Four years after the death of Charles Darwin, his research associate, George Romanes, invoked a mysterious process—“physiological selection”—that could often have secured reproductive isolation independently of, and prior to, natural selection, so leading to an origin of species. This postulate of two sequential selection modes can now be regarded as leading to modern “chromosomal,” as opposed to “genic,” speciation theories. Romanes’ abstractions—which confounded many, but not all, of his contemporaries—equate with divergences in parental DNA sequences that impede meiotic pairing in (...) their hybrid offspring, so rendering that offspring sterile. Unlike Darwin, Romanes saw hybrid sterility as a parental, rather than offspring, phenotype that would, within a species, reproductively isolate certain parents from each other while not impeding their crossing with other parents. This group selection would have empowered natural selection to act more advantageously than in its absence. Given suitable conditions, there could then be divergence from one species into two. The present essay introduces Romanes’ “Physiological Selection; an Additional Suggestion on the Origin of Species” 19:337–411; available as supplementary material in the online version of this essay) for the journal’s “Classics in Biological Theory” collection. (shrink)