 By universalizing the Malthusian “principle of population”, Darwin introduced an “inertial” principle into his theory, although such language is never used in his text. Newton’s first law, for example, established his physical system upon the tendency of a body in motion to persist either at rest or in uniform motion in a straight line, requiring a causal explanation for any deviations from this initial state. But Newton did not seek a deeper metaphysical explanation for this tendency. Similarly, the principle of population supplied Darwin with the assumption of an initial dynamic state of affairs that was not itself explained within the theory—there is no attempt to account causally for this tendency of living beings universally to reproduce geometrically. The principle of population might therefore be regarded as functioning axiomatically, defining a set of initial conditions from which any deviance from this ideal state demands explanation. This theoretical shift enabled Darwin to bracket his earlier efforts to develop a causal theory of life, and focus instead on the means by which the dynamic force of population was controlled. This allowed him to emphasize how this control on population worked in company with the phenomenon of slight individual variation and changing conditions of life to produce a gradual change of form and function. Darwin would later claim that he was able empirically to support the claim that living populations do tend to increase in this way, but he does not offer a causal theory of life to explain this tendency (Origin 1859: chp. 3, 63–68).