The origin of what Darwin referred to as ?organs of extreme perfection? represents a poorly understood aspect of evolution because the history of such origins (i.e. the intermediate stages) has often been lost through extinction. Here I propose a study of the evolution of the placenta, a complex organ of which the intermediate stages of evolution still exist. The fish family Poeciliidae has at least six independent origins of placentas with living, closely related species that either lack placentas or have them in different intermediate stages of development. I propose experimental work to characterize factors that are associated with, and potentially caused, the evolution of this trait. First, I will assess if (and how) certain environmental conditions (e.g. resource availability, predation pressure) experienced by the female may have favoured the evolution of matrotrophy (feeding of developing embryo?s by means of a placenta) over lecithotrophy (feeding of embryo?s by means of egg-yolk). Secondly, I will assess if placental-feeding (matrotrophy) may offer mothers a selective advantage by enabling them to allocate different amounts of resources to each developing offspring depending on its expected fitness (post-fertilization sexual selection).