Causes Of Mild Geological Climates

In discussions of climate, as of most subjects, a peculiar psychological

phenomenon is observable. Everyone sees the necessity of explaining

conditions different from those that now exist, but few realize that

present conditions may be abnormal, and that they need explanation just

as much as do others. Because of this tendency glaciation has been

discussed with the greatest fullness, while there has been much neglect

n t only of the periods when the climate of the earth resembled that of

the present, but also of the vastly longer periods when it was even

milder than now.



How important the periods of mild climate have been in geological times

may be judged from the relative length of glacial compared with

inter-glacial epochs, and still more from the far greater relative

length of the mild parts of periods and eras when compared with the

severe parts. Recent estimates by R. T. Chamberlin[58] indicate that

according to the consensus of opinion among geologists the average

inter-glacial epoch during the Pleistocene was about five times as long

as the average glacial epoch, while the whole of a given glacial epoch

averaged five times as long as the period when the ice was at a maximum.

Climatic periods far milder, longer, and more monotonous than any

inter-glacial epoch appear repeatedly during the course of geological

history. Our task in this chapter is to explain them.



Knowlton[59] has done geology a great service by collecting the evidence

as to the mild type of climate which has again and again prevailed in

the past. He lays special stress on botanical evidence since that

pertains to the variable atmosphere of the lands, and hence furnishes a

better guide than does the evidence of animals that lived in the

relatively unchanging water of the oceans. The nature of the evidence

has already been indicated in various parts of this book. It includes

palms, tree ferns, and a host of other plants which once grew in regions

which are now much too cold to support them. With this must be placed

the abundant reef-building corals and other warmth-loving marine

creatures in latitudes now much too cold for them. Of a piece with this

are the conditions of inter-glacial epochs in Europe, for example, when

elephants and hippopotamuses, as well as many species of plants from low

latitudes, were abundant. These conditions indicate not only that the

climate was warmer than now, but that the contrast from season to season

was much less. Indeed, Knowlton goes so far as to say that "relative

uniformity, mildness, and comparative equability of climate, accompanied

by high humidity, have prevailed over the greater part of the earth,

extending to, or into, polar circles, during the greater part of

geologic time--since, at least, the Middle Paleozoic. This is the

regular, the ordinary, the normal condition." ... "By many it is thought

that one of the strongest arguments against a gradually cooling globe

and a humid, non-zonally disposed climate in the ages before the

Pleistocene is the discovery of evidences of glacial action practically

throughout the entire geologic column. Hardly less than a dozen of these

are now known, ranging in age from Huronian to Eocene. It seems to be a

very general assumption by those who hold this view that these evidences

of glacial activities are to be classed as ice ages, largely comparable

in effect and extent to the Pleistocene refrigeration, but as a matter

of fact only three are apparently of a magnitude to warrant such

designation. These are the Huronian glaciation, that of the

'Permo-Carboniferous,' and that of the Pleistocene. The others, so far

as available data go, appear to be explainable as more or less local

manifestations that had no widespread effect on, for instance, ocean

temperatures, distribution of life, et cetera. They might well have been

of the type of ordinary mountain glaciers, due entirely to local

elevation and precipitation." ... "If the sun had been the principal

source of heat in pre-Pleistocene time, terrestrial temperatures would

of necessity have been disposed in zones, whereas the whole trend of

this paper has been the presentation of proof that these temperatures

were distinctly non-zonal. Therefore it seems to follow that the sun--at

least the present small-angle sun--could not have been the sole or even

the principal source of heat that warmed the early oceans."



