The Story of Evolution

by Joseph McCabe

In an earlier chapter it was stated that the story of life is a story of gradual and continuous advance, with occasional periods of more rapid progress. Hitherto it has been, in these pages, a slow and even advance from one geological age to another, one level of organisation to another. This, it is true, must not be taken too literally. Many a period of rapid change is probably contained, and blurred out of recognition, in that long chronicle of geological events. When a region sinks slowly below the waves, no matter how insensible the subsidence may be, there will often come a time of sudden and vast inundations, as the higher ridges of the coast just dip below the water-level and the lower interior is flooded. When two invading arms of the sea meet at last in the interior of the sinking continent, or when a land-barrier that has for millions of years separated two seas and their populations is obliterated, we have a similar occurrence of sudden and far-reaching change. The whole story of the earth is punctuated with small cataclysms. But we now come to a change so penetrating, so widespread, and so calamitous that, in spite of its slowness, we may venture to call it a revolution.

Indeed, we may say of the remaining story of the earth that it is characterised by three such revolutions, separated by millions of years, which are very largely responsible for the appearance of higher types of life. The facts are very well illustrated by an analogy drawn from the recent and familiar history of Europe.

The socio-political conditions of Europe in the eighteenth century, which were still tainted with feudalism, were changed into the socio-political conditions of the modern world, partly by a slow and continuous evolution, but much more by three revolutionary movements. First there was the great upheaval at the end of the eighteenth century, the tremors of which were felt in the life of every country in Europe. Then, although, as Freeman says, no part of Europe ever returned entirely to its former condition, there was a profound and almost universal reaction. In the 'thirties and 'forties, differing in different countries, a second revolutionary disturbance shook Europe. The reaction after this upheaval was far less severe, and the conditions were permanently changed to a great extent, but a third revolutionary movement followed in the next generation, and from that time the evolution of socio-political conditions has proceeded more evenly.

The story of life on the earth since the Coal-forest period is similarly quickened by three revolutions. The first, at the close of the Carboniferous period, is the subject of this chapter. It is the most drastic and devastating of the three, but its effect, at least on the animal world, will be materially checked by a profound and protracted reaction. At the end of the Chalk period, some millions of years later, there will be a second revolution, and it will have a far more enduring and conspicuous result, though it seem less drastic at the time. Yet there will be something of a reaction after a time, and at length a third revolution will inaugurate the age of man. If it is clearly understood that instead of a century we are contemplating a period of at least ten million years, and instead of a decade of revolution we have a change spread over a hundred thousand years or more, this analogy will serve to convey a most important truth.

The revolutionary agency that broke into the comparatively even chronicle of life near the close of the Carboniferous period, dethroned its older types of organisms, and ushered new types to the lordship of the earth, was cold. The reader will begin to understand why I dwelt on the aspect of the Coal-forest and its surrounding waters. There was, then, a warm, moist earth from pole to pole, not even temporarily chilled and stiffened by a few months of winter, and life spread luxuriantly in the perpetual semi-tropical summer. Then a spell of cold so severe and protracted grips the earth that glaciers glitter on the flanks of Indian and Australian hills, and fields of ice spread over what are now semitropical regions. In some degree the cold penetrates the whole earth. The rich forests shrink slowly into thin tracts of scrubby, poverty-stricken vegetation. The loss of food and the bleak and exacting conditions of the new earth annihilate thousands of species of the older organisms, and the more progressive types are moulded into fitness for the new environment. It is a colossal application of natural selection, and amongst its results are some of great moment.

In various recent works one reads that earlier geologists, led astray by the nebular theory of the earth's origin, probably erred very materially in regard to the climate of primordial times, and that climate has varied less than used to be supposed. It must not be thought that, in speaking of a "Permian revolution," I am ignoring or defying this view of many distinguished geologists. I am taking careful account of it. There is no dispute, however, about the fact that the Permian age witnessed an immense carnage of Carboniferous organisms, and a very considerable modification of those organisms which survived the catastrophe, and that the great agency in this annihilation and transformation was cold. To prevent misunderstanding, nevertheless, it will be useful to explain the controversy about the climate of the earth in past ages which divides modern geologists.

