century, it had become clear to many that "the existing dichotomy of the plant and animal kingdoms [had become] rapidly blurred at its boundaries and outmoded".

In 1860 John Hogg proposed the Protoctista, a third kingdom of life composed of "all the lower creatures, or the primary organic beings"; he retained Regnum Lapideum as a fourth kingdom of minerals. In 1866, Ernst Haeckel also proposed a third kingdom of life, the Protista,