time: the refreshing river 



recognition of the irreversibility which exists both in the second law 

 and in biological evolution. The pioneer work of the palaeontologist 

 Dollo^ led to the generalisation which has since been called by his 

 name. An organ which has been" reduced in the course of evolutionary 

 development never again reaches its original importance, and an organ 

 which has altogether disappeared never again appears. Further, if in 

 connection with adaptation to a new environment (such as aquatic, 

 terrestrial, or aerial life) an organ is lost which was valuable in the 

 previous environment, and if, as often happens, a secondary return 

 to the previous environment occurs, this organ will not reappear. In 

 its place some other organ will form a substitute. In a word, evolution 

 is reversible in the sense that structures which have been gained can 

 be lost, but it is irreversible in the sense that, once lost, these structures 

 can never be regained. Since Dollo's time it has been shown that his 

 generalisations hold good, not only for the morphological body 

 structures which he elucidated, but also for numerous physiological 

 and biochemical adaptations.^ 



In explanation, Dollo himself did not go much further than a 

 vague appeal to the "indestructibility of the past." "In the last 

 analysis," he wrote, "it is, like other natural laws, a question of 

 probability. Evolution is a summation of determined individual 

 variations in a determined order. For it to be reversible, there would 

 have to be as many causes, acting in the inverse sense, as those which 

 brought about the individual variations" (mutations, as we should 

 say to-day) "which were the source of the prior transformations and 

 their fixation. Such circumstances are too complex for us to suppose 

 that they ever exist." This idea has something in common with the 

 second law of thermodynamics. The universe is always passing from 

 less probable to more probable states. 



The position was further elaborated in the brilliant and unique 

 book of Lotka, who defined evolution as the history of any system 

 undergoing irreversible changes, and practically identified it with the 

 second law. But although his discussion is one of the three or four 

 greatest contributions to biological thought of the present century, 

 he never really faces the problem that the second law involves a 

 decrease, and the law of evolution an increase, of order. His pleasure 



^ L. Dollo, numerous papers, some of the more important of which are referred to 

 in Biol. Rev., 1938, 13, 225. 



^ By J. Needham, "Contributions of Chemical Physiology to the Problem of 

 Reversibility in Evolution," Biol. Rev., 1938, 13, 225. 



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