6 INTRODUCTION. 
or indirectly. Animals, therefore, differ from plants in requiring as 
food complex organic bodies which they ultimately reduce to very much 
simpler inorganic bodies. Whilst plants, then, are the great manu- 
facturers in nature, animals are the great consumers. Another distinc- 
tion arising from the nature of their food is, that whilst plants decom- 
pose carbonic acid, keeping the carbon and setting free the oxygen, 
animals absorb oxygen and give out carbonic acid, so that their reaction 
upon the atmosphere is the reverse of that of plants. 
As regards these general distinctions between plants and animals, there 
are three points which should be remembered :— 
1. That even if universally true, these distinctions can often not be 
applied in practice to the ambiguous microscopic organisms about which 
alone any doubt can be entertained. 
2. These general laws are certainly not cf universal application in the 
case of plants. Some fungi are known which in the matter of food are 
animals—that is to say, they cannot live upon inorganic materials alone, 
but require ready-made organic products for their support. 
3.. Recent researches have rendered it not unlikely that some of the 
lower animals have the power of acting as plants, and of manufacturing 
organic compounds out of inorganic materials. 
3. CLASSIFICATION. 
By the term classification is understood the arrangement of a 
number of dissimilar objects of any kind into larger or smaller 
eroups according as they exhibit more or less likeness to one 
another. The number of different animals is so enormous that 
it was long ago perceived that some classification of them, or 
method of arranging them into groups, was absolutely indispen- 
sable. Without some such arrangement it would have been 
utterly impossible to have ever acquired a clear notion of the 
animal kingdom asawhole. In the older arrangements animals 
were grouped in accordance with some particular character, 
which might or might not be a really essential one; and the 
result was that these classifications were “artificial,” and not 
“natural,” as they are when a@// the characters are taken into 
consideration. To take a familiar example of this: when we 
speak of “quadrupeds,” we really do so in consequence of our 
having, consciously or unconsciously, formed something like a 
rough classification of the animal kingdom. We have a dim 
idea that all animals with four legs belong together somehow, 
and forma single group. Our classification, however, is founded 
upon a single character only—the possession, namely, of four 
legs; and it is, therefore, a purely artificial arrangement. It 
