Darwin published in 1859 the argument that species change over time through natural selection, meaning that individuals vary, that variation is heritable, that more individuals are produced than can survive, and that those with advantageous variations leave more offspring, which changes the population's composition over generations.
The structure of the argument is worth noting for its method. Darwin opens with artificial selection in domesticated animals and plants, where humans have produced enormous variation within recorded time, to establish that populations are malleable. He then establishes variation in nature, then the struggle for existence drawn from Malthus, and only then states natural selection as the mechanism. He follows with chapters addressing the difficulties with his own theory, including the absence of intermediate forms in the fossil record, the evolution of complex organs such as the eye, and the existence of sterile castes in social insects.
He had no mechanism of inheritance, since Mendel's work was unknown to him, and his own hypothesis of blending inheritance was a genuine problem for the theory which he acknowledged. The reconciliation came with the modern synthesis in the 1930s and 1940s.
The book is long, the prose is Victorian, and much of it consists of accumulated observational detail from pigeons, barnacles and plant breeding. The evidence base is enormously larger now and better organised in modern textbooks. Its interest for a modern reader is the method: Darwin states his own objections at length, gives the strongest form of the counterarguments, and answers them, which is a model of how to make a contested argument.
Readers who want the primary text for its method of anticipating and answering objections, rather than for the biology, which modern sources cover better.
Chosen by shared topics, the entries that hold them, and the people who recommended both.