Taxonomists in biology have traditionally been concerned to delimit and classify actual (or previously actual) biological forms or kinds. But not all useful classification schemes are of actualised forms. This paper focuses on the need to delimit and classify non-actual forms when offering contrastive explanations in evolutionary biology. Such a classification scheme sorts actual and non-actual forms according to their modal status. Such a sorting has been offered by theoretical morphologists, but these efforts have paid insufficient attention to the metaphysics of modality. Contemporary approaches to the metaphysics and epistemology of modality are also found wanting. The paper ends by arguing that the needed intellectual resources are to be found in the imaginative use of Aristotle.