By Joule's great discovery (1840) that the same amount of work, whether mechanical or electrical, and however expended, always produced exactly the same amount of heat—that, in effect, heat and work were equivalent and interchangeable—the way was opened to the conclusion that the total energy of the material universe is constant in amount through all its changes.
A theory to account for the black lines crossing the coloured band of light, or spectrum, which is obtained by passing sunlight through a glass prism, originally suggested by Sir George Stokes, and subsequently reintroduced and verified by the German chemists, Bunsen and Kirchhoff, led to the important discovery that the sun and the stars are constituted of the very same elements as those of the earth beneath our feet. Spectrum analysis, moreover, soon detected new elements, e.g., helium, so-called because first observed as existing in the sun.
But great and stimulating as these discoveries were, their effect upon the thought of the age was not to be compared with that which was to be exercised by a theory which, starting in the domain of biological science, soon passed on to far more extended applications. The theory took its rise from a suggestion made in two papers, by Charles Darwin and Alfred Russel Wallace, which were read before the Linnean Society on July 1st, 1858.
The Darwinian theory—for so it was soon named—undertook to explain the formation of species by the principle of natural selection through the survival of the fittest in the struggle for life.[[1]] Darwin started from the admitted achievements of artificial selection; from the results attained by nurserymen and cattle breeders, who, by selecting the kinds they wished to perpetuate, had been able to vary and improve their stocks. He conceived that a like process had been carried on by Nature through vast spaces of time, and that it was this picking, choosing, continuing and abandoning of traits and qualities which had resulted in the preservation of the types which it had been best to retain—the reason in all cases being the fitness to correspond effectively to the conditions prescribed by environment.
It is important to remember that Darwin never claimed that his doctrine of evolution could account for the occurrence of variations. That it could do so he expressly denied. "Some," he said, in his great work, The Origin of Species (1859) "have, even imagined that natural selection induces variability, whereas it implies only the preservation of such variations as arise.... Unless such occur, natural selection can do nothing." What he saw, and proved by an amazing wealth of illustrative facts, was that any variation in structure or character which gave to an organism ever so slight an advantage might determine whether or not it would survive amid the fierce competition around it, and whether it would obtain a mate and produce offspring. He shewed that all innate variations (which are to be distinguished from the acquired characteristics upon the inheritance of which Lamarck had depended) tend to be transmitted, so that in this manner a favourable variation might be perpetuated, and in time a new species be developed.
Simple as this account of the matter sounds when once it has been clearly stated, the discovery—for such it was—opened an entirely new chapter in the history of science, inasmuch as it completely revolutionised the conceptions which had previously been entertained with regard to the relationships and the progress of all living things.
It was Darwinism, accordingly, that provided the principal subject of the controversy which was waged between the upholders and the assailants of the older opinions during the latter half of the nineteenth century.
[[1]] The actual phrase "Survival of the fittest" was Herbert Spencer's. Darwin had spoken of "The preservation of favoured races."