Knowlton is so strongly impressed by the widespread fossil floras that

usually occur in the middle parts of the geological periods, that as

Schuchert[3] puts it, he neglects the evidence of other kinds. In the

middle of the periods and eras the expansion of the warm oceans over the

continents was greatest, while the lands were small and hence had more

or less insular climates of the oceanic type. At such times, the marine

fauna agrees with the flora in indicating a mild climate. Large

colony-forming foraminifera, stony corals, shelled cephalopods,

gastropods and thick-shelled bivalves, generally the cemented forms,

were common in the Far North and even in the Arctic. This occurred in

the Silurian, Devonian, Pennsylvanian, and Jurassic periods, yet at

other times, such as the Cretaceous and Eocene, such forms were very

greatly reduced in variety in the northern regions or else wholly

absent. These things, as Schuchert[60] says, can only mean that Knowlton

is right when he states that "climatic zoning such as we have had since

the beginning of the Pleistocene did not obtain in the geologic ages

prior to the Pleistocene." It does not mean, however, that there was a

"non-zonal arrangement" and that the temperature of the oceans was

everywhere the same and "without widespread effect on the distribution

of life."



Students of paleontology hold that as far back as we can go in the study

of plants, there are evidences of seasons and of relatively cool

climates in high latitudes. The cycads, for instance, are one of the

types most often used as evidence of a warm climate. Yet Wieland,[61]

who has made a lifelong study of these plants, says that many of them

"might well grow in temperate to cool climates. Until far more is

learned about them they should at least be held as valueless as indices

of tropic climates." The inference is "that either they or their close

relatives had the capacity to live in every clime. There is also a

suspicion that study of the associated ferns may compel revision of the

long-accepted view of the universality of tropic climates throughout the

Mesozoic." Nathorst is quoted by Wieland as saying, "I think ... that

during the time when the Gingkophytes and Cycadophytes dominated, many

of them must have adapted themselves for living in cold climates also.

Of this I have not the least doubt."



Another important line of evidence which Knowlton and others have cited

as a proof of the non-zonal arrangement of climate in the past, is the

vast red beds which are found in the Proterozoic, late Silurian,

Devonian, Permian, and Triassic, and in some Tertiary formations. These

are believed to resemble laterite, a red and highly oxidized soil which

is found in great abundance in equatorial regions. Knowlton does not

attempt to show that the red beds present equatorial characteristics in

other respects, but bases his conclusion on the statement that "red beds

are not being formed at the present time in any desert region." This is

certainly an error. As has already been said, in both the Transcaspian

and Takla Makan deserts, the color of the sand regularly changes from

brown on the borders to pale red far out in the desert. Kuzzil Kum, or

Red Sand, is the native name. The sands in the center of the desert

apparently were originally washed down from the same mountains as those

on the borders, and time has turned them red. Since the same condition

is reported from the Arabian Desert, it seems that redness is

characteristic of some of the world's greatest deserts. Moreover, beds

of salt and gypsum are regularly found in red beds, and they can

scarcely originate except in deserts, or in shallow almost landlocked

bays on the coasts of deserts, as appears to have happened in the

Silurian where marine fossils are found interbedded with gypsum.



Again, Knowlton says that red beds cannot indicate deserts because the

plants found in them are not "pinched or depauperate, nor do they

indicate xerophytic adaptations. Moreover, very considerable deposits of

coal are found in red beds in many parts of the world, which implies the

presence of swamps but little above sea-level."



Students of desert botany are likely to doubt the force of these

considerations. As MacDougal[62] has shown, the variety of plants in

deserts is greater than in moist regions. Not only do xerophytic desert

species prevail, but halophytes are present in the salty areas, and

hygrophytes in the wet swampy areas, while ordinary mesophytes prevail

along the water courses and are washed down from the mountains. The

ordinary plants, not the xerophytes, are the ones that are chiefly

preserved since they occur in most abundance near streams where

deposition is taking place. So far as swamps are concerned, few are of

larger size than those of Seistan in Persia, Lop Nor in Chinese

Turkestan, and certain others in the midst of the Asiatic deserts.

Streams flowing from the mountains into deserts are almost sure to form

large swamps, such as those along the Tarim River in central Asia. Lake

Chad in Africa is another example. In it, too, reeds are very numerous.



