Huxley recognized that unlike the earlier transmutational ideas of Jean-Baptiste Lamarck and Vestiges of the Natural History of Creation, Darwin's theory provided a mechanism for evolution without supernatural involvement, even if Huxley himself was not completely convinced that natural selection was the key evolutionary mechanism.
Huxley mengakui bahwa tak seperti gagasan transmutasional sebelumnya dari Jean-Baptiste Lamarck dan Vestiges of the Natural History of Creation, teori Darwin menyediakan mekanisme untuk evolusi tanpa keterlibatan supranatural, bahkan jika Huxley sendiri tak sepenuhnya menyatakan bahwa seleksi alam adalah mekanisme evolusi penting.