ALTERNATION OF GENERATIONS. 20I 



attempted to treat all as cases of metamorphosis, thereby greatly 

 extending the meaning of that term. The labours of some of 

 the foremost naturalists have both extended Steenstrup's obser- 

 vations and rendered them more precise. We now know that 

 the phenomenon is of wider occurrence than was at first sup- 

 posed, and also that the title has been unduly extended to cover 



Diagrammatic representation of alternation of 

 generations, tis, asexual generation ; s, 

 sexual generation. 



II. Shows alternation of asexual («^) 

 and sexual {s) generations. 



In I. the sexual is becoming increas- 

 ingly subordinated to the asexual (as in 

 flowering plants). 



In III. the asexual is increasingly 

 subordinated to the sexual (in mosses). 



several entirely different sets of facts. It is necessary, therefore, 

 to notice the various forms which the rhythm of reproduction 

 may take. 



§ 2 . The Rhythm between Sexual and Asexual Reproduction. — 

 The clearest case to start with is that of many hydroids. 

 A sessile, plant-like zoophyte, which buds off numerous nutritive 

 persons, produces in the warm months modified individuals 

 which are set adrift as medusoid persons. Unlike the hydroid 

 which bore them, these become sexual; and from their fertilised 



