DISTINCTIVE CHARACTERS OF ANIMALS. 25 
great as tliat 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. Tor 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 
Yegetation, 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 Yege¬ 
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 
