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Here we must bring this study of the earth's evolution to a close. Its

fundamental principle has been that the present, if rightly understood,

affords a full key to the past. With this as a guide we have touched on

many hypotheses, some essential and some unessential to the general line

of thought. The first main hypothesis is that the earth's present

climatic variations are correlated with changes in the solar atmosphere.
/> This is the keynote of the whole book. It is so well established,

however, that it ranks as a theory rather than as an hypothesis. Next

comes the hypothesis that variations in the solar atmosphere influence

the earth's climate chiefly by causing variations not only in

temperature but also in atmospheric pressure and thus in storminess,

wind, and rainfall. This, too, is one of the essential foundations on

which the rest of the book is built, but though this cyclonic hypothesis

is still a matter of discussion, it seems to be based on strong

evidence. These two hypotheses might lead us astray were they not

balanced by another. This other is that many climatic conditions are due

to purely terrestrial causes, such as the form and altitude of the

lands, the degree to which the continents are united, the movement of

ocean currents, the activity of volcanoes, and the composition of the

atmosphere and the ocean. Only by combining the solar and the

terrestrial can the truth be perceived. Finally, the last main

hypothesis of this book holds that if the climatic conditions which now

prevail at times of solar activity were magnified sufficiently and if

they occurred in conjunction with certain important terrestrial

conditions of which there is good evidence, they would produce most of

the notable phenomena of glacial periods. For example, they would

explain such puzzling conditions as the localization and periodicity of

glaciation, the formation of loess, and the occurrence of glaciation in

low latitudes during Permian and Proterozoic times. The converse of this

is that if the conditions which now prevail at times when the sun is

relatively inactive should be intensified, that is, if the sun's

atmosphere should become calmer than now, and if the proper terrestrial

conditions of topographic form and atmospheric composition should

prevail, there would arise the mild climatic conditions which appear to

have prevailed during the greater part of geological time. In short,

there seems thus far to be no phase of the climate of the past which is

not in harmony with an hypothesis which combines into a single unit the

three main hypotheses of this book, solar, cyclonic, and terrestrial.

Outside the main line of thought lie several other hypotheses. Several

of these, as well as some of the main hypotheses, are discussed chiefly

in Earth and Sun, but as they are given a practical application in

this book they deserve a place in this final summary. Each of these

secondary hypotheses is in its way important. Yet any or all may prove

untrue without altering our main conclusions. This point cannot be too

strongly emphasized, for there is always danger that differences of

opinion as to minor hypotheses and even as to details may divert

attention from the main point. Among the non-essential hypotheses is the

idea that the sun's atmosphere influences that of the earth electrically

as well as thermally. This idea is still so new that it has only just

entered the stage of active discussion, and naturally the weight of

opinion is against it. Although not necessary to the main purpose of

this book, it plays a minor role in the chapter dealing with the

relation of the sun to other astronomical bodies. It also has a vital

bearing on the further advance of the science of meteorology and the art

of weather forecasting. Another secondary hypothesis holds that sunspots

are set in motion by the planets. Whether the effect is gravitational or

more probably electrical, or perhaps of some other sort, does not

concern us at present, although the weight of evidence seems to point

toward electronic emissions. This question, like that of the relative

parts played by heat and electricity in terrestrial climatic changes,

can be set aside for the moment. What does concern us is a third

hypothesis, namely, that if the planets really determine the periodicity

of sunspots, even though not supplying the energy, the sun in its flight

through space must have been repeatedly and more strongly influenced in

the same way by many other heavenly bodies. In that case, climatic

changes like those of the present, but sometimes greatly magnified, have

presumably arisen because of the constantly changing position of the

solar system in respect to other parts of the universe. Finally, the

fourth of our secondary hypotheses postulates that at present the date

of movements of the earth's crust is often determined by the fact that

storms and other meteorological conditions keep changing the load upon

first one part of the earth's surface and then upon another. Thus

stresses that have accumulated in the earth's isostatic shell during the

preceding months are released. In somewhat the same way epochs of

extreme storminess and rapid erosion in the past may possibly have set

the date for great movements of the earth's crust. This hypothesis, like

the other three in our secondary or non-essential group, is still so new

that only the first steps have been taken in testing it. Yet it seems to

deserve careful study.

In testing all the hypotheses here discussed, primary and secondary

alike, the first necessity is a far greater amount of quantitative work.

In this book there has been a constant attempt to subject every

hypothesis to the test of statistical facts of observation.

Nevertheless, we have been breaking so much new ground that in many

cases exact facts are not yet available, while in others they can be

properly investigated only by specialists in physics, astronomy, or

mathematics. In most cases the next great step is to ascertain whether

the forces here called upon are actually great enough to produce the

observed results. Even though they act only as a means of releasing the

far greater forces due to the contraction of the earth and the sun, they

need to be rigidly tested as to their ability to play even this minor

role. Still another line of study that cries aloud for research is a

fuller comparison between earthquakes on the one hand and meteorological

conditions and the wandering of the poles on the other. Finally, an

extremely interesting and hopeful quest is the determination of the

positions and movements of additional stars and other celestial bodies,

the faint and invisible as well as the bright, in order to ascertain the

probable magnitude of their influence upon the sun and thus upon the

earth at various times in the past and in the future. Perhaps we are

even now approaching some star that will some day give rise to a period

of climatic stress like that of the fourteenth century, or possibly to a

glacial epoch. Or perhaps the variations in others of the nearer stars

as well as Alpha Centauri may show a close relation to changes in the


Throughout this volume we have endeavored to discover new truth

concerning the physical environment that has molded the evolution of all

life. We have seen how delicate is the balance among the forces of

nature, even though they be of the most stupendous magnitude. We have

seen that a disturbance of this balance in one of the heavenly bodies

may lead to profound changes in another far away. Yet during the billion

years, more or less, of which we have knowledge, there appears never to

have been a complete cataclysm involving the destruction of all life.

One star after another, if our hypothesis is correct, has approached the

solar system closely enough to set the atmosphere of the sun in such

commotion that great changes of climate have occurred upon the earth.

Yet never has the solar system passed so close to any other body or

changed in any other way sufficiently to blot out all living things. The

effect of climatic changes has always been to alter the environment and

therefore to destroy part of the life of a given time, but with this

there has invariably gone a stimulus to other organic types. New

adaptations have occurred, new lines of evolutionary progress have been

initiated, and the net result has been greater organic diversity and

richness. Temporarily a great change of climate may seem to retard

evolution, but only for a moment as the geologist counts time. Then it

becomes evident that the march of progress has actually been more rapid

than usual. Thus the main periods of climatic stress are the most

conspicuous milestones upon the upward path toward more varied

adaptation. The end of each such period of stress has found the life of

the world nearer to the high mentality which reaches out to the utmost

limits of space, of time, and of thought in the search for some

explanation of the meaning of the universe. Each approach of the sun to

other bodies, if such be the cause of the major climatic changes, has

brought the organic world one step nearer to the solution of the

greatest of all problems,--the problem of whether there is a psychic

goal beyond the mental goal toward which we are moving with ever

accelerating speed. Throughout the vast eons of geological time the

adjustment of force to force, of one body of matter to another, and of

the physical environment to the organic response has been so delicate,

and has tended so steadily toward the one main line of mental progress

that there seems to be a purpose in it all. If the cosmic uniformity of

climate continues to prevail and if the uniformity is varied by changes

as stimulating as those of the past, the imagination can scarcely

picture the wonders of the future. In the course of millions or even

billions of years the development of mind, and perhaps of soul, may

excel that of today as far as the highest known type of mentality excels

the primitive plasma from which all life appears to have arisen.