Putting together the evidence on both sides in this disputed question,

it appears that throughout most of geological time there is some

evidence of a zonal arrangement of climate. The evidence takes the form

of traces of cool climates, of seasons, and of deserts. Nevertheless,

there is also strong evidence that these conditions were in general less

intense than at present and that times of relatively warm, moist climate

without great seasonal extremes have prevailed very widely during

periods much longer than those when a zonal arrangement as marked as

that of today prevailed. As Schuchert[63] puts it: "Today the variation

on land between the tropics and the poles is roughly between 110 deg. and

-60 deg.F., in the oceans between 85 deg. and 31 deg.F. In the geologic past the

temperature of the oceans for the greater parts of the periods probably

was most often between 85 deg. and 55 deg.F., while on land it may have varied

between 90 deg. and 0 deg.F. At rare intervals the extremes were undoubtedly as

great as they are today. The conclusion is therefore that at all times

the earth had temperature zones, varying between the present-day

intensity and times which were almost without such belts, and at these

latter times the greater part of the earth had an almost uniformly mild

climate, without winters."



It is these mild climates which we must now attempt to explain. This

leads us to inquire what would happen to the climate of the earth as a

whole if the conditions which now prevail at times of few sunspots were

to become intensified. That they could become greatly intensified seems

highly probable, for there is good reason to think that aside from the

sunspot cycle the sun's atmosphere is in a disturbed condition. The

prominences which sometimes shoot out hundreds of thousands of miles

seem to be good evidence of this. Suppose that the sun's atmosphere

should become very quiet. This would apparently mean that cyclonic

storms would be much less numerous and less severe than during the

present times of sunspot minima. The storms would also apparently follow

paths in middle latitudes somewhat as they do now when sunspots are

fewest. The first effect of such a condition, if we can judge from what

happens at present, would be a rise in the general temperature of the

earth, because less heat would be carried aloft by storms. Today, as is

shown in Earth and Sun, a difference of perhaps 10 per cent in the

average storminess during periods of sunspot maxima and minima is

correlated with a difference of 3 deg.C. in the temperature at the earth's

surface. This includes not only an actual lowering of 0.6 deg.C. at times of

sunspot maxima, but the overcoming of the effect of increased insolation

at such times, an effect which Abbot calculates as about 2.5 deg.C. If the

storminess were to be reduced to one-half or one-quarter its present

amount at sunspot minima, not only would the loss of heat by upward

convection in storms be diminished, but the area covered by clouds would

diminish so that the sun would have more chance to warm the lower air.

Hence the average rise of temperature might amount to as much at 5 deg. or

10 deg.C.



Another effect of the decrease in storminess would be to make the

so-called westerly winds, which are chiefly southwesterly in the

northern hemisphere and northwesterly in the southern hemisphere, more

strong and steady than at present. They would not continually suffer

interruption by cyclonic winds from other directions, as is now the

case, and would have a regularity like that of the trades. This

conclusion is strongly reenforced in a paper by Clayton[64] which came

to hand after this chapter had been completed. From his studies of the

solar constant and the temperature of the earth which are described in

Earth and Sun, he reaches the following conclusion: "The results of

these researches have led me to believe: 1. That if there were no

variation in solar radiation the atmospheric motions would establish a

stable system with exchanges of air between equator and pole and between

ocean and land, in which the only variations would be daily and annual

changes set in operation by the relative motions of the earth and sun.

2. The existing abnormal changes, which we call weather, have their

origins chiefly, if not entirely, in the variations of solar radiation."



If cyclonic storms and "weather" were largely eliminated and if the

planetary system of winds with its steady trades and southwesterlies

became everywhere dominant, the regularity and volume of the

poleward-flowing currents, such as the Gulf Stream and the Atlantic

Drift in one ocean, and the Japanese Current in another, would be

greatly increased. How important this is may be judged from the work of

Helland-Hansen and Nansen.[65] These authors find that with the passage

of each cyclonic storm there is a change in the temperature of the

surface water of the Atlantic Ocean. Winds at right angles to the course

of the Drift drive the water first in one direction and then in the

other but do not advance it in its course. Winds with an easterly

component, on the other hand, not only check the Drift but reverse it,

driving the warm water back toward the southwest and allowing cold water

to well up in its stead. The driving force in the Atlantic Drift is

merely the excess of the winds with a westerly component over those with

an easterly component.



