Darwin's On the Origin of Species linked processes observable in the present to patterns in the fossil record and formulated a coherent theory of evolution by natural selection. That link has been challenged by paleontologists on the grounds that the fossil record actually demonstrates long periods of little change in lineages interspersed with brief periods of relatively rapid change (punctuational equilibria). Stephen Jay Gould has proposed that evolutionary theory should include processes at three separable tiers of time: ecological moments, geological time (millions of years), and mass extinctions. Whatever progress is made at the first tier can be undone by the processes at the second tier (by punctuational equilibria) or third tier.