The Origin of Species (1859)

Darwin's masterwork is, undeniably, The Origin of Species, in which he introduced his theory of evolution by natural selection. Prior to its publication, the prevailing view was that each species had existed in its current form since the moment of divine creation and that humans were a privileged form of life, above and apart from nature. Darwin's theory knocked us from that pedestal. Wary of a religious backlash, he kept his ideas secret for almost two decades while bolstering them with additional observations and experiments. The result is an avalanche of detail—there seems to be no species he did not contemplate—thankfully delivered in accessible, conversational prose. A century and a half later, Darwin's paean to evolution still begs to be heard: "There is grandeur in this view of life," he wrote, that "from so simple a beginning endless forms most beautiful and most wonderful have been, and are being, evolved."

I'd replace Gaia with Douglas Hofstadter's Gödel, Escher, Bach: An Eternal Golden Braid.

Jorjor, I could not agree with you more. Lovelock is not a scientist and doesn't belong on that list.

Meanwhile, upon its publishing, GEB: EGB forever altered the way we perceived the underpinnings of our very own identities and those of others around us. It's a scientific masterpiece and my favorite modern-era book of all time.

I wore out my original copy and had to replace it. Same for "Guns, Germs & Steel".