Suppose that the numbers in Fig. 8 represent the strength of the winds

in a certain part of the North Atlantic or North Pacific, that is, the

total number of miles moved by the air per year. In quadrant A of the

left-hand part all the winds move from a more or less southwesterly

direction and produce a total movement of the air amounting to thirty

units per year. Those coming from points between north and west move

twenty-five units; those between north and east, twenty units; and those

between east and south, twenty-five units. Since the movement of the

winds in quadrants B and D is the same, these winds have no effect in

producing currents. They merely move the water back and forth, and thus

give it time to lose whatever heat it has brought from more southerly

latitudes. On the other hand, since the easterly winds in quadrant C do

not wholly check the currents caused by the westerly winds of quadrant

A, the effective force of the westerly winds amounts to ten, or the

difference between a force of thirty in quadrant A and of twenty in

quadrant C. Hence the water is moved forward toward the northeast, as

shown by the thick part of arrow A.




water.]



Now suppose that cyclonic storms should be greatly reduced in number so

that in the zone of prevailing westerlies they were scarcely more

numerous than tropical hurricanes now are in the trade-wind belt. Then

the more or less southwesterly winds in quadrant A' in the right-hand

part of Fig. 8 would not only become more frequent but would be stronger

than at present. The total movement from that quarter might rise to

sixty units, as indicated in the figure. In quadrants B' and D' the

movement would fall to fifteen and in quadrant C' to ten. B' and D'

would balance one another as before. The movement in A', however, would

exceed that in C' by fifty instead of ten. In other words, the

current-making force would become five times as great as now. The actual

effect would be increased still more, for the winds from the southwest

would be stronger as well as steadier if there were no storms. A strong

wind which causes whitecaps has much more power to drive the water

forward than a weaker wind which does not cause whitecaps. In a wave

without a whitecap the water returns to practically the original point

after completing a circle beneath the surface. In a wave with a

whitecap, however, the cap moves forward. Any increase in velocity

beyond the rate at which whitecaps are formed has a great influence upon

the amount of water which is blown forward. Several times as much water

is drifted forward by a persistent wind of twenty miles an hour as by a

ten-mile wind.[66]



In this connection a suggestion which is elaborated in Chapter XIII may

be mentioned. At present the salinity of the oceans checks the general

deep-sea circulation and thereby increases the contrasts from zone to

zone. In the past, however, the ocean must have been fresher than now.

Hence the circulation was presumably less impeded, and the transfer of

heat from low latitudes to high was facilitated.



Consider now the magnitude of the probable effect of a diminution in

storms. Today off the coast of Norway in latitude 65 deg.N. and longitude

10 deg.E., the mean temperature in January is 2 deg.C. and in July 12 deg.C. This

represents a plus anomaly of about 22 deg. in January and 2 deg. in July; that

is, the Norwegian coast is warmer than the normal for its latitude by

these amounts. Suppose that in some past time the present distribution

of lands and seas prevailed, but Norway was a lowland where extensive

deposits could accumulate in great flood plains. Suppose, also, that the

sun's atmosphere was so inactive that few cyclonic storms occurred,

steady winds from the west-southwest prevailed, and strong,

uninterrupted ocean currents brought from the Caribbean Sea and Gulf of

Mexico much greater supplies of warm water than at present. The

Norwegian winters would then be warmer than now not only because of the

general increase in temperature which the earth regularly experiences at

sunspot minima, but because the currents would accentuate this

condition. In summer similar conditions would prevail except that the

warming effect of the winds and currents would presumably be less than

in winter, but this might be more than balanced by the increased heat of

the sun during the long summer days, for storms and clouds would be

rare.



If such conditions raised the winter temperature only 8 deg.C. and the

summer temperature 4 deg.C., the climate would be as warm as that of the

northern island of New Zealand (latitude 35 deg.-43 deg.S.). The flora of that

part of New Zealand is subtropical and includes not only pines and

beeches, but palms and tree ferns. A climate scarcely warmer than that

of New Zealand would foster a flora like that which existed in far

northern latitudes during some of the milder geological periods. If,

however, the general temperature of the earth's surface were raised 5 deg.

because of the scarcity of storms, if the currents were strong enough so

that they increased the present anomaly by 50 per cent, and if more

persistent sunshine in summer raised the temperature at that season

about 4 deg.C., the January temperature would be 18 deg.C. and the July

temperature 22 deg.C. These figures perhaps make summer and winter more

nearly alike than was ever really the case in such latitudes.

