Fifty Billion Species

The number that anchors Segal's foreword and epilogue is Mayr's: approximately fifty billion distinct species have existed on Earth across four billion years. Exactly one of them produced the capacity for symbolic thought, recursive language, and abstract reasoning. One in fifty billion. The number is not rhetorical. It is Mayr's empirical argument against the thesis that intelligence is a general tendency of evolution. If high intelligence were favored by natural selection under common conditions, it should have evolved recurrently — the way eyes evolved independently more than forty times, the way flight evolved at least four times. It has not. The absence of convergence is evidence that intelligence is either not favored by selection or extraordinarily difficult to achieve.