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Exploration of theistic evolution within a Thomistic framework has taken on the label of Thomistic evolution. Although the conceptual boundaries of this approach are not precise, a historical trajectory of its scholastic methodology can be traced back to the views of Aquinas himself. An enduring central concept in this tradition has been the idea of substantial form. For Aquinas, distinct substantial forms were associated with the kinds of animals and plants that have no prior ancestry and are ascribed to God's production during the Genesis days of adornment. Later in the tradition, the early twentieth century Jesuit Erich Wasmann referred to these taxa as "natural species" with human beings serving as his paradigmatic example. As the evolutionary history of life becomes known in more and more detail, the existence of natural species and Wasmann's associated concept of polyphyletic evolution have become increasingly difficult to sustain. The historical linkage between substantial forms and natural species highlights the fact that, even if the idea of natural species is abandoned, the incorporation of substantial forms into the evolutionary process is problematic. Although recent philosophical reassessments of Aristotelian essentialism are relevant and intriguing, discrete substantial forms are not easily embedded within the evolutionary continuum of genetic mutation and phenotypic change.