It was left to Edgar Irmscher, Wegener’s young colleague at Hamburg, to go forward to try to reorganize his field of study—the distribution and diffusion of vascular plants—in terms of Wegener’s hypothesis. He had said (in the lecture that Wegener heard in 1919) that he intended to do this, and by 1922 he had completed this task to a first approximation in the publication of a book entitled Pflanzenverbreitung und Entwicklung der Kontinente (The diffusion of plants and the developmental history of the continents).77 This was a rewriting of paleobotany from the perspective of the permanence theory to that of the displacement theory which he intended to be read as a complement to Wegener’s work on continental displacements, as well as to Wegener and Köppen’s forthcoming work in paleoclimatology.78
It was a very ambitious book, 250 pages in length, which could serve as an introduction to Wegener’s theory, an introduction to paleobotany, and an introduction to the subject declared in the title: the diffusion of vascular plants. It concluded with a firm declaration that migration of the poles and large displacements of the continents were essential to the explanation of the current distribution of vascular plants, in combination with the plants’ own “efforts” to spread their range, develop, and interbreed with other species. At the very end of the book, Irmscher included a note of thanks to Wegener for his support and his generous sharing of his data and his unpublished maps (referring to the maps for the forthcoming book on climates of the past). These maps show that Wegener, by mid-1922, had begun to think about how to represent paleopoles and paleoequators.79
It was very convenient for Wegener that Irmscher was able to finish and publish this book so quickly, allowing Wegener to cite it as a source and use Irmscher’s mapping of the distribution of plants in his own book on climates of the geological past. The situation here was similar to that of Köppen and Milankovich, where they engaged in a collaboration, the younger author published separately and extensively based on his own research, but this work was prepared in collaboration with Köppen and Wegener, who could then cite work they had essentially “commissioned.”
While these various symposia on his work, plans for translations of his books, and endorsement of his theory proceeded, Wegener remained steadily and methodically at work on Klimate der geologischen Vorzeit throughout the autumn of 1922, all of 1923, and into the spring of 1924. He and Köppen had settled on that title in order to distinguish their work from many other books entitled “Climates of the Past,” “History of Earth,” “Climates of Earth,” “History of Climate,” and so on. There had already been a book titled Klimate der geologischen Vergangenheit (Climates of the Geological Past), published by the Dutch paleontologist Eugène Dubois (1858–1940) in 1893, and republished in English under that title in 1895.80 Dubois had a climate theory based on changes in solar radiation, as the Sun evolved into different phases of its history as a star. To avoid confusion, they chose another word for “the past”: die Vorzeit, with its sense of past ages of Earth.
In working out the geologic history of climate, there were a few textbooks that Wegener depended on heavily to supplement and frame the information in the various volumes of the Handbuch der Regionalen Geologie and individual monographs from the geological literature. In 1922–1924 his most comfortable resource was Erdgeschichte (History of Earth) by Melchior Neumayr (1845–1890) and Viktor Uhlig (1857–1911), from the 1895 edition of that work.81 Wegener referred to this book throughout as “Neumayer–Uhlig,” reminiscent of the way classical scholars, then and now, refer to “Pauly–Wissowa,” an encyclopedic work on Greek and Roman classical antiquity.
Wegener searched for a book with a detailed treatment of the regional geology of North America, a book that could play a similar role to that of Neumayr–Uhlig, and he decided on Thomas C. Chamberlin and Rollin D. Salisbury’s three-volume Geology (1905–1907), a standard undergraduate textbook for many years after its publication.82 It covered basic science, physical geology, paleontology, stratigraphy, tectonics, and large questions concerning the origin of Earth. If Chamberlin had been a European, he would probably have been called a “cosmic physicist” because although he had made a great reputation in unraveling the advance and retreat of the Quaternary ice sheets in the northern half of the United States, he was insistent that geology should extend itself back to the origin of Earth and the origin of the solar system. This was certainly congenial to Wegener, and it was an excellent choice for a standard North American reference. Almost as important for Wegener was Eliot Blackwelder’s (1880–1969) Handbuch volume (1912) for the United States. Blackwelder had worked on both desert and glacial deposits, had traveled with Bailey Willis in China, and had written on Meteor Crater, Arizona; Wegener already knew of him.83
Wegener depended, of course, as he had since 1915, on the work of Dacqué, and both he and Köppen were also entirely confident in the paleobotanical treatise of Henry Potonié, brought up to date in a 1921 edition by Walther Gothan (1879–1954) and generally referred to as “Potonié–Gothan.”
