River Out of Eden reveals Dawkins to have a lighter, witty side as well. The central metaphor of Dawkins's new book is a river of genes, flowing through geological time rather than space. Each river is a population of genes held together by sexual interbreeding. The river may fork in time. Each branch slowly drifts apart to give rise to a new species.
Some branches dry up in extinction.
Others split repeatedly, generating the 30 million branches that exist today. The River of Eden has a majestic ring with its borrowed Biblical authority. Its illumination of biology is, however, somewhat limited. After all, rivers flow downhill, joining together, becoming larger and deeper, the reverse of the effect of diversifying speciation. The river metaphor hardly illuminates this old conundrum.