Popular Science Monthly/Volume 74/April 1909/Darwin and Geology
| DARWIN AND GEOLOGY[1] |
By Professor JOHN JAMES STEVENSON
NEW YORK UNIVERSITY
CHARLES DARWIN was born in a time of intellectual unrest. Explorers, students of chemistry and workers in mines had been adding to actual knowledge for nearly one third of a century and thoughtful men had been forced to recognize the worthlessness of many conceptions which had long passed current. Nowhere was this unrest more manifest than among the younger geologists; but they were compelled to express themselves cautiously, for, fettered by a false chronology, the church dignitaries who controlled the universities rebuked investigation and branded as infidels those who recorded obnoxious facts. Little more than a year prior to Darwin's birth, the Geological Society of London had been founded as a protest against subjective study of this globe, but already many adherents to the principles of that society had appeared on the continent, proclaiming that actual knowledge of conditions must precede attempts to explain them.
The development of opinion was so rapid that before Darwin reached his majority the geological pendulum had made its great swing from the doctrine of cataclysms to that of uniformity; from the belief that this globe is less than 6,000 years old to an abiding faith that its age can not be measured in years. It was amid such conditions that toward the close of his university studies, he came under the influence of Henslow and Sedgwick, the latter being engaged at that time along with Murchison in an effort to unravel the tangle of Welsh geology. Some have said that these men taught him how to observe; not so; he was already a keen observer and they merely led him into wider fields.
In 1831, Captain Fitzroy was assigned to command H. M. S. Beagle, a little brig of 240 tons, and was commissioned to complete the coast survey of southern South America as well as to run a line around the globe. When he expressed the wish to be accompanied by a naturalist, Darwin, then only twenty-two years old, promptly volunteered his services, which were accepted, and he was enrolled as a supernumerary member of the staff. The Beagle left England on December 27, 1831, and returned on October 2, 1836, bringing with it Charles Darwin, now grown intellectually to man's stature and bearing a notable cargo of material collections as well as of accumulated observations. There was no haste in publication; aside from some very brief communications to societies, nothing appeared until 1839, when the Journal of Researches was printed. Owen's descriptions of the fossil mammalia was issued in 1840 with an introduction by Darwin and the final publication of results was made in three parts, dated 1842, 1844 and 1846. Thus early in his career, Darwin showed that caution which characterized him throughout life, an indifference to priority which was the outgrowth of his love of accuracy.
Part 2 of the "Geological Observations," dated 1844, relates chiefly to volcanic islands. In most cases the stay at those was brief and the studies were fragmentary; yet Darwin saw enough to let him discuss the origin of volcanic cones, to determine some cardinal points respecting the distribution of the islands, to distinguish submarine from subaerial lava flows and to prove that experimental studies on metamorphosis of limestones had led to very nearly true conceptions of the process.
As the coast survey of southern South America was the important object of Captain Fitzroy's expedition, there was ample time for a good reconnaissance of that region and Darwin spent nearly six months in studying the pampas from the Parana and Uraguay rivers southward almost to Magellan's Strait. A synopsis was given as an introduction to Owen's Memoir, but the details did not appear until 1846, when they were published as Part 3 of the "Geological Observations." The whole subject was discussed attractively in the second edition of the Journal of Researches.
The superficial deposit of the great plains is a "reddish argillaceous earth" containing concretions of indurated marl, which, at times become continuous layers or even replace much of the red earth. In the northerly part of the plains-area, this pampas deposit, which passes downward into sands, limestones and clays of late Tertiary age, yielded no marine shells to Darwin; its infusoria, studied by Ehrenberg, proved to be partly marine, partly freshwater, while the marly concretions resemble some freshwater limestones seen in Europe; but this paucity of invertebrate life was unimportant, for the whole of that region proved to be one vast cemetery, in which the skeletons of gigantic extinct mammals are so numerous that a line could not be drawn in any direction without passing through some bones. In northern Patagonia the red deposit is bound closely to an overlying gravel, containing marine forms belonging to species now existing on the coast, while in southern Patagonia marine shells occur in the pampas deposit itself.
