CHAPTER XXVI 



ALTERNATION OF GENERATIONS. 

 PARTHENOGENESIS 



Alternation of Generations is usually defined as the 

 appearance in any Individual of two kinds of component 

 organism, actually or virtually independent, one of which 

 is sexless and whose chief functions are those of nutrition 

 or support, while the other develops sexual organs with 

 a view to the perpetuation of the species. But while such 

 a definition is correct as far as it goes, it only describes 

 one limited aspect of a phenomenon exhibited in all living 

 Individuals. For our contention is that Alternation of 

 Generations in the generally accepted sense is simply one 

 aspect of the everywhere-obtaining Alternation of Inter- 

 mediate and Terminal Cycle-stages. 



In itself the term " Alternation of Generations " is not 

 appropriate to the phenomena exhibited, for these take 

 place within the Individual, which is one generation and 

 no more ; nor are the asexual organisms produced by any 

 act of generation, but by gemmation or fission. This is 

 obvious if we turn to the alternation as it is displayed by 

 a hydrozoal colony. 



The development of the normal Individual is for the 

 restoration of the lost personalities of the originally com- 

 bining elements, and it is just because this restoration is 

 a very gradual process that cycle-stages of the Individual 

 in which it is going on can be recognised, to contrast with 

 the terminal or sexual stages marking restoration. 



Naturally, where the Individual develops in cellular 

 discontinuity the sexless or intermediate cycle-stages are 

 evanescent, and the alternation is not strikingly apparent ; 

 but where Continuity unites and arrests definite numbers 

 of cell-stages of the Individual to form zooids or higher 



240 



