Jan Boeke and the harmony of the organism. A case-study of holism in twentieth-century Dutch biology. In the 1920s and 1930s many biologists adopted a holistic view of the organism. In this article the scientific work of the morphologist and neurohistologist Jan Boeke (1874-1956) is discussed as an example of the influence of holistic ideas in Dutch biology. It is shown that Boeke's opposition to the neuron theory of the nervous system — he remained a reticularist all his life — was closely linked with his holistic conception of the organism. Boeke was also critical of the Mendelistic-Morganistic programme of genetic research and of Darwin's gradualistic and selectionistic evolutionary theory. Here, too, his objections derived from his holistic outlook. Not all implications of Boeke's holism were negative, though. As a morphologist, he was a strong supporter of the experimental biological disciplines, which began to gain ground in the Netherlands in the 1920s. It is argued that the holistic idea of the unity of form and function which was propagated by Boeke and other Dutch biologists, H.J. Jordan for instance, facilitated the introduction of the experimental disciplines in Dutch biology. It also explains the long continuation of the morphological tradition in the Netherlands. From the holistic point of view, descriptive and experimental work were equally important, so that there was no reason for animosity between morphologists and experimentalists or for a 'revolt from morphology'.