The fall of the nineteenth century abounded in discoveries of natural laws, that is, strict and unequivocal rules of the development of life. These laws stated for example that ontogeny of individuals repeats, or recapitulates, phylogeny of the species; that big and specialized forms originate in evolution from small and unspecialized ones, while the reverse is not true; that endothermic animals are larger-sized and more compact in cool than in warm climates, and so on. Louis Dollo, a Belgian palaeontologist, proposed the so-called law of irreversibility of evolution which stated that an organic structure lost in evolution cannot reappear with all the peculiarities of its design. This law seems indeed to be true but not particularly original. It was Heraclitus who first observed that one could not enter the same river twice. Dollo's law says essentially the same thing. The flow of evolutionary events combines so many processes in so many ways that past evolutionary events cannot recur exactly. “What is done, is done forever”. Time itself introduces asymmetry into the evolutionary process.