes increasingly unsatisfactory as the phenomena are
developed into fuller appreciation. The more we consider the asymmetry
of the ice distribution in latitude and longitude, and its disparity
in elevation, the more difficult it becomes to explain the phenomena
upon any astronomical basis. If we were at liberty to disregard the
considerations forced upon us by physicists and astronomers, and
permit ourselves simply to follow freely the apparent leadings of the
phenomena, it appears at this hour as though we should be led upon an
old and forbidden trail,--the hypothesis of a wandering pole. It is
admitted that there is a _vera causa_ in elevations and depressions of
the earth's crust, but it is held inadequate. It is admitted that the
apparent changes of latitude shown by the determinations of European
and American observatories are remarkable, but their trustworthiness
is challenged. Were there no barriers against free hypotheses in
this direction, glacial phenomena could apparently find adequate
explanation; but debarred--as we doubtless should consider ourselves to
be at present--from this resource, our hypotheses remain inharmonious
with the facts, and the riddle remains unsolved."
It should be observed here that the unsolved "riddle" is that of a
continental ice sheet. This, as we have already seen, is probably
insoluble in any way, but fortunately needs no solution, being merely
imaginary. If we adopt a moderate view as to the actual conditions of
the Pleistocene, the geographical theory will be found quite sufficient
to account for the facts.
Let it be observed here also, in connection with the above thoughtful
and frank avowal of one of the ablest of American glacialists, that
the geographical theory provides for that "asymmetry "'or irregular
distribution of glacial deposits to which he refers; since, at every
stage of continental elevation and depression, there must have been
local changes of circumstances; and the same inequality of temperature
in identical latitudes which we observe at present must have existed,
probably in a greater degree, in the Glacial age.
The sufficiency of the Lyellian theory to account for the facts, in so
far as plants are concerned, may, indeed, be inferred from the course
of the isothermal lines at present. The south end of Greenland is on
the latitude of Christiania, in Norway, on the one hand, and of Fort
Liard, in the Peace River region, on the other; and while Greenland
is clad in ice and snow, wheat and other grains, and the ordinary
trees of temperate climates, grow at the latter places. It is evident,
therefore, that only exceptionally unfavourable circumstances prevent
the Greenland area from still possessing a temperate flora, and these
unfavourable circumstances possibly tell even on the localities with
which we have compared it. Further, the mouth of the McKenzie River
is in the same latitude with Disco, near which are some of the most
celebrated localities of fossil Cretaceous and Tertiary plants. Yet
the mouth of the McKenzie River enjoys a much more favourable climate,
and has a much more abundant flora than Disco. If North Greenland were
submerged, and low land reaching to the south terminated at Disco,
and if from any cause either the cold currents of Baffin's Bay were
arrested, or additional warm water thrown into the North Atlantic by
the Gulf Stream, there is nothing to prevent a mean temperature of 45°
Fahrenheit from prevailing at Disco; and the estimate ordinarily formed
of the requirements of its extinct floras is 50°, which is probably
above, rather than below, the actual temperature required.
We thus know that the present distribution of land and water greatly
influences climate, more especially by affecting that of the ocean
currents and of the winds, and by the different action of land as
compared with water in the reception and radiation of heat. The
present distribution of land gives a large predominance to the Arctic
and sub-Arctic regions, as compared with the equatorial and with
the Antarctic; and we might readily imagine other distributions that
would give very different results. But this is not an imaginary
case, for we can to some extent restore, on geological grounds, the
ancient geography of large regions, and can show that it has been
very different from that prevailing at present. We know also that,
while the forms and positions of the great continents have been fixed
from a very early date, they have experienced many great submergences
and re-elevations, and that these have occurred in somewhat regular
sequence, as evidenced by the cyclical alternations of organic
limestones and earthy sediments in the successive great geological
periods, each of which, as may be seen in any geological text-book,
presents a dip of the continental plateaus, with subsequent elevation,
as if the land was subject to a series of regular pulsations.[182]
[182] See "Acadian Geology"--Introduction to the Carboniferous System.
Finally, the Lyellian theory tends to abate the tendency to imagine
portentous and impossible climatal changes; and it inclines geologists
to give more attention to the connection of palæogeography with changes
in the life history of the earth.
References:--"Acadian Geology," 1st ed., 1855; 4th ed., 1892.
Icebergs of Belle-Isle, and Glaciers of Mont Blanc, _Canadian
Naturalist_, 1865. "Notes on Pleistocene of Canada," Montreal,
1871. Papers at various dates in the _Canadian Naturalist_ and
_Canadian Record of Science_. "The Ice Age in Canada," Montreal,
1892. Canadian Pleistocene, _London Geological Magazine_, March,
1883. Flora of the Pleistocene, Dawson and Penhallow. _Bulletin of
Geological Society of America_, vol. i., 1890, p. 311.
_THE DISTRIBUTION OF ANIMALS AND PLANTS AS RELATED TO
GEOGRAPHICAL AND GEOLOGICAL CHANGES._
DEDICATED TO THE MEMORY OF MY LATE FRIEND,
MR. GWYN JEFFRIES,
who so ably investigated the Distribution
of Oceanic Molluska,
more especially in the North Atlantic.
Changes of Climate and of Land and Water with Reference to
Distribution of Life--Regions of the Continents--Insular Faunas and
Floras--Their History Applications to Geology and to Man--Geological
Time--Theories of Introduction And Migration
[Illustration: Distribution of Animals in Time. (p. 420.)
_Vertebrata._ 1, Ganoid Fishes; 2, Teliort Fishes; 3, Batrachians; 4,
Reptiles; 5, Birds; 6, Mammals.
_Invertebrata._ 1, Trilobites, etc.; 2, Worms; 3, Bivalve and Univalve
Shellfishes; 4, Nautiloid Shellfishes; 5, Cuttlefishes; 6, Brachiopods.
It will be noticed that Nos. 2 and 5 in the first table, and 3 and
5 in the second, follow a different order of curve from the others,
indicating their exceptional culmination in modern times.]