Why Life Must Change Continually
( Originally Published Early 1900's )
The Record of the Rocks is like a great book that has been carelessly misused. All its pages are torn, worn and defaced, and many are altogether missing. The outline of the story that we sketch here has been pieced together slowly and pain-fully in an investigation that is still incomplete and still in progress. The Carboniferous Rocks, the "coal-measures," give us a vision of the first great expansion of life over the wet lowlands. Then come the torn pages known as the Permian Rocks (which count as the last of the Palaeozoic), that preserve very little for us of the land vestiges of their age. Only after a long interval of time does the history spread out generously again.
It must be borne in mind that great changes of climate have always been in progress, that have sometimes stimulated and sometimes checked life. Every species of living thing is always adapting itself more and more closely to its conditions. And conditions are always changing. There is no finality in adaptation. There is a continuing urgency towards fresh change.
About these changes of climate some explanations are necessary here. They are not regular changes; they are slow fluctuations between heat and cold. The reader must not think that because the sun and earth were once incandescent, the climatic history of the world is a simple story of cooling down. The centre of the earth is certainly very hot to this day, but we feel nothing of that internal heat at the surface; the internal heat, except for volcanoes and hot springs, has not been perceptible at the surface since first the rocks grew solid. Even in the Azoic or Archaeozoic Age there are traces in ice worn rocks and the like of periods of intense cold. Such cold waves have always been going on everywhere, alternately with warmer conditions. And there have been periods of great wetness and periods of great dryness throughout the earth.
A complete account of the causes of these great climatic fluctuations has still to be worked out, but we may perhaps point out some of the chief of them.' Prominent among them is the fact that the earth does not spin in a perfect circle round the sun. Its path or orbit is like a hoop that is distorted; it is, roughly speaking, elliptical (eve-elliptical), and the sun is nearer to one end of the ellipse than the other. It is at a point which is a focus of the ellipse. And the shape of this orbit never remains the same. It is slowly distorted by the attractions of the other planets, for ages it may be nearly circular, for ages it is more or less elliptical. As the ellipse becomes most nearly circular, then the focus becomes most nearly the centre. When the orbit becomes most elliptical, then the position of the sun becomes most remote from the middle or, to use the astronomer's phrase, most eccentric. When the orbit is most nearly circular, then it must be manifest that all the year round the earth must be getting much the same amount of heat from the sun ; when the orbit is most distorted, then there will be a season in each year when the earth is nearest the sun (this phrase is called Perihelion) and getting a great deal of heat comparatively, and a season when it will be at its farthest from the sun (Aphelion) and getting very little warmth. A planet at aphelion is travelling its slowest, and its fastest at perihelion; so that the hot part of its year will last for a much less time than the cold part of its year. (Sir Robert Ball calculated that the greatest difference possible between the seasons was thirty-three days.) During ages when the orbit is most nearly circular there will therefore be least extremes of climate, and when the orbit is at its greatest eccentricity, there will be an age of cold with great extremes of seasonal temperatures. These changes in the orbit of the earth are due to the varying pull of all the planets, and Sir Robert Ball declared himself unable to calculate any regular cycle of orbital change, but Professor G. H. Dar-win maintained that it is possible to make out a kind of cycle between greatest and least eccentricity of about 200,000 years.
But this change in the shape of the orbit is only one cause of the change of the world's climate. There are many others that have to be considered with it. As most people 'know, the change in the seasons is due to the fact that the equator of the earth is inclined at an angle to the plane of its orbit. If the earth stood up straight in its orbit, so that its equator was in the plane of its orbit, there would be no change in the seasons at all The sun would always be overhead at the equator, and the day and night would each be exactly twelve hours long throughout the year everywhere. It is this inclination which causes the difference in the seasons and the unequal length of the day in summer and winter. There is, according to Laplace, a possible variation of nearly three degrees (from 22° 6' to 24° 50') in this inclination of the equator to the orbit, and when this is at a maximum, the difference between summer and winter is at its greatest. Great importance has been attached to this variation in the inclination of the equator to the orbit by Dr. Croll in his book Chinate and T nie. At present the angle is 23 27'. Manifestly when the angle is at its least, the world's climate, other things being equal, will be most equable.
And as a third important factor there is what is called the precession of the equinoxes. This is a slow wobble of the pole of the spinning earth that takes 25,000 odd years. Any one who watches a spinning top as it "sleeps," will see its axis making a slow circular movement, exactly after the fashion of this circling movement of the earth's axis. The north pole, therefore, does not always point to the same north point among the stars; its pointing traces out a circle in the heavens every 25,000 years.
Now, there will be times when the earth is at its extreme of aphelion or of perihelion, when one hemisphere will be most turned to the sun in its midsummer position and the other most turned away at its midwinter position. And as the precession of the equinoxes goes on a time will come when the summer-winter position will come not at aphelion and perihelion, but at the half-way points between them. When the summer of one hemisphere happens at perihelion and the winter at aphelion, it will be clear that the summer of the other hemisphere will happen at aphelion and its winter at perihelion. One hemisphere will have a short hot summer and a very cold winter, and the other a long cold summer and a briefer warmish winter.. But when the summer-winter positions come at the half-way point of the orbit, and it is the spring of one hemisphere and the autumn of the other that is at aphelion or perihelion, there will not be the same wide difference between the climate of the two hemispheres.
Here are three wavering systems of change all going on independently of each other; the precession of the equinoxes, the change in the obliquity of the equator to the orbit, and the changes in the eccentricity of the orbit. Each system tends by itself to produce periods of equability and periods of greater climatic contrast. And all these systems of change interplay with each other. When it happens that at the same time the orbit is most nearly circular, the equator is at its least inclination from the plane of the earth's orbit, and the spring and autumn are at perihelion and aphelion, then all these causes will be conspiring to make climate warm end uniform; there will be least difference of summer and winter. When, on the other hand, the orbit is in its most eccentric stage of deformation, when also the equator is most tilted up and when further the summer and winter are at aphelion and perihelion, then climates will be at their extremest and winter at its bitterest. There will be great accumulations of ice and snow in winter; the heat of the brief hot summer will be partly reflected back into space by the white snow, and it will be unequal to the task of melting all the winter's ice before the earth spins away once more towards its chilly aphelion. The earth will ac-cumulate cold so long as this conspiracy of extreme conditions continues.
So our earth's climate changes and wavers perpetually as these three systems of influence come together with a common tendency towards warmth or severity, or as they contradict and cancel each other.
We can trace in the Record of the Rocks an irregular series of changes due to the interplay of these influences ; there have been great ages when the separate rhythms of these three systems kept them out of agreement and the atmosphere was temperate, ages of world-wide warmth, and other ages when they seemed to concentrate bitterly to their utmost extremity, to freeze out and inflict the utmost stresses and hardship upon life.
And in accordance we find from the record in the rocks that there have been long periods of expansion and multiplication when life flowed and abounded and varied, and harsh ages when there was a great weeding out and disappearance species, genera, and classes, and the learning of stern lessons by all that survived. Such a propitious conjunction it must have been that gave the age of luxuriant low-grade growth of the coal measures ; such an adverse series of circumstances that chilled the closing aeons of the Palaeozoic time.
It is probable that the warm spells have been long relatively to the cold ages. Our world today seems to be emerging with fluctuations from a prolonged phase of adversity and extreme conditions. Half a million years ahead it may be a winterless world with trees and vegetation even in the polar circles. At present we have no certainty in such a fore cast, but later on, as knowledge increases, it may be possible to reckon with more precision, so that our race will make its plans thousands of years ahead to meet the coming changes.