CHAP'l'ER 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 {Sal pa) 

 that a solitary form gave birth to embryos of a different char- 

 acter, connected 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 progress of marine zoology and the study of 

 parasitic worms gave naturalists like Sars, Dalyell, Loven, Von 

 Siebold, and Leuckart, early glimpses of many alternations in 

 life-history, but Steenstrup was the first to generalise the results. 

 This he did (1842) some twenty years after Chamisso, in a 

 work entitled " On the Alternation of Generations ; or. The 

 Propagation 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 phenomena of an animal producing 

 an offspring, which at no time resembles its parent, but which 

 itself brings forth a progeny that returns in its form and nature 

 to the parent." The interpolated generation he distinguished 

 by the name of "Amme" or "wet-nurse." In 1849, Owen 

 submitted Steenstrup's essay to stern criticism, rejecting especi- 

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

 and proposing to explain what he also called "alternation 

 of generations," along with parthenogenesis and other pheno- 

 mena, by the supposition of a residual germ force or spermatic 

 power in the cells of the apparently asexual offspring. In 

 this he partially prophesied the modern conception of a 

 residual persistent germ-plasma. Soon afterwards Leuckart 



