  His understanding of induction figures also in his defense of Darwin. In a review essay of 1870, he commented on the almost universal acceptance of Darwin's theory by the scientifically minded and attributed its success to “the skillful combination of inductive and deductive proofs with hypothesis.” This combination must rely, however, on a preceding simpler induction, he said. The near simultaneous discovery by Wallace and Darwin of the principle behind biological evolution testifies to their ability as naturalists to appreciate “the force of obscure and previously little studied facts” (PD 99). In this context, he also insisted upon the importance to science of investigating principles operating at a level in nature comparable to the level of political economy. He said that to fail to investigate a principle operating at the level of the whole organism or at the level of populations would go against the “Aristotelian” tendency of mind of the scientific culture. The scientific mind cannot regard the intricate system of adaptations in nature as arbitrary and is not satisfied “so long as any explanation, not tantamount to arbitrariness itself, has any probability in the order of nature” (PD 100).