4 DISTINCTIONS BETWEEN ANIMALS AND PLANTS. 
the oxygen to the atmosphere or water. Animals, on the 
contrary, during the process of respiration take into their 
tissues free oxygen, and return, in its place, to the surrounding 
medium in which they live, carbon-dioxide, the result of the 
combination of the superfluous carbon in the animal system 
with the oxygen which has been inhaled. Plants, therefore, in 
assimilation absorb carbon-dioxide and eliminate oxygen ; while 
animals in respiration absorb oxygen and eliminate carbon- — 
dioxide. 
Fourthly, while all plants and animals are made up of cells, 
those of the latter do not develop upon their exterior any sub- 
stance materially differing from the more internal protoplasm ; 
but the whole substance of the cell is more or less homogeneous, 
and consists throughout of matter which is essentially composed 
of the four elements, Carbon, Oxygen, Hydrogen, and Nitrogen, 
together with some Sulphur and Phosphorus. The protoplasmic 
mass of the cells of plants, which is also essentially composed of 
the same constituents, on the other hand, sooner or later, as a 
general rule, becomes changed on its outer surface, where a 
membranous covering is developed termed the cell-wall, com- 
posed of cellulose, and therefore consisting entirely of the three 
elements, Carbon, Oxygen, and Hydrogen. Plants, then, are 
made up of cells, the protoplasm of which is enclosed in a cell- 
wall of cellulose, while animals are made up of cells which 
have no such cell-wall. 
Fifthly, the presence of starch was also formerly considered 
as a diagnostic character of plants; but it is now known that 
this substance, or at least one isomeric with, and presenting the 
same general appearances as it, is also to be found in the tissues 
of animals. 
In reference to the above distinctive characters, therefore, it 
should be especially noticed that they are only general, namely, 
those derived from comparing together, as a whole, the members 
of the Animal and Vegetable Kingdoms ; and that to all such 
characters some exceptions may be found when we compare 
particular individuals. We arrive accordingly at the conclusion 
that it is impossible to give a complete and perfect definition of 
a plant, or, in other words, to lay down any single character by 
which plants can in all cases be distinguished from animals. In 
determining, then, whether an organism under investigation be 
a plant or an animal, the naturalist must first take into his con- 
sideration, not any one character alone, but the sum of all the 
characters which it may exhibit. 
Since there are many organisms which it is very difficult to 
assign with any certainty either to the Vegetable or Animal 
Kingdom, as some of their characters indicate that they belong 
to the one and some to the other kingdom, Haeckel proposes 
that all these should be grouped together into a third kingdom 
to be called ‘ Protista.’ 
