IN his concern with re-delivering Kammerer's work, Koestler has assumed that if Kammerer's experimental integrity were accepted, his Lamarckian conclusions would be reasonable1, a flaw unnoticed by a recent review2. However, known phenomena are sufficient to explain Kammerer's results. Most controversy followed work on the terrestrial midwife toad, eggs from which were immersed in water for several generations. Most eggs died but eventually some male descendants were produced with nuptial pads, characteristic of aquatic anurans. Kammerer suggested acquired inheritance, but intensive selection had occurred and a third explanation is that an environmentally-triggered gene switch had operated.