LIFE AND DEATH. 121 



absent, and in which reproduction consisted only in an often- 

 repeated process of rejuvenescence among existing individuals, 

 without any increase in their number. Such a condition is in- 

 conceivable because it would involve a rapid disappearance of the 

 species, and the whole consideration clearly shows us that division 

 of un-encysted individuals must have existed from the first, and 

 that this, and not a vague and mysterious rejuvenescence, has 

 always been the real and primitive reproduction of the Mono- 

 plastides. The fact that encystment does not always lead to the 

 division of the contents of the cyst proves, in my opinion, that not 

 reproduction but preservation against injury from without, was the 

 primitive meaning of encystment. It is possible that at the present 

 time there are but few Monoplastides which are able to go through 

 an infinite number of divisions without the interposition of the 

 resting condition implied by encystment ; although it has not yet 

 been demonstrated for all species 1 . But it is not right to conclude 

 from this that there is an internal necessity which leads to encyst- 

 ment, that is to say to identify this process with rejuvenescence. It 

 is much more probable that encystment is merely an adaptation 

 to continual changes in the external Conditions of life, such as 

 drought and frost, and perhaps also the want of food which arises 

 from the over-population of small areas. The same phenomenon is 

 known in certain low Crustacea the Daphnidae which possess 

 an ephippium or protective case for their winter-eggs. This case is 

 only developed after a. certain definite number of generations has 

 been run through, an event which may happen at any time in 

 the year in species living in pools which are liable to be often 

 dried-up ; but only in the autumn in such as live in lakes which 

 are never dry. No one ever doubted that the periodical formation 

 of the ephippium in certain generations was an adaptation to 

 changes in the external conditions of life. 



Even if the process of rejuvenescence in the Monoplastides were 

 really equivalent to the death of the higher animals, we could 

 not conclude from this that it is necessarily associated with re- 

 production. Encystment alone is not reproduction, and it first 



1 Among the Rhizopoda encystment is only known in fresh-water forms, and 

 not in a single one of the far more numerous marine forms which possess shells (see 

 Biitschli, ' Protozoa,' p. 148) ; the marine Rhizopoda are not exposed to the effects 

 of desiccation or frost, and thus the strongest motives for the process of encystment 

 do not exist, at least among forms possessing a shell. 



