DISTINCTIVE CHAEACTERS OF ANIMALS. 25 



great as that which, exists among the lowest animals ; so that 

 no positive line can be drawn between the two kingdoms on 

 the basis of this distinction alone. There is another very 

 important physiological difference, however, between the two 

 kingdoms, which seems to afford an adequate means of 

 settling the true place of those tribes whose position would 

 otherwise be doubtful. This lies in the nature of their food, 

 and the source from which it is obtained. If or although it .is 

 now known that the primary tissues of plants are originally 

 formed of the same albuminous material as are those of 

 animals (the cellulose layers which constitute the great 

 bulk of the vegetable fabric being a subsequent deposit), yet 

 this material is generated in the Plant by the combination of 

 the elements which it obtains from the carbonic acid, water, 

 and ammonia of the soil or of the atmosphere ; whilst the 

 Animal is destitute of all power of thus forming it for itself, 

 and is hence entirely dependent upon the plant for its sup- 

 plies of nutriment. Thus, whilst the very humblest forms of 

 Vegetation, in common with the highest, are found to have 

 the power of decomposing carbonic acid under the influence 

 of sunlight, setting free its oxygen and retaining its car- 

 bon, the humblest forms of Animal life, in common with 

 the highest, derive their nutriment either directly from 

 plants, or from the bodies of other animals which have sub- 

 sisted on vegetable food, whilst they produce a converse 

 change in the atmosphere by their respiration, absorbing 

 from it oxygen, and giving forth to it carbonic acid. This 

 criterion will serve, it is believed, to distinguish the very 

 lowest forms of Animal life from those humble forms of Vege- 

 tation which they most closely resemble in the simplicity of 

 their organization ( 128); and its application will generally 

 be found to be very easy. There is now no longer any doubt 

 that a large proportion of the beings formerly ranked as 

 Animalcules, are really to be regarded as Plants, notwithstand- 

 ing that they possess a power of active and apparently 

 spontaneous movement, far greater than that of many unques- 

 tionable animals. And generally it may be said that the 

 presence of a bright-green or bright-red colour in any of these 

 simple organisms, where it is not derived from coloured sub- 

 stances taken- in as food, affords a strong probability of their 

 vegetable character; these colours being produced in the 



