INTRODUCTORY NOTE TO THE INFUSORIA. 19 
ach, but consist of a homogeneous mass, are true animals. If these 
premises are correct, nothing will remain, as I conceive, for a distinctive 
characteristic, but voluntary motion. This, when positive, 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 Kélli- 
ker’s view of a stomachless animal prove correct, the inverse condition of 
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. Stebold, Kolliker, 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 Kolliker’s observations and statements upon Gregari- 
uae.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 + has raised the point of their belonging to 
the Worms. But, aside from such grounds, I was led, somestime since, 
after considerable study of infusoria-forms, to veuture an opinion quite at 
variance with that just mentioned of Szebold and Kciliker. 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. This statement was made more than two 
years since; and subsequent observations, some of them of a special char- 
acter, have not led me toa 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 thisaccount 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 zodlogical 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. Eprror. 
* Siebold. Siebold and Kélliker’s Zeitsch. + Bruch. Siebold and Kélliker’s Zeitach. 
I. p. 270. IL. p. 110. 
+ Kélliker. Siebold and Kélliker’s Zeitsch. § Burnett. Proceed. Boston Soc. Nat 
I. p.1. Hist. V. p. 124. 
