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reliable, are only metamorphosing vegetable structures. In- 

 deed, Kiitzing, well known in this department, has sought refuge 

 from the many perplexing questions here arising, in the view 

 which he has advanced, that the distinctions so widely separating 

 the animal from the vegetable world, with all the higher forms, 

 cease to exist with these very lowest creations. Nevertheless, 

 careful observations are made, from year to year, upon the spe- 

 cial forms ; and, although their history is rarely made out com- 

 pletely, yet the bearing of these studies clearly indicates that, at 

 last, the class Infusoria may be swallowed up in other classes of 

 both the vegetable and the animal world. Thus, I scarcely need 

 say, that already most of the Polygastrica^ as the Bacillaria, 

 the Diatomacece^ the Vihrionia, and many other families are, 

 indisputably, only vegetable forms; and the individual character 

 of Vorticellina, Trachelina, KaJpodea, and others, all of which 

 have held as firm a position as any in this whole class, seems to 

 me to be quite disputable. Many of them have been shown to be 

 the imperfect, immature forms of Acalephs, Hehninths^ Crusta- 

 ceans, and other classes of the Invertebrata. That they may well 

 be such, is now more easily comprehended since we understand, 

 in part, the phenomena of the so-called Alternation of Gene- 

 ration, which show that an animal form may live, for a period, inde- 

 pendently, or as a parasite, and without any special resemblance 

 to the ulterior condition it is finally to attain. In this interme- 

 diate condition they may be, as ihey often have been, taken for 

 true adult, specific forms. They may be of a cell-like form, or 

 even to all appearances only simple, nucleated cells, freely mov- 

 ing about by means of cilia, and unless the microscopist is able 

 to watch them through all the phases of an entire generation to 

 the reproduction of their own by a new one, he is not justified, 

 I think, in describing them as distinct animals. But what are to 

 constitute the phenomena of reproduction in these lowest crea- 

 tions ? Generation and fissuration, so much insisted upon here, 

 I do not regard to have been satisfactorily shown to exist ; at 

 least, as these processes occur among the lower invertebrates. 

 There are, to be sure, in Volvox, PoramcEciiim, Bursaria, and 

 others, phenomena of a somewhat similar character, apparently ; 

 but much of this so-called fissuration, by which new cells are 

 formed, appears to me to belong to the process of real vitelline 



