Just as biologists have their favored places for doing research, so do historians. As someone who likes working in archives, the most surprising thing the present author ever found was a particular letter that had been written to him by the ethologist Niko Tinbergen-but that Tinbergen had never sent. The letter included a detailed critique of the intellectual style and conceptual shortcomings of Tinbergen's career-long friend and colleague Konrad Lorenz. The present author first saw the letter 3 years after Tinbergen's death and 10 years after the letter was composed. Here we discuss the contents and historical context of that letter.
Keywords: Archives; Ernst Mayr; Ethology; Field naturalist; Franz Liszt; Konrad Lorenz; Niko Tinbergen; Orangutan.