  Wright labored in his essays and review articles to make Darwin's theory understandable to the educated American public by countering the questions about what kind of explanation natural selection offered. Realizing that utility as a principle provided the logical form for Darwin's theory, he insisted that natural selection could not submit to requirements of demonstration. It could not serve as an axiom from which deduction starts. Indeed, it should be compared to the principle of gravitation not as this concept figured in celestial mechanics or even in the laboratory but as gravitation is manifest “in the concrete courses of outward nature, in meteorology and physical geology.” Natural selection could be compared to the fundamental laws of political economy, as these laws actually emerge in the fixing of value and prices through demand and supply (PD 137). Here we see both the influence of utilitarianism and Wright's belief in the interdependence of different levels of explanatory principles.