Genetics is the science of heredity and variation in living organisms. Knowledge of the inheritance of characteristics has been implicitly used since prehistoric times for improving crop plants and animals through selective breeding. However, the modern science of genetics, which seeks to understand the mechanisms of inheritance, only began with the work of Gregor Mendel in the mid-1800s. Although he did not know the physical basis for heredity, Mendel observed that inheritance is fundamentally a discrete process with specific traits that are inherited in an independent manner — these basic units of inheritance are now called genes.