Correspondence patterns

(a) Historical record of the number of letters sent (Darwin, black; Einstein, green) and received (Darwin, red; Einstein, blue) each year by the two scientists. An anomalous drop in Einstein's correspondence marks the Second World War period (1939-45, boxed). Arrows indicate birth dates of Darwin (left) and Einstein (right).
(b, c) Distribution of response times to letters by Darwin and Einstein, respectively. Note that both distributions are well approximated with a power-law tail that has an exponent of 3/2, the best fit over the whole data for Darwin giving α=1.45 plus or minus 0.1 and for Einstein α=1.47 plus or minus 0.1 (image and text: ">Nature 437 1251 ).

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