Fireflies out on a warm summer's night, seeing the urgent, flashing, yellow-white phosphorescence below them, go crazy with desire; moths cast to the winds an enchantment potion that draws the opposite sex, wings beating hurriedly, from kilometers away; peacocks display a devastating corona of blue and green and the peahens are all aflutter; competing pollen grains extrude tiny tubes that race each other down the female flower's orifice to the waiting egg below; luminescent squid present rhapsodic light shows, altering the pattern, brightness and color radiated from their heads, tentacles, and eyeballs; a tapeworm diligently lays a hundred thousand fertilized eggs in a single day; a great whale rumbles through the ocean depths uttering plaintive cries that are understood hundreds of thousands of kilometers away, where another lonely behemoth is attentively listening; bacteria sidle up to one another and merge; cicadas chorus in a collective serenade of love; honeybee couples soar on matrimonial flights from which only one partner returns; male fish spray their spunk over a slimy clutch of eggs laid by God-knows-who; dogs, out cruising, sniff each other's nether parts, seeking erotic stimuli; flowers exude sultry perfumes and decorate their petals with garish ultraviolet advertisements for passing insects, birds, and bats; and men and women sing, dance, dress, adorn, paint, posture, self-mutilate, demand, coerce, dissemble, plead, succumb, and risk their lives. To say that love makes the world go around is to go too far. The Earth spins because it did so as it was formed and there has been nothing to stop it since. But the nearly maniacal devotion to sex and love by most of the plants, animals, and microbes with which we are familiar is a pervasive and striking aspect of life on Earth. It cries out for explanation. What is all this in aid of? What is the torrent of passion and obsession about? Why will organisms go without sleep, without food, gladly put themselves in mortal danger for sex? ... For more than half the history of life on Earth organisms seem to have done perfectly well without it. What good is sex?... Through 4 billion years of natural selection, instructions have been honed and fine-tuned...sequences of As, Cs, Gs, and Ts, manuals written out in the alphabet of life in competition with other similar manuals published by other firms. The organisms become the means through which the instructions flow and copy themselves, by which new instructions are tried out, on which selection operates. 'The hen,' said Samuel Butler, 'is the egg's way of making another egg.' It is on this level that we must understand what sex is for. ... The sockeye salmon exhaust themselves swimming up the mighty Columbia River to spawn, heroically hurdling cataracts, in a single-minded effort that works to propagate their DNA sequences into future generation. The moment their work is done, they fall to pieces. Scales flake off, fins drop, and soon--often within hours of spawning--they are dead and becoming distinctly aromatic. They've served their purpose. Nature is unsentimental. Death is built in.
— Carl Sagan, Shadows Of Forgotten Ancestors: A Search For Who We Are