122 LIFE AND DEATH. [III. 



encystment ; although it has not yet been demonstrated for all 

 species'. But it is not right to conclude from this that there 

 is an internal necessity which leads to encystment, 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 Daph- 

 nidae — 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 reproduction. Encystment alone is not reproduction, and 

 it first becomes a form of reproduction when it is associated 

 with the division of the encysted animal. Simple division was 

 the true and original form of reproduction in Monoplastides, 

 and even now it is the principal and fundamental form. 



Hence we see that among the Monoplastides reproduction is 

 not connected with death, even if we accept Gotte's view and 

 allow that encj^stment represents death. I shall return later 

 on to the relation between death and reproduction in the 

 Metazoa ; but the question first arises whether encystment, 

 if it is not death, has any analogue in the higher animals, and 

 further whether death takes that place in their development 

 which is occupied by encystment in the Monoplastides. 



Among the higher Metazoa there can be no doubt as to what 



^ 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 Biitschh, ' 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. — "-'^e:-^^ 