Wegener also depended heavily on the compendious treatise Unsere Erde (Our Earth), authored by the paleontologist and economic geologist Lukas Waagen (1877–1959) with the help of Jacob van Bebber (1847–1909), a meteorological colleague of Köppen at Hamburg, and also Damian Kreichgauer. It was an odd conjugation of scientists: a very young paleontologist, a very old meteorologist, and a monk who was even then turning away from the history of climate toward an attempt to translate the Mayan codices.84 Yet there was a good deal of experience and novelty mixed together in this volume, which pursued the same theme and the same problems that interested Wegener and Köppen.
Certain figures—Chamberlin and Salisbury; Neumayer and Uhlig; Potonié and Gothan; Waagen, van Bebber, and Kreichgauer—Wegener employed as “authorities.” They had written major texts of established reputation, and their citation structure was reliable. Often Wegener’s reference to a primary monograph would be not to the monograph itself but to the page where it was cited in one of these “authorities.”
He used the Handbuch volumes in the same way to cite literature, and even some of the these emerged as “authorities,” especially Blackwelder for the United States, Paul Lemoine (1879–1940) on West Africa, and Max Blanckenhorn (1861–1947) for the Middle East and Egypt. Beyond these there were still others, such as Darashaw Wadia (1883–1969), an expert in many areas of geology, author of a major textbook on the stratigraphical history of India, and a great student of the structure of the Himalayas. Wadia had a long section in Geology of India (1919) on the Glossopteris and Gangamopteris florae and on economic geology, including the location of Gondwana coalfields and of gypsum.85
The tendency to cite authorities led to an uneven citation pattern in the finished book. Sometimes Wegener gave a direct page reference; sometimes he just referred to the name of his authority, expecting that the structure of these textbooks would carry the reader to the correct page (generally it does). At times this looseness of citation reflects the time pressure he felt while working on this book. The facsimile edition of his research notebook shows that he often would record the necessary geological information and the name of the author, but very rarely the full citation or the page number. This notebook, which he carried with him in his time-consuming commutes from one worksite to another, was a basis for rumination, reading, and thinking about the distributions of his geological markers, but if he decided to use them as citations, there was no time for him to go back and redo all the research to find the page numbers. Since, however, he was referring in almost every case not to a theory of Earth but to some list of geological strata, or the occurrence of coal, gypsum, desert sandstone, or some other marker, the exact page number for a reference was not as important as it would have been in a work that was stringing together arguments rather than plotting data points.
Wegener in 1922–1923 was overcoming his discomfort with geological reports now that he could “reduce” them as a physicist might reduce physical data, in order to plot them on his globe. With his
tissue paper cutouts of the continents in their conjectural position for the period of time he was working on, he could “pinpoint” coal, gypsum, sandstone, coral, and traces of ice and mark them on his half-meter-diameter globe with flags of colored paper, as he had done in 1920. Wegener adapted his meteorological technique for mapping layer boundaries at altitude by temperature into a geological technique for mapping geological boundaries at latitude by using specific mineral markers. In a two-dimensional map, a latitude line appears as an “altitude” above or below the equator. Wegener established the distribution of coal, desert sandstone, gypsum, and traces of ice for the most securely documented and mapped locations. He discussed, for each period of geological time, each of these locations (each item that ended up as a plot point on a map) and referenced it to the literature. Each letter “K” (coal) indicated a geologically mapped coal deposit registered in the literature for that period of time; the same was true for all the other indicators.