The root of the difference of opinion and the character of the conflicting parties have already been indicated. It is a protest of the "Planetesimalists" against the older, and still general, view of the origin of the earth. As we saw, that view implies that, as the heavier elements penetrated centreward in the condensing nebula, the gases were left as a surrounding shell of atmosphere. It was a mixed mass of gases, chiefly oxygen, hydrogen, nitrogen, and carbon-dioxide (popularly known as "carbonic acid gas"). When the water-vapour settled as ocean on the crust, the atmosphere remained a very dense mixture of oxygen, nitrogen, and carbon-dioxide—to neglect the minor gases. This heavy proportion of carbon-dioxide would cause the atmosphere to act as a glass-house over the surface of the earth, as it does still to some extent. Experiment has shown that an atmosphere containing much vapour and carbon-dioxide lets the heat-rays pass through when they are accompanied by strong light, but checks them when they are separated from the light. In other words, the primitive atmosphere would allow the heat of the sun to penetrate it, and then, as the ground absorbed the light, would retain a large proportion of the heat. Hence the semi-tropical nature of the primitive earth, the moisture, the dense clouds and constant rains that are usually ascribed to it. This condition lasted until the rocks and the forests of the Carboniferous age absorbed enormous quantities of carbon-dioxide, cleared the atmosphere, and prepared an age of chill and dryness such as we find in the Permian.

But the planetesimal hypothesis has no room for this enormous percentage of carbon-dioxide in the primitive atmosphere. Hinc illoe lachrymoe: in plain English, hence the acute quarrel about primitive climate, and the close scanning of the geological chronicle for indications that the earth was not moist and warm until the end of the Carboniferous period. Once more I do not wish to enfeeble the general soundness of this account of the evolution of life by relying on any controverted theory, and we shall find it possible to avoid taking sides.

I have not referred to the climate of the earth in earlier ages, except to mention that there are traces of a local "ice-age" about the middle of the Archaean and the beginning of the Cambrian. As these are many millions of years removed from each other and from the Carboniferous, it is possible that they represent earlier periods more or less corresponding to the Permian. But the early chronicle is so compressed and so imperfectly studied as yet that it is premature to discuss the point. It is, moreover, unnecessary because we know of no life on land in those remote periods, and it is only in connection with life on land that we are interested in changes of climate here. In other words, as far as the present study is concerned, we need only regard the climate of the Devonian and Carboniferous periods. As to this there is no dispute; nor, in fact, about the climate from the Cambrian to the Permian.

As the new school is most brilliantly represented by Professor Chamberlin, [*] it will be enough to quote him. He says of the Cambrian that, apart from the glacial indications in its early part, "the testimony of the fossils, wherever gathered, implies nearly uniform climatic conditions... throughout all the earth wherever records of the Cambrian period are preserved" (ii, 273). Of the Ordovician he says: "All that is known of the life of this era would seem to indicate that the climate was much more uniform than now throughout the areas where the strata of the period are known" (ii, 342). In the Silurian we have "much to suggest uniformity of climate"—in fact, we have just the same evidence for it—and in the Devonian, when land-plants abound and afford better evidence, we find the same climatic equality of living things in the most different latitudes. Finally, "most of the data at hand indicate that the climate of the Lower Carboniferous was essentially uniform, and on the whole both genial and moist" (ii, 518). The "data," we may recall, are in this case enormously abundant, and indicate the climate of the earth from the Arctic regions to the Antarctic. Another recent and critical geologist, Professor Walther ("Geschichte der Erde und des Lebens," 1908), admits that the coal-vegetation shows a uniformly warm climate from Spitzbergen to Africa. Mr. Drew ("The Romance of Modern Geology," 1909) says that "nearly all over the globe the climate was the same—hot, close, moist, muggy" (p. 219).