Clubbed digits resemble the human embryonic fingers and toes, which look like the digits of a claw. Clubbed digits, thus, may represent the return of the embryonic claw and may even represent the claws man has lost during evolution, if ontogenesis really recapitulates phylogenesis. We put forward the hypothesis that secondary clubbing, like gynecomastia, is caused by a pathologic condition, which alters hormone levels in the blood, leading to the activation of 'dormant' genes, resulting in the development of an organ. However, the nature of the diseases that cause clubbing suggests that these hormones may actually be cytokines, acting as hormones. The nature of these cytokines is not known. They may be identified by comparing their blood levels or the combination of their blood levels to the presence or absence of clubbing, but also to the degree of clubbing and its disappearance after treatment of the primary disease.