INTRODUCTION. 5 
test, since it occurs regularly in some undoubted animals (¢.¢., Stestor 
amongst the /uzfusoria, and the Hydra viridis, or green fresh-water 
polype, amongst the Ce/lenterata). 
Fourthly, As regards locomotive power, or the ability to effect changes 
of place at will, the results of observation are singularly at variance with 
our preconceived notions. Before the invention of the microscope, no 
instances of independent voluntary movements. were known in plants, if 
we except the voluntary opening and closure of flowers and their turning 
. towards the sun, the drooping of the leaves of sensitive plants under 
irritation, and some other phenomena of a like nature. Now, however, 
we know of many plants which are endowed, either when young or 
throughout life, with the power of effecting voluntary movements appar- 
ently as spontaneous and independent as those exhibited by the lower 
animals. In some cases the movements are brought about by means of 
little vibrating hairs or cilia with which a part or the whole of the sur- 
face is furnished. In other cases the movements seem to be certainly 
not produced by cilia, but their exact cause is obscure (¢.g., in the 
Diatomacee and Desmidie, two of the lower orders of plants, all of 
which are microscopic in size). When it is added that many animals 
are permanently fixed and rooted to solid objects in their fully-grown 
condition, it will be seen that no absolute distinction can be drawn 
between animals and plants merely on the ground of the presence or 
absence of independent locomotive power. 
Fifthly, We have shortly to consider one of the most reliable of all the 
tests by which an animal may be separated from a plant—namely, the 
nature of the food, and the products which are formed out of the food 
within the body. 
The differences between animals and plants in this respect may be 
roughly stated as follows :— 
. I. Plants live upon purely inorganic substances, such as water, car- 
bonic acid, and ammonia—and they have the power of making out 
of these true organic substances, such as starch, cellulose, sugar, &c. 
Plants, therefore, take as food very simple bodies, and manufacture 
them into much more complex substances, so that plants are the great 
producers in nature. 
2. Plants in the process of digestion break up carbonic acid into the 
two elements of which it is composed—namely, carbon and oxygen, 
keeping the carbon and setting free the oxygen. As carbonic acid 
occurs always in the air in small quantities, the result of this is that 
plants remove carbonic acid from the atmosphere and give out oxygen. 
3. Animals, on the other hand, have no power of living on inorganic 
matters, such as water, carbonic acid, and ammonia. They haye no 
power of converting these into the complex organic substances of which 
their bodies are composed. On the contrary, animals require to be 
supplied with ready-made organic compounds if their existence is to be 
maintained. These they can only get in the first place from plants, and 
therefore animals are all dependent upon plants for food either directly 
