84 Studies in the Theory of Descent. 



dividual do not in these cases serve as the point 

 of departure for alternation of generation, but the 

 different contemporary forms living gregariously 

 into which the species has become divided through 

 functional differentiation of the various individuals 

 of the same stock. Individuals are here produced 

 which alone acquire the power of sexual reproduc- 

 tion, and metagenesis is thus brought about, these 

 individuals detaching themselves from the stock 

 on which they originated, while the rest of the 

 individuals remain in combination, and retain the 

 asexual mode of propagation. No sharp distinc- 

 tion can be otherwise drawn between this and the 

 cases previously considered. 4 The difference con- 

 sists only in the whole cycle of reproduction being 

 performed by one stock ; both classes have the 

 common character that the different phyletic stages 

 never appear in the same individual (metamor- 

 phosis), but in the course of further phyletic de- 

 velopment metagenesis at the same time arises, 

 i.e. the division of these stages among a succession 

 of individuals. We are therefore able to distin- 

 guish this primary metagenesis from the secondary 

 metagenesis arising from metamorphosis. 



4 The idea that alternation of generation is derived from 

 polymorphism (not the reverse, as usually happens ; i. e. poly- 

 morphism from alternation of generation) is not new, as I find 

 whilst correcting the final proof. Semper has already ex- 

 pressed it at the conclusion of his interesting memoir, " Uber 

 Generationswechsel bei Steinkorallen," &c. See " Zei-tschrift f. 

 wiss. Zool." vol. xxii. 1872. 



