Depending on the definition of the word, monism has been around for some time. If we define monism as the theory that all spirit and matter are united in a single original substance, we are reminded of Spinoza and even of Eastern thought. The heritage of monism, then, lies deep in the past. Still, monism is most commonly encountered as a phenomenon of the later nineteenth century. It is usually associated with Ernst Haeckel, since it was from his class="EmphasisTypeItalic ">General Morphology and the Natural History of Creation that the word came into general parlance.¹