Nevertheless, they show that a diminution of storms and a consequent

strengthening and steadying of the southwesterlies might easily raise

the temperature of the Norwegian coast so high that corals could

flourish within the Arctic Circle.



Another factor would cooeperate in producing mild temperatures in high

latitudes during the winter, namely, the fogs which would presumably

accumulate. It is well known that when saturated air from a warm ocean

is blown over the lands in winter, as happens so often in the British

Islands and around the North Sea, fog is formed. The effect of such a

fog is indeed to shut out the sun's radiation, but in high latitudes

during the winter when the sun is low, this is of little importance.

Another effect is to retain the heat of the earth itself. When a

constant supply of warm water is being brought from low latitudes this

blanketing of the heat by the fog becomes of great importance. In the

past, whenever cyclonic storms were weak and westerly winds were

correspondingly strong, winter fogs in high latitudes must have been

much more widespread and persistent than now.



The bearing of fogs on vegetation is another interesting point. If a

region in high latitudes is constantly protected by fog in winter, it

can support types of vegetation characteristic of fairly low latitudes,

for plants are oftener killed by dry cold than by moist cold. Indeed,

excessive evaporation from the plant induced by dry cold when the

evaporated water cannot be rapidly replaced by the movement of sap is a

chief reason why large plants are winterkilled. The growing of

transplanted palms on the coast of southwestern Ireland, in spite of its

location in latitude 50 deg.N., is possible only because of the great

fogginess in winter due to the marine climate. The fogs prevent the

escape of heat and ward off killing frosts. The tree ferns in latitude

46 deg.S. in New Zealand, already referred to, are often similarly protected

in winter. Therefore, the relative frequency of fogs in high latitudes

when storms were at a minimum would apparently tend not merely to

produce mild winters but to promote tropical vegetation.



The strong steady trades and southwesterlies which would prevail at

times of slight solar activity, according to our hypothesis, would have

a pronounced effect on the water of the deep seas as well as upon that

of the surface. In the first place, the deep-sea circulation would be

hastened. For convenience let us speak of the northern hemisphere. In

the past, whenever the southwesterly winds were steadier than now, as

was probably the case when cyclonic storms were relatively rare, more

surface water than at present was presumably driven from low latitudes

and carried to high latitudes. This, of course, means that a greater

volume of water had to flow back toward the equator in the lower parts

of the ocean, or else as a cool surface current. The steady

southwesterly winds, however, would interfere with south-flowing surface

currents, thus compelling the polar waters to find their way equatorward

beneath the surface. In low latitudes the polar waters would rise and

their tendency would be to lower the temperature. Hence steadier

westerlies would make for lessened latitudinal contrasts in climate not

only by driving more warm water poleward but by causing more polar water

to reach low latitudes.



At this point a second important consideration must be faced. Not only

would the deep-sea circulation be hastened, but the ocean depths might

be warmed. The deep parts of the ocean are today cold because they

receive their water from high latitudes where it sinks because of low

temperature. Suppose, however, that a diminution in storminess combined

with other conditions should permit corals to grow in latitude 70 deg.N. The

ocean temperature would then have to average scarcely lower than 20 deg.C.

and even in the coldest month the water could scarcely fall below about

15 deg.C. Under such conditions, if the polar ocean were freely connected

with the rest of the oceans, no part of it would probably have a

temperature much below 10 deg.C., for there would be no such thing as ice

caps and snowfields to reflect the scanty sunlight and radiate into

space what little heat there was. On the contrary, during the winter an

almost constant state of dense fogginess would prevail. So great would

be the blanketing effect of this that a minimum monthly temperature of

10 deg.C. for the coldest part of the ocean may perhaps be too low for a

time when corals thrived in latitude 70 deg..



