CLASSIFICATION OF MEDUSiE. 83 



3d. The case of Coryne, Sfc. 



a. The zoophyte produces medusae by gemination. 



b. The medusse produce eggs. 



c. The eggs produce infusoria. 



d. The infusoria fix and become zoophytes. 



4th. 27ie case of Li%%ia and Sarsia. 



a. The medusa produces medusae by gemmation. 

 (The remaining stages as yet unobserved, but probably) — 



b. The medusae produce eggs. 



c. The eggs produce infusoria. 



d. The infusoria fix as polypes, and produce medusae. 



With such facts — unquestioned facts — before us, it seems to me that we have no choice 

 between theories, and that we must admit the idea of " Alternation of Generations" to be true. 

 Steenstrup was assuredly the first naturalist who announced that idea as a general fact 

 dependent on a law. "The special subject of this Essay" — I quote from the author's preface 

 to the German version of his celebrated work, as translated by Mr. Busk — " is the funda- 

 mental idea expressed by the words ^Alternation of Generations^ or the remarkable, and 

 till now inexplicable, natural phenomenon of an animal producing an oflfspring, which at no 

 time resembles its parent, but which, on the other hand, itself brings forth a progeny, which 

 returns in its form and nature to the parent animal, so that the maternal animal does not meet 

 with its resemblance in its own brood, but in its descendants of the second, third, or fourth 

 degree of generation ; and this always takes place in the different animals which exhibit the 

 phenomena in a determinate generation, or with the intervention of a determinate number of 

 generations. This remarkable precedence of one or more generations, whose function it is, as 

 it were, to prepare the way for the later succeeding generation of animals destined to attain 

 a higher degree of perfection, and which are developed into the form of the mother, and 

 propagate the species by means of ova, can, I believe, be demonstrated in not a few instances 

 in the animal kingdom." 



The main position thus stated appears to me sound and true : the assumption of a definite 

 regularity in the alternations is a secondary and non-essential one, and true probably when 

 disturbing conditions are not at work. But numerous observations, especially those of 

 Dalyell,* Reid,t and Price,:}: show that under pecuhar circumstances, in what may be termed 

 unnatural situations, the polype generations may go on continually producing polype genera-, 

 tions ; and those of Sars and myself, on the other hand, that a Medusa generation may go on 

 producing Medusa generations ; although, under normal conditions in each instance, there is 

 every reason to suppose that zoophytic and Medusoid forms would have regularly alternated. 



I am anxious to bear testimony to the value of the idea enunciated by Steenstrup, 

 because I believe it has given a strong impulse in a right direction to Invertebrate Zoology. 



* Remarkable Animals of Scotland. 



t Annals of Nat. Hist., Second Series, vol. i. 



X British Association Report, 1846, p. 86. 



