"Very scanty acquaintance with practical geology, I'm exceedingly interested in all wider problems with which it deals"

Alfred Russel Wallace (1896)

When Charles Darwin published "The Origin of Species" in November 1859 geologists were still discussing the age of the earth. Deep time was an essential prerequisite to explain the recent biodiversity by gradual and slow changes in the remote past. However the calculations and criticism by physicists of the reconstructed geological age posed a great problem to evolutionists like Darwin and Wallace. Darwin didn't address this problem in public, but he was convinced that the thickness of the stratigraphical column could only be explained by an ancient earth and the calculations of the physicists were based on wrong assumptions.

A.R. Wallace was also interested in geological time and , much more open-minded to radical new ideas and not afraid to discuss them in public, choose an interesting solution for this problem. He assumed that abrupt climatic changes in the past significantly increased the evolutionary pressure and therefore the speed of evolutionary changes of organisms. So even on an earth not billions, but millions of years old, evolution was fast enough to explain the recent plants and animals. Wallace became especially interested in the most recent and profound climatic changes: the ice ages.

In 1842 the French mathematician Joseph Alphonse Adhémar argued that diminished solar radiation could explain sudden cooling of earth´s surface. Based on this premise he calculated the variations of earth's orbit around the sun, concluding that earth had experienced many ice ages (not only one or four, as proposed by geologists at the time) in the recent past.

The self-taught mathematician and janitor James Croll (1821-1890) became interested in the ice ages problem after 1864 and re-calculated the occurrence of ice ages based on the newest astronomical data. In 1875 he will publish his results in the book "Climate and Time, in Their Geological Relations".

Fig.1. Variations in the eccentricity of the Earth's orbit for three million years before 1800 A.D. and one million years in the future, from CROLL 1875 (image in public domain).

"The astronomical theory seems to me the best solution to the present ice age riddle. It bears in it all the decisive factors for the occurrence of alternating cold and warm periods, and accounts for the peculiar character of glacial and interglacial climates."

Geologist Sir Archibald Geikie

Wallace became aware of Croll's work by a lecture hold by T.H. Huxley in February 1869 at the Geological Society of London and in the coming years exchanged many letters with Croll on this topic.

Wallace published his main arguments supporting the occurrence of cyclic ice ages in two papers in 1879 and then in 1893. It may surprise that the main criticism to Wallace's support for ice ages as important geological and biological factors involved lakes. A fierce discussion arouse between Wallace and geologists if the great lakes of the Alps and in North America were formed by geologically recent glaciers or by ancient tectonic forces. Wallace didn´t hesitate to address the critics with a bit of sarcasm, arguing "that a few words on the other side seem desirable".

To Wallace it seems strange that tectonic forces would create lakes only on some parts of earth´s surface (coincidentally covered with evidence of former glacial quarrying, as he noted even in his native Welsh country) and not others.

Later research and modern radiometric dating techniques will show that indeed there were many glacial periods that resculpted earth´s crust and earth is far older we imagined - and that ice ages can indeed influence evolution.

Bibliography:

FLEMING, J.R. (2006): James Croll in Context: The Encounter between Climate Dynamics and Geology in the Second Half of the Nineteenth Century. History of Meterology 3: 43 - 54

TINKLER, K. (2008): Wallace and the Great Ice Age. In. SMITH, C.H. & BECCALONI, G. (eds.) Natural Selection and beyond - the intellectual Legacy of Alfred Russel Wallace. Oxford University Press: 186-200