The temperature of the ocean depths cannot permanently remain lower than

that of the coldest parts of the surface. Temporarily this might indeed

happen when a solar change first reduced the storminess and strengthened

the westerlies and the surface currents. Gradually, however, the

persistent deep-sea circulation would bring up the colder water in low

latitudes and carry downward the water of medium temperature at the

coldest part of the surface. Thus in time the whole body of the ocean

would become warm. The heat which at present is carried away from the

earth's surface in storms would slowly accumulate in the oceans. As the

process went on, all parts of the ocean's surface would become warmer,

for equatorial latitudes would be less and less cooled by cold water

from below, while the water blown from low latitudes to high would be

correspondingly warmer. The warming of the ocean would come to an end

only with the attainment of a state of equilibrium in which the loss of

heat by radiation and evaporation from the ocean's surface equaled the

loss which under other circumstances would arise from the rise of warm

air in cyclonic storms. When once the oceans were warmed, they would

form an extremely strong conservative force tending to preserve an

equable climate in all latitudes and at all seasons. According to the

solar cyclonic hypothesis such conditions ought to have prevailed

throughout most of geological time. Only after a strong and prolonged

solar disturbance with its consequent storminess would conditions like

those of today be expected.



In this connection another possibility may be mentioned. It is commonly

assumed that the earth's axis is held steadily in one direction by the

fact that the rotating earth is a great gyroscope. Having been tilted to

a certain position, perhaps by some extraneous force, the axis is

supposed to maintain that position until some other force intervenes.

Cordeiro,[67] however, maintains that this is true only of an absolutely

rigid gyroscope. He believes that it is mathematically demonstrable that

if an elastic gyroscope be gradually tilted by some extraneous force,

and if that force then ceases to act, the gyroscope as a whole will

oscillate back and forth. The earth appears to be slightly elastic.

Cordeiro therefore applies his formulae to it, on the following

assumptions: (1) That the original position of the axis was nearly

vertical to the plane of the ecliptic in which the earth revolves around

the sun; (2) that at certain times the inclination has been even greater

than now; and (3) that the position of the axis with reference to the

earth has not changed to any great extent, that is, the earth's poles

have remained essentially stationary with reference to the earth,

although the whole earth has been gyroscopically tilted back and forth

repeatedly.



With a vertical axis the daylight and darkness in all parts of the earth

would be of equal duration, being always twelve hours. There would be no

seasons, and the climate would approach the average condition now

experienced at the two equinoxes. On the whole the climate of high

latitudes would give the impression of being milder than now, for there

would be less opportunity for the accumulation of snow and ice with

their strong cooling effect. On the other hand, if the axis were tilted

more than now, the winter nights would be longer and the winters more

severe than at present, and there would be a tendency toward glaciation.

Thus Cordeiro accounts for alternating mild and glacial epochs. The

entire swing from the vertical position to the maximum inclination and

back to the vertical may last millions of years depending on the earth's

degree of elasticity. The swing beyond the vertical position in the



other direction would be equally prolonged. Since the axis is now

supposed to be much nearer its maximum than its minimum degree of

tilting, the duration of epochs having a climate more severe than that

of the present would be relatively short, while the mild epochs would be

long.



Cordeiro's hypothesis has been almost completely ignored. One reason is

that his treatment of geological facts, and especially his method of

riding rough-shod over widely accepted conclusions, has not commended

his work to geologists. Therefore they have not deemed it worth while to

urge mathematicians to test the assumptions and methods by which he

reached his results. It is perhaps unfair to test Cordeiro by geology,

for he lays no claim to being a geologist. In mathematics he labors

under the disadvantage of having worked outside the usual professional

channels, so that his work does not seem to have been subjected to

sufficiently critical analysis.



