40 THEORY OF ORGANIC EVOLUTION. 



in all others they show differences which become 

 continually greater and more numerous. Thus 

 alternation of generations in cells exists, because 

 the successive generations become more and 

 more complicated at each succeeding period. 

 Among these periods the ontogenetic period or 

 ontogeny embraces all generations from one cell to 

 the return of the exactly similar kind of cell. In 

 the lowest forms of cell differentiation the cells of 

 successive generations are all independent; the 

 ontogenetic period consists of a cycle of generations 

 of unicellular plants. Later the cell generations of 

 an ontogeny are united by parts into plant individ- 

 uals; the ontogenetic period consists of a cycle of 

 multicellular and unicellular, or only of multicellular 

 plant generations. If all the cell generations of an 

 ontogenetic period have been united into a single 

 individual, the successive plant generations are alike 

 and alternation of generations has ceased. 



The unlikeness of the generations arises either 

 from inner causes of temporary differentiation 

 alone, or by temporary differentiation which receives 

 a definite imprint by the change of seasons. But in 

 the latter case the characteristic of adaptation is 

 again lost in the course of the phylogeny and alter- 

 nation of generations follows then without regard 

 to the season. If the given adaptation is united in 

 the lower plants with alternation of generations 

 during the ontogenetic periods, one of the unlike 



