III.] LIFE AND DEATH. II7 



before. Rejuvenescence has also been mentioned in con- 

 nection with the process of conjugation, but this is quite another 

 thing. It is quite reasonable, at least in a certain sense, to 

 maintain the connection of rejuvenescence with conjugation ; 

 for a fusion of the substance of two individuals takes place, 

 to a greater or lesser extent, in conjugation, and the matter 

 which composes each individual is therefore really altered. 

 But in simple encystment, rejuvenescence can only be under- 

 stood in the sense in which we speak of the fable of the 

 Phoenix, which, when old, was believed to be consumed by 

 fire, and to rise again from its own ashes as a young bird. 

 I doubt whether this idea is in agreement with the physiology 

 of to-day, or with the laws of the conservation of energy. 

 It is easy to pull down an old house with rotten beams and 

 crumbling walls, but it would be impossible to build it anew 

 with the old material, even if we used new mortar, repre- 

 sented in Gotte's hypothesis by water and oxygen. P^or 

 these reasons I consider the idea of rejuvenescence of the en- 

 cysted individual to be contrary to our present physiological 

 knowledge. 



It is much more simple and natural to regard encystment 

 as adapted for the protection of certain individuals in a colony 

 from destruction by being dried up or frozen, or for the pro- 

 tection of the individual during multiplication by division, when 

 it is helpless, and would easily fall a prey to enemies, or to 

 secure advantages in some other way\ The case of Ac- 

 tinosphaeriiwi mentioned by Gotte, clearly demonstrates that 

 rejuvenescence of the individual is not the only event which 

 happens during encystment, for this would scarcely require 

 six months. The long duration of latent life, from summer to the 

 next spring, clearly proves that encystment is of the highest 

 importance for the species, in order to maintain the life of the 

 individual through the dangers of an unfavourable season 2. 



^ Professor Gruber informs me that among the Infusoria of the harbour 

 of Genoa, he has observed a species which encysts upon one of the free- 

 swimming Copepoda. He has often found as many as ten cysts upon 

 one of these Copepods, and has observed the escape of their contents 

 whenever the water under the cover-glass began to putrefy. Here 

 advantage is probably gained in the rapid transport of the cyst by the 

 Crustacean. 



^ The views of most biologists who have worked at this subject agree 