Depending on how much coal, salt, gypsum, and desert sandstone he could reliably locate for a period, he might begin by establishing the equator directly, especially in periods for which he had a very large number of data points indicating tropical coal, as in the Carboniferous. He might also begin by establishing the spread of sandstone, salt, and gypsum in two widely separated regions—say, across North America and Europe on the one hand and middle South America and Central Africa on the other—and then using these data to establish latitudes 30° north and 30° south, interpolating the equator between them. From these 30° lines one could establish the 60° latitude line and the position of the poles. On the other hand, for heavily glaciated periods, the large number of markers indicating glacial ice might be the controlling factor, leading him to establish the 60° latitudes and pole positions first, and then 30° and equatorial rainfall zones later.
Working through Wegener’s text and maps, one can see how important were Potonié’s ideas about coal formation, because sometimes these coal deposits are so widely scattered that unless one accepted that coal could and would form in tropical, temperate, and even periglacial environments, it would be impossible to make sense of the distributions. Similarly, one may see that gypsum and sandstone were important indicators of rainfall and not temperature, as these are sometimes found in high-latitude deserts where precipitation is scarce.
Wegener’s mapping was iterative and self-correcting, and sometimes the climate data also indicated the need to relocate continental positions and even to rotate continents. For instance, Wegener was able to use geological information on India for different periods to calculate that India had moved about 49° of latitude (5,400 kilometers [3,355 miles]) and rotated about 30° counterclockwise since the beginning of the Tertiary period.86
We have to think of these maps as the result of two huge data sets superimposed on one another. The first is the map of continental positions dictated by faunal and floral continuities and discontinuities, leading to the establishment of a time of separation of continental fragments and a rate of separation calculated backward from their current distance apart. This allowed the interpolation of intermediate positions for different periods of time since the separation. Superimposed on that fossil data are the geological data for latitude zones, which allowed calculation of the pole positions, this time following geological conjunctions more than floral or faunal disjunctions.
While Wegener assumed the “truth” of continental displacements in establishing the climates of the past, this book contains the most detailed reconstructions of continental displacement he ever produced. Prior to this effort he had mapped the Carboniferous, the Eocene (early Tertiary), and the Quaternary. In this book he calculated paleolatitudes and pole positions, as well as the arrangement and exact distance apart of the continents, for the Devonian, Carboniferous, Permian, Triassic, Jurassic, Cretaceous, Eocene, Miocene, and Pliocene and early Quaternary (the last two mapped as one period). He produced additional special maps using the same (Hammer) projection, for floral distributions in the Carboniferous and Permian, land and sea distributions in the Jurassic, and tropical corals in the Cretaceous.
Wegener’s climate maps of Earth (on a Hammer equal-area elliptical projection) for the Carboniferous, the Triassic, and the Pliocene and early Quaternary. E = traces of ice; K = coal; S = salt; G = gypsum; W = desert sandstone; punktierte Räume (dotted regions) = arid areas. The maps show the arrangement of the continents (with Africa arbitrarily in its current position), as well as the positions of the poles and the paleoequator, with arrows showing the direction and magnitude of the displacement of the pole during each period. Note the absence of traces of ice in the map for the Triassic. From Wladimir Köppen and Alfred Wegener, Die Klimate der geologischen Vorzeit (Berlin: Gebrüder Bornträger, 1924).
The concluding chapter of this extraordinary research effort and imaginative tour de force consisted of only four pages, of which three are maps and charts of latitude and longitude data. Using the Lambert oblique projection, Wegener produced a map showing the path of the poles from the Devonian to the present, against a background showing the continents in bold outline (their current positions) and in shaded outline (their position in the Carboniferous). The polar displacements are shown relative to Africa fixed in its current position (Polwege bezogen auf Afrika).87
Wegener’s map of the path of the poles (relative to Africa) from the Archaean to the Quaternary, on a Lambert oblique orthographic equal-area map. The current outlines are shown with solid lines, and their Carboniferous positions with shaded lines. The Southern Hemisphere is to the left, the Northern Hemisphere to the right. From Köppen and Wegener, Die Klimate der geologischen Vorzeit.
