No. 644] 



ARRESTED EVOLUTION 



leads to an internal disturbance, thereby correcting the 

 conditions of uniformity. 



Applying these conclusions to our case of the persis- 

 tent types it could be conceived that the reduction of fer- 

 tilization to rare intervals, or its entire suppression, in 

 the numerous persistent types that reproduce by fission, 

 budding, parthenogenesis or hermaphroditism, produces 

 a perpetual senescent condition that while not leading 

 to death as in the rapidly dividing and sensitive Para- 

 mecium, finds its expression in the rigidity of the foi*ms, 

 recognizable in their lack of response to external stimuli 

 and of further evolution, i.e., in their persistence. Or in 

 other words, infrequency or entire lack of rejuvenation 

 through fertilization favors the persistent condition, at 

 least among those persistent terminals that do not live 

 under" stable physical conditions. Those living under 

 stable conditions may require fertilization as a necessary 

 rejuvenating process counteracting progressive senes- 

 cence and final extinction through lack of external stimu- 

 li. It would then appear that these lower modes of re- 

 production and very stable external conditions could not 

 verj^ well exist together. 



However, as pointed out by Kellicott, there has been 

 an evolution both of the process and of the consequences 

 of fertilization, and the various possibilities as to the 

 significance of fertilization are not mutually exclusive. 

 It is therefore possible that the large percentage of per- 

 sistent types among forms with more or less suppressed 

 fertilization finds its explanation in some cases in the 

 resulting lack of variation, in others in the resulting 

 senescent and rigid condition of the race, and in still 

 others it may be sought in the process of heredity, con- 

 nected with fertilization. This last possibility will be 

 dealt with in the following chapter. 



Reductiox of Factors to FrxDA>rEXTAL Causes 

 The investigation of the various croujis of persistent 

 types has indicated that there are a variety of factors 



