In his Principles of Biology, published in 1864, Spencer outlines a complementarity between the mitigated transformational cuvierianism that was in the basis of his idea of direct equilibration, and the theory of the natural selection that Darwin had already formulated in 1859. Spencer called the mechanism of natural selection indirect equilibration. According to him, this second form of equilibration enabled the explanation of some evolutionary phenomena that could not be caused by direct equilibration. However, for Spencer, it was clear that the operation of this second equilibration had to be always subordinated to the control of the first one.
Spencer; Darwin; Cuvier; Direct equilibration; Indirect equilibration; Evolution; Natural selection; Organic correlations; Progress