Conventional wisdom has it that Darwin’s theory of natural selection needed Mendel’s theory of inheritance to become workable, and relatedly, that had Darwin read Mendel’s 1866 paper on his experiments with crossbred peas, the necessary fix would have come around 1870 rather than decades later. This chapter shows that, on closer inspection, neither of these propositions should be accepted. From Darwin’s perspective, when it came to inheritance, his theory depended only on an undoubted fact: that offspring on the whole inherit their parents’ characters. Even when a character gets transmitted in a diluted form, due to blending, the struggle for existence ensures, as Darwin saw it, that such dilution is minimal, since only organisms that vary in similarly advantageous directions will live long enough to reproduce. Against the idea that Darwin would have instantly embraced Mendel’s paper as putting inheritance on a new, theory-saving basis, thus saving evolutionary biology from decades of sterile debate, the chapter emphasizes three points: first, the similarity between Mendel’s results and ones that Darwin was already familiar with from his own snapdragon crosses; second, the differences between Mendel’s results and ones reported in 1866 by Darwin’s pea expert, Thomas Laxton; and third, Mendel’s criticisms of Darwin on whether, as Darwin believed, variation under domestication is much greater in extent than variation in the wild.

James Clerk Maxwell’s field theory of electromagnetism had important and previously unrecognized roots in the cable industry of the mid-nineteenth century. When he took up electrical physics in 1854, the subject was permeated by a concern with cable problems. Guided by William Thomson, Maxwell soon adopted Faraday’s field approach, which in 1861 he sought to embody in a mechanical model of the electromagnetic ether. Seeking evidence to bolster the electromagnetic theory of light to which this model had led him, Maxwell joined the British Association Committee on Electrical Standards, which had been formed in 1861 largely to meet the needs of the submarine telegraph industry. Maxwell’s work on the committee between 1862 and 1864 brought home to him the value of framing his theory in terms of quantities he could measure in the laboratory—particularly the “ratio of units”—rather than relying on a hypothetical mechanism. Maxwell’s shift from his mechanical ether model of 1861 to his seemingly abstract “Dynamical Theory of the Electromagnetic Field” of 1864 thus reflected the often overlooked role concerns rooted in cable telegraphy played in the evolution of his thinking.

An agreed system of electrical units and standards was crucial to building a workable cable systemin the 1860s, as well as to advancing electrical science. Without such standards, it was almost impossible to extend accurate electrical knowledge beyond a single laboratory or testing room. Amid conflicts over competing standards and in response to rising demands from the telegraph industry, in 1861 William Thomson called on the British Association for the Advancement of Science to establish a Committee on Electrical Standards. The committee proved very influential, and its work marks one of the most important points of intersection between electrical science and technology in the mid-nineteenth century. Led by James Clerk Maxwell ,and Fleeming Jenkin, the committee determined the value of the ohm experimentally in 1862–64 and distributed standard resistance coils around the world. Standard ohms soon became a key part of quality control in the cable industry; indeed, the aim in manufacture became to make a cable that was, in effect, a chain of standard ohms strung end to end, its properties at each point known and recorded.