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POPULAR SCIENCE REVIEW. 
rapidly about the smaller Volvox, were four such cells endowed 
with the power of locomotion, in order that they might move 
about in search of their proper pabulum, the scientific word 
implying food, but a more appropriate one than the latter in 
this case, for we can hardly say then* ‘ food/ inasmuch as they 
are, as I have already told you, not animals but plants. Before 
our investigations are complete, I dare say I shall have an 
opportunity of showing you similar forms possessing the pro- 
perties of animals.” 
“ But tell me, pray,” my young pupil asked, interrupting 
me, “ by what means are you able to distinguish between a 
plant and an animal, if both possess hfe and motion ? and both, 
in this instance at least, consist of a single cell. If you place 
before me a man and a tree, I have various means of distinguish- 
ing which is animal and which vegetable ; but in this case I 
confess I should be puzzled to know one from the other.” 
“Ay; and wiser persons than you or I are puzzled every 
day with the same problem. As I told you before, there is a 
vast number of living forms whose nature, so far as its being 
animal or vegetable, is still undecided ; and I should have great 
difficulty in referring you to any work on Natural History that 
would enlighten you in this respect. One author will tell you 
that the distinction consists in the animal possessing a stomach 
for the digestion of food, whilst the plant has no such cavity. 
Another, that sensation distinguishes the animal from the plant; 
a third, that in vegetable substances we always find starch, but 
not in animal forms ; a fourth will say animals inhale oxygen 
and exhale carbon, whilst in the vegetable kingdom the reverse 
is the case; and a fifth will cut the Grorclian knot by boldly 
asserting that there is no appreciable difference between the 
lowest forms of animal and vegetable existence. 
But now to what c new’ conclusion do you think naturalists 
are hastening in this respect ? 
Simply this. Animals are nourished by organized sub- 
stances, either alive or in a state of partial decomposition, 
whilst plants derive their sustenance from in organic substances, 
such as phosphate of lime, ammonia, water, &c. This defini- 
tion of the difference between the two classes of living objects is 
not quite satisfactory, but it is, I think, the best that you will 
find in the present state of our knowledge. And, curiously 
enough, it accords with the oldest theories on the subject ; for, 
as plants were first created and then animals, plants must have 
derived their support, as they do now, from inorganic sub- 
stances — carbon, water, nitrogen, &c. — which they extract 
from the earth and atmosphere ; whilst animals feed upon 
organized compounds, which they ingest and work up into 
their own substance, either in stomachs perfectly formed and 
