Natural History of the Animalcules. 103 
highly complex compounds manufactured from simple 
inorganic materials supplied to them as food — a charac- 
teristic not possessed by animals, which require complex 
organic compounds for their nourishment — yet very 
many plants, and chiefly those of the great group of the 
Fungous plants, are unable to assimilate inorganic food, 
and are as dependent as animals on organic produCts. 
An important faCtor in the difficulty of distinguishing 
the lowest plants and animals, is to be found in the 
conditions under which they must be examined, which, 
in conjunction with the very minute size of the organisms, 
render it extremely difficult, or even impossible, to apply 
tests and perform experiments, which, under ordinary 
circumstances, would be practicable and decisive. 
Considering the fundamental distinctions which were 
once thought to underlie plant and animal life, it was 
natural that the basis or ground substance of life, the 
protoplasm, should be considered essentially different 
in the two kingdoms ; w T ithin the last thirty years, 
however, it has been shown that no such difference exists, 
but that protoplasm is essentially the same both in plants 
and animals. 
The intimate structures of plants and animals are also 
essentially the same : they are all made up of, or derived 
from, morphological units, known as cells. The cells, 
which are always minute, generally consist, in plants, of 
portions of protoplasm surrounded with a covering of 
I cellulose ; but, as we have seen, this coat of cellulose is 
1 absent in many humble plants ; and the essential com- 
ponent of a cell is, therefore, simply the protoplasm 
' itself. In animals, the cell also presents itself in the 
form of a speck of protoplasm, though usually it is 
