Induction and deduction are the two methods for establishing and explaining the relationship between features or for deriving one feature from another, such as form from function. This chapter shows that structures can be explained by deduction. It presents an example, the structures in the facial part of cichlid fishes, that show how deduction can be used, where it fails or falls short, and consequently what kind of new information or models are required to be obtained in further investigations. The chapter explains structures in the facial part of the head in relation to the parameters of the niche of the fish. It shows the strength of deductive explanation in functional morphology and in morphological transformations. Deduction in functional morphology, and also in constructional and ecological morphology, aims to explain animal form and features from function, behavior, and environmental conditions.