Theodosius Dobzhansky has long been recognized by historians as a pioneer in the combining of the 'field natural history' and 'laboratory experimentalist' traditions in biology (Allen 1994). The following essay analyzes two papers in his well-known Genetics of Natural Populations series, GNP IX and GNP XII, which demonstrate how Dobzhansky combined field and laboratory work in the pursuit of an evolutionary question. The analysis reveals the multiple and complementary roles field observations and experiments played in his investigations. But it also identifies several interpretive problems associated with the use of intervention that limited the effectiveness of his approach. The essay argues that these problems reflect a fundamental tension between the amount of control Dobzhansky had over the circumstances of his experiments and the applicability of his results to natural populations. It concludes that this trade-off represents an important distinction between experiments in biology and most other sciences.