Darwin believed that this pampas material was deposited within a vast estuary, into which great rivers carried from the surrounding region carcasses of the animals whose skeletons were entombed in muds tranquilly accumulating on the bottom. All conditions go to show that the mammalia became extinct after the sea had received its present fauna; and there is nothing to suggest that a period of overwhelming violence swept away and destroyed the inhabitants of the land; everything supports the contrary belief. The only noteworthy change in conditions has been a gradual elevation of the continent; but that was not enough to modify the climate or to bring about a change in the land fauna.
Several of the important genera collected by Darwin had been found in North America long prior to his time. This similarity of the Quaternary faunas induced him to speculate on the causes which had divided the American continent into two well-defined and somewhat contrasting zoological provinces. He does not hesitate to suggest recent elevation of the Mexican platform or more probably, recent submergence of the West Indian Archipelago as a conceivable cause of this separation. It seems to him most probable that the elephants, the mastodons, the horses and the hollow horned ruminants of North America "migrated, on land since submerged near Behring Straits, from Siberia into North America, and thence, on land since submerged in the West Indies, into South America, where for a time they mingled with forms characteristic of that southern continent and have since become extinct." Had this American Museum of Natural History existed in Darwin's day, study of the remarkable exhibits in its Mammalian Hall would have enabled him to extend his list of extinct forms common to both continents, and possibly he might have anticipated some of the all-important generalizations for which the world is indebted to the former president of this academy, who now is president of the museum.
Nothing in South America, east or west, escaped Darwin; from glaciers to peat bogs, from earthquakes to climatal variations, everything was important; but what impressed him most on both sides of the continent were the evidences of extremely slow secular movement in the earth's crust. This was the preparation for that study of the coral islands which resulted in his chief contribution to philosophical geology.
Many voyagers prior to 1833 had observed and had tried to account for the strange atolls, or low ring-like coral reefs, each enclosing a lagoon which communicates with the sea by a narrow channel; but Darwin discovered other forms of reefs which were equally perplexing. Many islets of rock are fringed by coral growth, while vast barrier reefs, separated from the land by channels of varying depth, extend at times for hundreds of miles along coasts. All explanations by previous observers were defective as they seemed to ignore these types as well as other features, not less important.
Reef-making corals can not endure exposure to the air and they can not thrive at a depth of more than 20 fathoms, so that their vertical range is about 115 feet; yet hooks and anchors brought up coral rock and sand from many hundreds of feet below the limit of growth; in a great number of instances, the atolls or ring-like reefs are mere peaks rising with abrupt slopes from "fathomless" abysses. Coral-bearing areas within the Indian and Pacific Oceans are of vast extent, there being chains of archipelagos, 1,000 to 1,500 miles long. The reefs are rudely circular or elliptical in the islands but are linear along the coasts; in the one case, the reef encloses a lagoon, in the other, a lagoon-like channel separates the reef from the coast. These are fundamental elements of the problem, not one of which may be neglected in the solution. A clue to the explanation was found by this keen observer when he saw an islet of old rock, fringed with coral, rising from the lagoon of an atoll, so that the atoll-ring resembled in many respects the barrier reef of a continent and the lagoon itself resembled the lagoon-like channel seen on the Australian and other coasts.
Chamisso's suggestion that coral reefs had been formed on banks of sedimentary material seemed wholly incompetent to meet the conditions, for the areas are too vast, and Darwin was compelled to believe that the atolls rest on rocky bases; but even on this supposition, it appears incredible that peaks of several great mountain chains should all come to within less than 180 feet of the surface and that not one rose any higher. The long study in South America had prepared him to seek an explanation in mobility of the earth's crust; but it was clear that elevation could not bring about the conditions, as that would destroy the corals themselves; subsidence alone can account for the phenomena. And thus Darwin presents his case:
The origin of the ring as well as that of the barrier reef seemed to be easily explained by this hypothesis. The corals on the outer side of the reef grew with greater rapidity than did those within, as the supply of food is constant; those on the inner side became starved and eventually the interior growth ceased and the lagoon was shallowed by wind-drifted material from the shores.