These pole-wandering paths are considerably shorter than those he had calculated and sketched in 1921 to be published along with Köppen’s article for Petermanns. The distances between the Carboniferous and Permian pole positions are only about half of what they had been in 1921, and while he still mapped very large excursions of the pole in the Tertiary, similar large excursions of the pole for the Mesozoic (Triassic, Jurassic, and Cretaceous) had entirely disappeared, with the pole remaining at nearly the same latitude between the Triassic and the Eocene, and with a reduction of its shift in longitude from 75° down to about 45°.
In addition to this map, he produced a chart of the latitude and longitude (using the Greenwich Meridian and the current position of Africa) of the North and South Poles for every period from the Carboniferous to the Quaternary. He produced a graph of the latitude of Tokyo, Leipzig, Cairo, Punta Arenas (Chile), and Hobart (Tasmania) from the Precambrian (Algonkian) to the present. On the facing page he constructed a chart for the total change in latitude of twenty-seven locations on Earth since the Carboniferous. For variety and the interest of his readers he distributed these among North America, Europe, Asia, South America, Africa, Australia, and Antarctica, so one could find the change in location of (among others) New York, the Panama Canal, Madrid, Colombo (Sri Lanka), the Cape Colony (South Africa), Perth (Australia), and Mount Erebus, Antarctica’s active volcano.88
It is of some interest to determine how accurate this method of work was, especially since so many of Wegener’s opponents attacked his reconstructions as absurd. Paleomagnetic measurements made in the 1950s by Edward Irving, on a selection of rock samples from India, suggested that India had traveled 6,000 kilometers (3,728 miles) through 55° of latitude and 30° of counterclockwise rotation; Wegener’s accuracy approached the limits of resolution of a novel geophysical technique thirty years after his publication. Irving did not publish these measurements at that time, but they cemented his conviction that latitude zonation was crucial to understanding the character and extent of continental mobility; Irving’s research later became a crucial piece of evidence in the assemblage of scientific work which emerged as “plate tectonics” in the 1970s.89
In assessing the impact of Köppen and Wegener’s book and its relationship to the theory of continental displacements, we should probably treat it
, as we have already suggested, as two separate books, one on the “climates of the geological past” and one on the “climatology and causes of the North hemispheric Quaternary ice ages.” The former book was by Alfred Wegener, and the latter by Wladimir Köppen and Milutin Milankovich. The two books under one cover differ in style, subject, method of approach, organization, citation conventions, depth of coverage, cartographic style, and mathematical content.
The former book is a descriptive and “morphological-empirical” reconstruction of continental positions, latitude zones, and movements of Earth’s pole of rotation, based on geological and paleontological data. The latter book extends Köppen’s scheme for the contemporary climate of Earth back one million years into the Quaternary and unites this climatic scheme with geological (and even Paleolithic archeological) data marking the advance and retreat of the ice sheets. These are, in turn, united and explained via Milankovich’s mathematical theory of the astronomical forcing of ice ages by variation in the amount of solar radiation reaching the surface of Earth. One of these books starts on page 1 and extends through page 157; the other begins on page 158 and extends through page 256, plus the foldout at the end of the book graphing the advance and retreat of the ice sheets in rhythm with the fluctuations in solar radiation calculated by Milankovich.
Wegener and Köppen insisted in print, in person, and in correspondence that this was their joint work from the beginning to the end, and thus each accepted all the conclusions from the part written by the other. Yet, there was a serious theoretical contradiction between the first half of this book and the second, which had to be finessed.
In 1921, in Wegener’s map for Köppen’s article in Petermanns, the North Pole moved through a huge series of spiraling oscillations from the Bering Strait, vibrating between latitudes 70° and 80° north many times while shifting across 120° of longitude in a matter of one million years: more polar motion in the past one million years than in the previous 500 million.
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