Stephen Jay Gould: What we really amended was a subsidiary, but very strong, belief of Darwin’s, and most of his century and most of subsequent evolutionary thought, that change in the large-scale history of life should be cumulative, slow, steady and gradual, based on the transformation of entire populations. I don’t think we amended the basic statements of the theory of natural selection, which is a different area, but that’s still an important part of Darwinian traditions. The theory of punctuated equilibrium argues that most species are stable, most of the time. And that’s true, look at human beings for example. People always ask me, “Where is human evolution going?” But the only answer to that, and it’s not a special answer for humans but for all species, is that successful species don’t go anywhere. They tend to be stable for long periods of time. We’ve only been around for 200,000 years or so, so our period of stability is far from over, that’s the normal state of species. When evolutionary change occurs, it occurs co-incident with an event of speciation, that is, a branching. And although speciation isn’t overnight — it would seem slow by the scale of our lives — it may take thousands of years. Thousands of years, compared to the millions of years of subsequent stability, is a tiny fraction of one percent. So that even an event that is the branching of speciation, which would seem slow by the scale of our lives, in geological parlance it’s instantaneous. And that’s the punctuation in punctuated equilibrium. Punctuated equilibrium argues a geological perspective. Evolutionary change is concentrated in geologically sudden — but actually slow by the scale of our lives — bursts of speciation, and then stability’s the norm for species in-between. That has a lot of implications for evolutionary theory that are quite different from conventional views, including the idea that evolutionary trends are not pushing a ball up an inclined plane by slow and steady and continuous adaptations. More like climbing a staircase, and the reasons why you take a lateral step on the staircase are very different from why you slowly and steadily push the ball up.