substantiate this conclusion Baer extracted material from 

 Steenstrup's ftBER DEN GENERATIONSWECHSEL , 26 and other 

 sources. He rated highly Steenstrup's work, in which the 

 wide distribution of the phenomenon of alternation of 

 generations is shown, which indicated its general biological 

 significance. Baer turned his attention to the fact that 

 Steenstrup had stated almost nothing about the alternation 

 of generations in plants. Attempting to fill this gap, 

 Baer cited many clear examples related to sporophytic and 

 phanerogamous plants. These examples show that the alterna- 

 tion of two forms of reproduction — sexual and asexual (i.e. 

 occurring without fertilization) — is inherent to the entire 

 organic world. The alternation of generations, during which 

 the ability to reproduce asexually appears (Baer related it 

 not only to division, as in budding and other forms of 

 vegetative reproduction, but also to development from un- 

 fertilized ova, i.e. parthenogenesis and paedogenesis) , he 

 considered not an exception but a rule. The essential dif- 

 ference between sexual reproduction and all forms of asexual 

 reproduction Baer concluded from the fact that sexual 

 reproduction takes place only in the adult condition, while 

 asexual reproduction occurs in different stages of ontogenesis. 

 In particular, adult individuals are capable of parthenogenesis 

 and reproduction by means of fertilization, while the paedo- 

 genesis of dipterous insects occurs only in larvae. 



The ability to reproduce in a sexually immature condition 

 is characteristic also for other invertebrates — Coelenterata, 

 parasitic Platyhelminthes, and Tunicata; therefore, all 

 sexually immature forms of different animals — rediae, cercariae, 

 brachiolariae, tornariae, strobiliae, scyphistomae, and so 



on—must also be called larvae. 



In comparing the individual life of the organism — i.e. 

 its development with reproduction — with the life of the species, 



26. J. J. Steenstrup, (JBER DEN GENERATIONSWECHSEL, ODER 

 DIE FORTPFLANZUNG DURCH ABWEICKENDE GENERATIONEN, 

 EIN EIGENTHUMLICHE FORM DER BRUTSFLAGE IN DEN 

 NIEDEREN THIERCLASSEN (Copenhagen, 1842), 140 pp. 

 This edition represents the translation of Forentsen 

 from a manuscript published at the same time in Danish. 



477 