Without expressing any opinion as to the value of Cordeiro's results we

feel that the subject of the earth's gyroscopic motion and of a possible

secular change in the direction of the axis deserves investigation for

two chief reasons. In the first place, evidences of seasonal changes and

of seasonal uniformity seem to occur more or less alternately in the

geological record. Second, the remarkable discoveries of Garner and

Allard[68] show that the duration of daylight has a pronounced effect

upon the reproduction of plants. We have referred repeatedly to the tree

ferns, corals, and other forms of life which now live in relatively low

latitudes and which cannot endure strong seasonal contrasts, but which

once lived far to the north. On the other hand, Sayles,[69] for example,

finds that microscopical examination of the banding of ancient shales

and slates indicates distinct seasonal banding like that of recent

Pleistocene clays or of the Squantum slate formed during or near the

Permian glacial period. Such seasonal banding is found in rocks of

various ages: (a) Huronian, in cobalt shales previously reported by

Coleman; (b) late Proterozoic or early Cambrian in Hiwassee slate; (c)

lower Cambrian, in Georgian slates of Vermont; (d) lower Ordovician, in

Georgia (Rockmart slate), Tennessee (Athens shale), Vermont (slates),

and Quebec (Beekmantown formation); and (e) Permian in Massachusetts

(Squantum slate). How far the periods during which such evidence of

seasons was recorded really alternated with mild periods, when tropical

species lived in high latitudes and the contrast of seasons was almost

or wholly lacking, we have as yet no means of knowing. If periods

characterized by marked seasonal changes should be found to have

alternated with those when the seasons were of little importance, the

fact would be of great geological significance.



The discoveries of Garner and Allard as to the effect of light on

reproduction began with a peculiar tobacco plant which appeared in some

experiments at Washington. The plant grew to unusual size, and seemed to

promise a valuable new variety. It formed no seeds, however, before the

approach of cold weather. It was therefore removed to a greenhouse where

it flowered and produced seed. In succeeding years the flowering was

likewise delayed till early winter, but finally it was discovered that

if small plants were started in the greenhouse in the early fall they

flowered at the same time as the large ones. Experiments soon

demonstrated that the time of flowering depends largely upon the length

of the daily period when the plants are exposed to light. The same is

true of many other plants, and there is great variety in the conditions

which lead to flowering. Some plants, such as witch hazel, appear to be

stimulated to bloom by very short days, while others, such as evening

primrose, appear to require relatively long days. So sensitive are

plants in this respect that Garner and Allard, by changing the length of

the period of light, have caused a flowerbud in its early stages not

only to stop developing but to return once more to a vegetative shoot.



Common iris, which flowers in May and June, will not blossom under

ordinary conditions when grown in the greenhouse in winter, even

under the same temperature conditions that prevail in early summer.

Again, one variety of soy beans will regularly begin to flower in

June of each year, a second variety in July, and a third in August,

when all are planted on the same date. There are no temperature

differences during the summer months which could explain these

differences in time of flowering; and, since "internal causes" alone

cannot be accepted as furnishing a satisfactory explanation, some

external factor other than temperature must be responsible.



The ordinary varieties of cosmos regularly flower in the fall in

northern latitudes if they are planted in the spring or summer. If

grown in a warm greenhouse during the winter months the plants also

flower readily, so that the cooler weather of fall is not a

necessary condition. If successive plantings of cosmos are made in

the greenhouse during the late winter and early spring months,

maintaining a uniform temperature throughout, the plantings made

after a certain date will fail to blossom promptly, but, on the

contrary, will continue to grow till the following fall, thus

flowering at the usual season for this species. This curious

reversal of behavior with advance of the season cannot be attributed

to change in temperature. Some other factor is responsible for the

failure of cosmos to blossom during the summer months. In this

respect the behavior of cosmos is just the opposite of that observed

in iris.



Certain varieties of soy beans change their behavior in a peculiar

manner with advance of the summer season. The variety known as

Biloxi, for example, when planted early in the spring in the

latitude of Washington, D. C., continues to grow throughout the

summer, flowering in September. The plants maintain growth without

flowering for fifteen to eighteen weeks, attaining a height of five

feet or more. As the dates of successive plantings are moved forward

through the months of June and July, however there is a marked

tendency for the plants to cut short the period of growth which

precedes flowering. This means, of course, that there is a tendency

to flower at approximately the same time of year regardless of the

date of planting. As a necessary consequence, the size of the plants

at the time of flowering is reduced in proportion to the delay in

planting.



