To a modern listener, what was not known may be as interesting as what was. With the 100-inch Mt. Wilson reflector the largest telescope in the world, the existence of galaxies outside the Milky Way was suspected but not confirmed. Neutrons, soon to become important in the field of nuclear energy and atomic bombs, were as yet unguessed-at, yet the prospect of liberating the immense energy of the atom was already a keen interest. Although the famous Michaelson-Morley experiment had already been seen as disproof of an all-pervading "ether" which facilitated the flow of energy across empty space, scientists still retained ether as a place-holder for properties they could measure but not explain – an approach very similar to the "dark matter" of modern cosmology.

Regardless of your personal sentiments on Darwin's theory of evolution, Thomson provides well-chosen examples that illustrate why this theory arose. He examines not only the fossil record but the evidences present in modern living beings that the process of evolution is by no means finished, but ongoing.