Darwin's hypothesis and the facts on which it was based have become so familiar that students sometimes express surprise that so much praise has been awarded to the author. The conditions as presented in his discussion are so clear that certainly no man could reach any other conclusion. That is true, but it is true only because Darwin marshalled his facts in a manner so masterly; in any event, it is always easy to do a thing when another has done it well and told us how. But it must be remembered that an hypothesis of this sort, though normal enough in our day, was very abnormal in that day; indeed, it was contrary to Darwin's own underlying conceptions, for, though a uniformitarian, he had seen many phenomena which, for a time, made him only a halting disciple. Yet his hypothesis was a monumental contribution in support of the uniformitarian doctrine, which, under the leadership of Lyell, was gaining sturdy adherents. That the hypothesis met with uncompromising opposition need not be said. The material of coral origin extended to vast depths alongside of the islands, in some cases apparently to 4,000 feet. The upward growth of the reef was known to be extremely slow. If the subsidence and the upward growth kept pace, as was essential to the hypothesis, evidently the required period, belonging to the latest portion of the earth's existence, was immensely long. It is difficult now to understand how great moral courage was needed by the man who published such a doctrine; sixty years ago, the educated man of Great Britain had not learned to distinguish between faith and prejudice.
This effort to explain the origin of coral reefs has been regarded, justly, as Darwin's especial contribution to geology. It has been opposed strenuously by careful students during the last twenty years and even now it is a bone of contention; but the most strenuous opponent concedes that it is logical and a fair induction from the facts as then known. Be it true or not, be it a competent explanation or not, no matter. In influence on geology it has been as far-reaching as the doctrine of natural selection has been on biology. It involves every important problem in dynamics of the earth's crust; in testing it, men have been led into paths of investigation, which, but for Darwin, might still be untrodden. The influence went farther. The hypothesis was presented at a time when men's minds were warped by prejudice, when men were extremists, when too many were defenders of dogmas in science and too few were searchers after truth. Darwin's discussion was a model of frankness; suggestions offered by his predecessors were dealt with courteously; he searched far and wide for objections to his own suggestions, and when objections were found he stated them in detail, concealing nothing and urging further investigation. His conclusions were, for him, merely tabulations of observed facts. One can not overestimate the importance of this method; it was a chief factor in changing the tone of scientific literature, in leading to replacement of subjective by objective modes of investigation.
Darwin's work as geologist practically ended with these publications of the Beagle results. It is true that in later years he made some contributions possessing much interest, but they were merely incidental to studies in other directions; the greater part of his long life was devoted to biological problems. At the same time, his whole mode of thinking and of observing was that of the geologist, so that if one were treating of his later years the topic might well be the influence of geology upon Darwin. In his later works, one finds constantly recurring consideration of geological conditions as potent factors in biological change, while on the other hand he emphasized the influence of life as a factor in bringing about geological changes. To him nature was always one; and he, in great measure, was responsible for the broadness of view characterizing the geologists who were his contemporaries as well as for the remarkable change in attitude of the community toward scientific discussion. Nowadays, when workers are so many and knowledge is so increased, men have been forced into narrow lanes of investigation; students, perplexed by phenomena within their limited vision, too often think little and know less of what neighbors are doing. And this must continue until some important problems have been solved, at least in part, and some positive results have been obtained in many directions. Then another Darwin will come, will gather loose strands floating in the wind and will weave from them a new system, once more binding nature studies into one and providing a safe platform, whence men may start anew to fathom the unknown by means of the known.
- ↑ An address given at the American Museum of Natural History on February 12.