The bearing of this on geological problems lies in a query which it

raises as to the ability of a genus or family of plants to adapt itself

to days of very different length from those to which it is wonted. Could

tree ferns, ginkgos, cycads, and other plants whose usual range of

location never subjects them to daylight for more than perhaps fourteen

hours or less than ten, thrive and reproduce themselves if subjected to

periods of daylight ranging all the way from nothing up to about

twenty-four hours? No answer to this is yet possible, but the question

raises most interesting opportunities of investigation. If Cordeiro is

right as to the earth's elastic gyroscopic motion, there may have been

certain periods when a vertical or almost vertical axis permitted the

days to be of almost equal length at all seasons in all latitudes. If

such an absence of seasons occurred when the lands were low, when the

oceans were extensive and widely open toward the poles, and when storms

were relatively inactive, the result might be great mildness of climate

such as appears sometimes to have prevailed in the middle of geological

eras. Suppose on the other hand that the axis should be tilted more than

now, and that the lands should be widely emergent and the storm belt

highly active in low latitudes, perhaps because of the activity of the

sun. The conditions might be favorable for glaciation at latitudes as

low as those where the Permo-Carboniferous ice sheets appear to have

centered. The possibilities thus suggested by Cordeiro's hypothesis are

so interesting that the gyroscopic motion of the earth ought to be

investigated more thoroughly. Even if no such gyroscopic motion takes

place, however, the other causes of mild climate discussed in this

chapter may be enough to explain all the observed phenomena.



Many important biological consequences might be drawn from this study of

mild geological climates, but this book is not the place for them. In

the first chapter we saw that one of the most remarkable features of the

climate of the earth is its wonderful uniformity through hundreds of

millions of years. As we come down through the vista of years the mild

geological periods appear to represent a return as nearly as possible to

this standard condition of uniformity. Certain changes of the earth

itself, as we shall see in the next chapter, may in the long run tend

slightly to change the exact conditions of this climatic standard, as we

might perhaps call it. Yet they act so slowly that their effect during

hundreds of millions of years is still open to question. At most they

seem merely to have produced a slight increase in diversity from season

to season and from zone to zone. The normal climate appears still to be

of a milder type than that which happens to prevail at present. Some

solar condition, whose possible nature will be discussed later, seems

even now to cause the number of cyclonic storms to be greater than

normal. Hence the earth's climate still shows something of the great

diversity of seasons and of zones which is so marked a characteristic of

glacial epochs.



FOOTNOTES:



[Footnote 58: Rollin T. Chamberlin: Personal Communication.]



[Footnote 59: F. H. Knowlton: Evolution of Geologic Climates; Bull.

Geol. Soc. Am., Vol. 30, 1919, pp. 499-566.]



[Footnote 60: Chas. Schuchert: Review of Knowlton's Evolution of

Geological Climates, in Am. Jour. Sci., 1921.]



[Footnote 61: G. R. Wieland: Distribution and Relationships of the

Cycadeoids; Am. Jour. Bot., Vol. 7, 1920, pp. 125-145.]



[Footnote 62: D. T. MacDougal: Botanical Features of North American

Deserts; Carnegie Instit. of Wash., No. 99, 1908.]



[Footnote 63: Loc. cit.]



[Footnote 64: H. H. Clayton: Variation in Solar Radiation and the

Weather; Smiths. Misc. Coll., Vol. 71, No. 3, Washington, 1920.]



[Footnote 65: B. Helland Hansen and F. Nansen: Temperature Variations in

the North Atlantic Ocean and in the Atmosphere; Misc. Coll., Smiths.

Inst., Vol. 70, No. 4, Washington, 1920.]



[Footnote 66: The climatic significance of ocean currents is well

discussed in Croll's Climate and Time, 1875, and his Climate and

Cosmogony, 1889.]



[Footnote 67: F. J. B. Cordeiro: The Gyroscope, 1913.]



[Footnote 68: W. W. Garner and H. A. Allard: Flowering and Fruition of

Plants as Controlled by Length of Day; Yearbook Dept. Agri., 1920, pp.

377-400.]



[Footnote 69: Report of Committee on Sedimentation, National Research

Council, April, 1922.]