CONTRIBUTIONS OF CHEMICAL PHYSIOLOGY TO THE PROBLEM OF REVERSIBILITY IN EVOLUTION

Abstract

Summary1. It has been the object of this article to draw attention to biochemical and physiological evidence which supports the generalization known as Dollo's Law, side by side with palaeontological and morphological evidence.2. Evolution is reversible in the sense that structures or functions once gained can be lost, but irreversible in the sense that once lost they can never be regained. Loss of properties is illustrated, (a) by the enzymic equipment of bacteria and Protozoa, (b) by the relative uraemia of marine and fresh–water elasmobranch fishes, (c) by the uricotelism of terrestrial and fresh–water molluscs. Failure to regain properties once lost is illustrated, (a) by the cleidoicity of the eggs of aquatic birds, (b) by the glomerular status of the kidneys of marine and fresh–water teleostean fishes.3. Why there should be this irreversibility in evolution is a problem of fundamental interest. If the second law of thermodynamics is responsible, we may ponder the curious paradox that while on the one hand there is in evolution an increase of organization as ever higher groups and phyla appear, there is also a diminution of organization associated with the shuffling process of entropy increase‐the only thing nature can never undo.