INTRODUCTORY NOTE TO THE INFUSORIA. 19 



ach, but consist of a liomogcneous mass, are true animals. If these 

 premises are coi'rect, nothing will remain, as I conceive, for a distinctive 

 characteristic, but voluntary inotion. This, when j^ositive, is indubitable 

 evidence of any given form being of an animal character ; and it must 

 remain for each individual observer to determine what is, and what is not, 

 voluntary action, in each particular case. Moreover, even should Kclli- 

 ker's view of a stomachless animal prove correct, the inverse condition of 

 a true stomachal cavity being present, must, I think, be regarded as posi- 

 tive evidence of the animal nature of the form in question ; for this must 

 always be a distinctive characteristic of the two kingdoms, when present. 



In regard to the other point, What constitutes an animal ? observers are 

 very far from being agreed. Siebold, Ki'Uiker, and others, have taken 

 the ground that individual animal forms may be unicellular ; or, in other 

 words, that an animal may be composed of only a single cell.* This view 

 is principally due to Kclliker's observations and statements upon Gregari- 

 nae.t The facts are indeed striking, but the evidence does not appear to 

 me sufficient, as yet, to settle such a vexed and important question ; and 

 more especially so since Bruch X has raised the point of their belonging to 

 the Worms. But, aside from such grounds, I was led, some time since, 

 after considerable study of infusoria- forms, to venture an opinion quite at 

 variance with that just mentioned of Siebold and K'dliker. I then made 

 the following statement : In regard to the question. What characteristic in 

 organic animal matter shall constitute an individual ? I feel satisfied of 

 this much, — that cell processes, however closely interwoven they may be 

 with the expressions of individual life, cannot be considered as constituting 

 the ground-work of its definition. "v* This statement was made more than two 

 years since ; and subsequent observations, some of them of a special char- 

 acter, have not led me to a change of opinion. True individual animal 

 life seems to involve a cycle of relations not implied in simple cells ; in 

 other words, these last must always lose their character as such, in a definite 

 form which belongs to the individual. 



On this account I regard the Infusoria proper, or those which have been 

 shown to be of an undoubted animal character, as in a completely transition 

 state ; and, although it may be well to arrange these forms systematically, 

 for the sake of convenience, yet they cannot be considered as holding 

 fixed zoological positions. Further research in this direction, and upon 

 " Alternation of Generation," will, I think, widely clear up this obscure, 

 yet most interesting field of study. Editou. 



* Siebold. Siebold and Kolliker's Zcitsch. J Bnick. Siebold and Kolliker^s Zeitsch. 



I. p. 270. II. p. 110. 



^ Kulliker. Siebold and Kulliker's ZcitAch. ^Burnett. Proceed. Boston Soc. Nat. 



I. p. 1. Hist. V. p. 124. 



