96 PinJc Monads and their Enemies. 



and we still find genera, to which animal terms are applied, 

 mixed up with other genera concerning which nothing ani- 

 mal can "be predicated. It is not, indeed, easy to say what is 

 an animal or what is a vegetable, and Dr. Burnett observed 

 some years ago, in an introductory note to his translation of 

 Yon Siebold-'s Anatomy of the Invertebrata, that "all the older 

 criteria by which animals were separated from plants have 

 long since been regarded as invalid ; and some of those which 

 in late years have been regarded among the most constant, 

 have, quite recently, been declared as equally unsound. Cel- 

 lulose has been shown to be a component of animal as well as 

 of vegetable structures, and Kolliker has insisted that some 

 forms that have neither mouth nor stomach, but consist of a 

 homogeneous mass, are true animals. If these premises are 

 correct, nothing will remain, I conceive, for a distinctive cha- 

 racteristic, but voluntary motion." Afterwards he says, that 

 if stomachless creatures be admitted as animals, still the 

 existence of a stomach should be regarded as proving that an 

 object is not vegetable. Whether or not motion is voluntary, 

 is often an extremely difficult problem to decide, and where 

 no distinct animal organs are present, the class to which the 

 creature belongs must be decided by its preponderating affini- 

 ties rather than by any one characteristic, and particular 

 attention must be paid to its mode of origin and embryonic 

 forms. A stomachless, organless thing, like the Gregarina, 

 is recognized as an animal partly on account of its young 

 form resembling an Amoeba, and thrusting out similar pro- 

 cesses. The negative evidence against this, the possessing 

 neither mouth nor stomach, does not deanimalise it; as the 

 tape-worm, which no one would dream of supposing a plant, 

 has no such organs. Moreover, the nutrition of the Grega- 

 rina is that of an animal ; but as it lives in other animals in 

 contact with their juices, it can imbibe and assimilate them, 

 without the machinery for taking in solid matter, and reducing 

 to a fluid state, which, is requisite for the mode of life of other 

 animals higher in the scale. 



The absence of a mouth does not suffice to indicate that a 

 living object is not an animal; but the possession of a true 

 mouth is evidence the other way ; and none of the various 

 things called Monads, in which mouths can be detected, ought 

 to be associated with other creatures in which nothing but 

 vegetable characteristics can be traced. When an object is 

 not in its complete form, but only a germ, or early stage of 

 something else, it should not be classed with others that may 

 resemble it, but which do not develope into anything else. 

 Until, therefore, the life history of all the objects called Monads 

 is ascertained, their true classification cannot be effected, and 



