212 Proceedings of the Royal Pliysiccd Society. 



stability and stability of anabolism. Conjugation, fertilisa- 

 tion, and crossing would be attempts further and further 

 afield to gain this result. The continual loss of these powers 

 through specialisation would be aggravated by restriction in 

 this field, and might be produced in the first place com- 

 paratively rapidly through too close in-breeding, and in the 

 second place slowly in geological time through natural 

 selection restricting the field of variation within narrower 

 and narrower limits. 



Dr Ash worth has kindly pointed out to me one of a series 

 of papers called " Studies on the Life-History of Protozoa," 

 by Mr Gary N. Calkins, which have appeared in the 

 Biological Bulletin of the Marine Biological Laboratory, 

 Woods Holl, Mass. From experiments on cultures of Para- 

 mceciurii caudcitum, Mr Calkins has come to the conclusion 

 that rejuvenescence in the Maupas sense can be brought 

 about without the aid of conjugation by means of a suitable 

 change in the nutrition when depression of vitality begins 

 to show itself. He believes that the rejuvenescence induced 

 by conjugation of forms that have lived under rather 

 different conditions can be accounted for on the grounds of 

 chemical differences in the composition of the protoplasm 

 of the different individuals. The importance of these results 

 is evident, as showing that there may be a relation between 

 conjugation and metabolism in the production of rejuvenes- 

 cence. Mr Calkins decides " to side with Weissmann in the 

 contention that these organisms have the potential of end- 

 less existence, and this without conjugation. When other 

 things fail, the organisms may resort to conjugation, but we 

 may consistently doubt whether this mode of recuperation 

 for Protozoa is as widespread in nature as hitherto 

 supposed." From the point of view of the other side, it 

 might well be asked why conjugation ever came into exist- 

 ence unless it was necessary for the persistence of the race. 



In a few words I will summarise the hypotheses put 

 forward. 



1. That plants and animals have diverged in their evolu- 

 tion owing to a difference in their kind of anabolism. 



2. That in plants the specialisation has been dominated 



