The evolution of placental mammals. 
Based on morphological, virological, biochemical and molecular biological data, it is proposed that the presence of endogenous retrovirus particles in the placental cytotrophoblasts of many mammals is indicative of some beneficial action provided by the virus in relation to cell fusion, syncytiotrophoblast formation and the creation of the placenta.  Further, it is hypothesised that the germ line retroviral infection of some primitive mammal-like species resulted in the evolution of the placental mammals.