THE NATURE AND VARIETIES, &C. 109 



IV. 



THE NATURE AND VARIETIES OF ALTERNATION 

 OF GENERATIONS. 



1. THE two modes of propagation by gemmae capable of 

 spontaneous evolution, and by germs dependent on impreg- 

 nation as has been already observed, are frequently asso- 

 ciated with no less remarkable diversities in the immediate 

 result of the development, leading in cases of periodic recur- 

 rence or alternation of the former, to a corresponding mu- 

 tation or alternation of dissimilar forms in the same species. 

 It is only, however, quite recently that this has been ad- 

 mitted generally by zoologists, who were not unnaturally in- 

 disposed to it, by observing the constant succession of like 

 to like in the higher animals. But since the time that 

 Chamisso called the attention of naturalists to the recur- 

 rence of two forms in Salpa, as a case of " Alternation of 

 Generations," analogous phenomena have been abundantly 

 brought forward in other tribes of organised beings. Steen- 

 strup was the first to group together these cases, applying 

 to them the same term as was used by the former naturalist, 

 for which some later writers would substitute that of Me- 

 tagenesis, proposed originally by Professor Owen. * 



In all these cases we may admit so much as this in 

 common that an act of digenesis recurs with greater regu- 

 larity in the interval of the acts of monogenesis ; and that 

 the products of the former differ more or less in their con- 

 formation from the organisms budded off in the latter. 



Hence, as both forms must be taken into account to 

 complete our idea of the perfection of the species, it has 

 been proposed to term them zooids in the case of animals, 



