It is a commonplace in Aristotelian scholarship that the forms of living beings and the animal species to which they give rise are “fixed.” However, Aristotle’s biological works often stress the flexibility of nature during the development of animals. The purpose of this paper is twofold: first, to delineate the range of flexibility that Aristotle takes natures to have in the design of animals; and second, to draw out the implications of this for Aristotle’s embryology and theory of natural teleology.