Variations: Darwin's finches, sea barnacles and the side effects of antidepressants.

"It may metaphorically be said," Darwin wrote, "that natural selection is daily and hourly scrutinizing, throughout the world, the slightest variations; rejecting those that are bad, preserving and adding up all that are good; silently and insensibly working, whenever and wherever opportunity offers..." Variation is a principle of nature, without which natural selection could not operate, and life exist. Darwin believed that natural selection would make nature "more and more diversified." Variation occurs in the clutch sizes of birds, the color of hair and skin, the annual temperature, in language and speech, the direction of local Magnetic North and True North, and the variation of pathogens (antigenic variation). Antidepressants act as probes, burrowing into the deepest recesses of cells, and signaling physiological and pathological information to observers. They have at least forty side effects that are not only variations, but often paradoxes that would have fascinated Charles Darwin, who had the keenest interest in the variation of the beaks of finches and in sea barnacles.