32 ACALEPIIS IN GENERAL. Part I. 



lephs, by tlie iniexpectod views under which he presented the facts recoi-ded )\y 

 his predecessors, so much so tliat a new era may be dated from the pubhcation 

 of his httle work, for the history of the Acalephs not only, but also ior the 

 inverteljrate animals in general. The wdiole aim of Steenstrup's investigations is 

 fully expressed in the title of his work, ''■ On the alternation of generations." ' He 

 expres.'^es himself uj)on that point very clearly and in very few Avords, in his 

 prefoce : " The substance of this paper is the fundamental idea expressed by alter- 

 nation of generations. It is a, remarkal)le, and, thus far, unexplained phenomenon 

 of nature, that an animal brings forth ;i Ijrood neither similar, nor growing to be 

 similar, to the parent, but differing from it, and producing liy itself anothei' lu'ood, 

 that returns to the I'orm and relations of the mother animal, in such a manner 

 that a mother annual does not rear the like of itself, Ijut reappears only in its 

 descendants of the second or third or a following generation ; and this appears 

 always, in difi'erent animals, in a definite generation, and with definite intermediate 

 generations." 



Next to Sars and Steenstrnp, Sir John Dalyell has been most successful in tracing 

 the phenomena here alluded to. This author, Avdiom Ed. Forbes, Avith his (juick 

 appreciation of every kind of merit in others, justly calls the Spallanzani of Scot- 

 land, has done more for the elucidation of the early history of the Medus;v than 

 any other writer, although, from want of method in his descriptions and owing to 

 his disregard of the modern systematic forms of presenting such subjects, his obser- 

 A'ations are onl}- intelligiljle upon very careful perusal, and not available for a 

 connected study of the gradual growth and successiAc phases of their development. 

 For instance, it has not occurred to Sir John Dalyell, that what he calls "Hydra 

 tul)a" may ])e the oflspring of scA'eral distinct genera of Medusa\ and so he con- 



' Steenstiui- (,Tun. .Iai'ETIts Sm.), Ueber <lrii coiisidcr ns .1 simple metanini-pliosis of :i liima, 



Gencratiuiiswrcliscl, oder die Fortpllanzuiig mid but as llic metam(ir]di(isis nf .a nrw ui'iua-ation 



Entwickrlmig diiixdi abwiadistdnde (nairratiiauai, derived IVdiu the progeny of a Jledusa. lie goes 



fibersetzt von C. II. Lorenzen, Co]:ieidiagen, LS42, even so far as to eoiisider this mode of repro- 



8vo. lig. English translation by George Busk, pul)- dnetion as a eas,. parallel to tliat of Salpa, tirst 



lished by the Eay .Soeiely : On the Alternation of observed by Chaniisso, and to vindieate the aeeu- 



(ienerations, Lomhjn, 181."., Svo. tig. Although the racy of the inveMigations of the genial jioel. Tlins 



question of alternate generations is for the tirst the groimdwork upon which the theory of alternate 



time dislinetly raised by .Steenstruji, and i.resented generations eould be reared is .already laid out by 



by him as a phenomenon oeeurring not only among Sai's, when lie says (Wiegmann's Arehiv, 1S41, 



Radiates, but also among Mollusks and Articulates, ^,,l. 1, p. I'S), "It is, therefore, not the larva, or 



it would be doing injustice to Sars not to remember, (he indivi.lual hatched from the egir, that dev<-Iops 



tlmt, as far as the Medusa> are conci'rneil, he hail into a perfect Acale]di, but tlie brood arising from 



already correctly ap]ireciated tlie character of ihi' this larva by transverse division." 

 development of Aurelia aurita, wdiich he does not 



