AL TERN A TION OF GENERA TIONS. 



185 



CHAPTER XV. 



ALTERNATION OF GENERATIONS. 



I. History of Discovery. — Early in the century the poet 

 Chamisso, accompanying Kotzebue on his circumnavigation of the 

 globe, observed in one of the locomotor tunicates (Salpa) that a 

 solitary iorm gave birth to embryos of a different character, con- 

 nected together in chains, and that each link of the chain again 

 produced a solitary form. Chamisso' s observation does not seem 

 to have been quite accurate, but 

 there is no doubt that he first called 

 attention to what is by no means an 

 uncommon fact, that an organism 

 produces an offspring very unlike 

 itself, which by and by gives origin 

 to a form like the parent. The pro- 

 gress of marine zoology and the 

 study of parasitic worms gave natu- 

 ralists like Sars, Dalyell, Loven, Von 

 Siebold, and Leuckart, early glimpses 

 of many alternations in life-history, 

 but Steenstrup was the first to gen- 

 eralize the result. This he did (1842) 

 some twenty years after Chamisso, 

 in a work entitled ' ' On the Alterna- 

 tion of Generations; or, The Propa- 

 gation and Development of Animals 

 through Alternate Generations, a 

 peculiar form of fostering the young 

 in the lower classes of animals." 

 From hydroids and flukes, he gave 

 illustrations of the ' ' natural phe- 

 nomena of an animal producing an offspring, which at no time resem- 

 bles its parent, but which itself brings forth a progency that returns in 

 its form and nature to the parent." The interpolated generation he 

 distinguished by the name of "Amine" or "wet-nurse." In 1849, 

 Owen submitted Steenstrup' s essay to stern criticism, rejecting espe- 

 cially the metaphorical name "nurse" as but a verbal explanation, 

 and proposing to explain what he also called ' ' alternation of genera- 

 tions ' ' along with parthenogenesis and other phenomena, by the 

 supposition of a residual germ-force or spermatic power in the cells 



Fk 



JL I 



61. — Diagrammatic representation of 

 alternation of generations, as, asexual 

 generation ; s, sexual generation. 



II. Shows alternation of asexual (as) 

 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). 



