lOOli.J NATURAL SCIE>'CES OF PIIILADELnilA. 201 



There remain to be considered the cases of alternation of genera- 

 tions (metagenesis), where the individuals of one generation differ 

 struct urally from those of the next generation, as in many Hydro- 

 zoa, where from the polyp is produced a medusa, from the medusa 

 a polyp. Each generation here is reproductively active, so that the 

 state of such activity does not decide for us whether the classification 

 should consider primarilythe polyp or the medusa condition. Kow, 

 in the Hydrozoa many variations are fouud in the cycle of genera- 

 tion. Hydra can produce new polyps asexually, by buddiug, and 

 also by fertilized eggs, i. e., sexually; here there is change of gen- 

 eration, but not polymorphism of the successive generations. In 

 certain Hydromedusse there is true change of generation, with free- 

 swimming medusse budded off from the polyp individual (" meta- 

 genetic " Hydrozoa j; in others the change of generation is not so 

 clearly marked, but the medusoid individuals remain attached to 

 the polyp as medusoid buds or gonophores (Hydrozoa with masked 

 change of generation) ; in still others there is no polyp stage, but 

 a medusa develops from the egg, and the latter becomes another 

 medusa ( " hypogenetic " forms, with suppression of one genera- 

 tion). Again, while most medusse are incapable of asexual divis- 

 ion, and reproduce by ova and spermatozoa, certain of them repro- 

 duce also asexually, by budding. In the Hydrozoa is fouud a fund 

 of differences in the degree and kind of generation, such as no 

 other metazoan group evinces (not even the Trematoda), and on 

 this account they furnish a broad field for examination. Now in 

 such Hydrozoa as show no change of generation, the individual 

 should be classified at its last stage of reproductive activity, in 

 order to represent its degree of racial development. Where true 

 change of generation occurs and the different generations aie sepa- 

 rated individuals, such individuals should be classified as stand at 

 the end of each metagenetic cycle. This is a consequence of the 

 argument given before, to show that the organization as a whole is 

 most advanced and perfect, and therefore expresses most fully the 

 phyletic divergence of the organism from its ancestors, at the time 

 of the last period of reproduction. Where the change of genera- 

 tion appears less pronounced, by reason of the egg-producing indi- 

 viduals remaining attached as gonophores to the body of the polyp, 

 the same conclusion would hold — i. e., that the egg-producing indi- 

 vidual should be tlie one classified, provided tliat such a medusoid 



