A given explanatory theory T falls into circular reasoning if the only way to determine its explanandum is through the application of T. To find an underlying theory T′ that determines T′s explanandum helps us save T from this accusation of circularity. We follow the structuralist view of theories in presenting and dealing with this issue, by applying it to particular theories. More specifically, we focus on the relationship between the Darwinian theory of common ancestry and the determination of homologies.