VITALITY 



283 



continue indefinitely (see Child, Hartmann, Belaf, Jollos, etc.). 

 Such is the explanation that I would give of continued life without 

 fertilization of animal flagellates, aided here possibly by changes 

 which may take place during the periods of encystment. On the 

 same basis we find an explanation of the long-continued isolation 

 cultures without fertilization of organisms which, under usual 

 conditions, undergo fertilization. Some types of organization are 

 evidently able under appropriate conditions of the environment to 

 return to the same labile organization after each division. Such 

 types would thus have a prolonged asexual cycle, possibly, as 

 Enriques asserts, as long as the observer cares to continue the 

 culture. In such cases it is not improbable, as M. Robertson (1929) 



Fig. 145. — Paramecium caudatum in a period of depression and recovery by treat- 

 ment with salts. (After Calkins, i 



concludes, that the environment is so stabilized that its stimuli do 

 not call out the cyclical changes which might be expected with an 

 irritable and adaptable protoplasmic organization. 



If, however, reorganization as effected by division does not leave 

 the protoplasm in its original labile condition, then inter-divisional 

 activity of the progeny starts with a different organization than did 

 the previous generation and this, continued generation after genera- 

 tion produces an accumulative effect. This is manifested by physi- 

 ological activities and by structural modifications not shown before. 

 The decline in the division-rate for example may indicate that the 

 living substances are becoming relatively stabile and more and 

 more irreversible in phase, as was the case with one race of Para- 

 mecium caudatum in which the individuals became homogeneous 



