This dissertation shows that the central conceptual feature and explanatory motivation of theories of evolutionary directionality between 1890 and 1926 was as follows: morphological variation in the developing organism limits the possible outcomes of evolution in definite directions. Put broadly, these theories maintained a conceptual connection between development and evolution as inextricably associated phenomena. This project develops three case studies. The first addresses the Swiss-German zoologist Theodor Eimer's book Organic Evolution (1890), which sought to undermine the work of noted evolutionist August Weismann. Second, the American paleontologist Edward Drinker Cope's Primary Factors (1896) developed a sophisticated system of inheritance that included the material of heredity and the energy needed to induce and modify ontogenetic phenomena. Third, the Russian biogeographer Leo Berg's Nomogenesis (1926) argued that the biological world is deeply structured in a way that prevents changes to morphology taking place in more than one or a few directions. These authors based their ideas on extensive empirical evidence of long-term evolutionary trajectories. They also sought to synthesize knowledge from a wide range of studies and proposed causes of evolution and development within a unified causal framework based on laws of evolution. While being mindful of the variation between these three theories, this project advances "Definitely Directed Evolution" as a term to designate these shared features. The conceptual coherence and reception of these theories shows that Definitely Directed Evolution from 1890 to 1926 is an important piece in reconstructing the wider history of theories of evolutionary directionality.