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the twentieth century, but they were not spurred by the need to add absolute
age dates to the geological time scale. Radiometric dating, the discipline of
assigning dates to rocks, developed in response to an entirely different ques-
tion: finding the age of the earth.
By the late 1800s, the search for a reliable means of dating nonsedi-
mentary rocks was a major goal of science. Economic motives were still im-
portant, just as they had been at the start of the century. But as the century
came to a close, a more purely scientific reason for the better dating of rocks
emerged. Scientists of the time became much interested in determining the
age of the earth itself. There were many motives for this pursuit, but perhaps
none so pressing as those related to Darwin's newly proposed theory of evo-
lution. This theory necessitated an earth of great antiquity, and both Dar-
win's supporters and his detractors clamored for better information about the
age of the earth.
Discovering the age of the earth was perhaps the most technically dif-
ficult challenge facing scientists of the late nineteenth centuty. Two schools
of thought prevailed. Most geologists, recognizing the immense thickness of
sedimentary rocks scattered over the globe, believed that
billions
of years had
transpired since the origin of the earth. The great diversity of fossils encased
in all these strata seemed to support this view, because Darwin's theory of or-
ganic evolution required that great expanses of time must have elapsed since
the origin of the earth to allow fot the evolution of organisms now present in
such great diversity. By contrast, physicists thought the earth was much
younger than this, perhaps a few hundreds of millions of years old. These
clashes, the first to pit against each other the physical scientists and those
who styled themselves "naturalists," were a harbinger of the future, and they
were immensely bitter.
At this time, the man most notable for addressing the question of the
age of the earth was William Thomson, better known by the title of his peer-
age: Baron Kelvin of Largs Ayrshire—Lord Kelvin for short. Thomson has
been described as the most honored British scientist in history, and he was
certainly the most famous scientist in the wotld at the end of the nineteenth
century. He was elected president of the Royal Society and was re-elected
