206 THE STRUGGLE FOR LIFE. 
I intend now to confine myself to the problems of Animal, 
ignoring those of Vegetable, life. These in the higher 
members of each kingdom differ too widely to be treated 
together, I say in the higher members, because the lower 
animals and plants approach each other so closely, that it 
has been proposed to form a third or intermediate Kingdom 
of Nature, the ‘‘ Protista,’’ to include them both. 
The simplest animal consists of a single cell, and the cell 
is the unit of the animal body. Every part of the highest 
animal, however modified it may be in form, and however 
exalted may be the function it subserves, depends for its 
origin on one or more cells. In fact every animal has been 
at one stage of its existence nothing more or less than a 
single cell. As the cell plays a leading part in what I 
have to say, I will briefly describe a typical animal cell. 
An animal cell is a microscopic mass composed of a sub- 
stance named Protoplasm. The cell is generally originally 
roundish or ovoid in shape, and under the microscope seems 
to be made up of three distinct structures, viz: the Nucleus, 
the Cell-contents, and the Cell-wall. Of these the Nucleus 
is the essential portion, the others being probably derived 
from it, the contents being highly vitalised near the Nucleus, 
while they are nearly if not quite dead at the periphery, 
where they form the cell-wall. 
The simplest forms of animal life consist of a single cell 
such as the Monas crepusculum, this is so minute, that eight 
millions of them would only occupy the same space as is 
filled by a grain of mustard seed. The Amebe are masses 
of protoplasm, constantly varying in shape, and they feed 
literally by getting outside their prey, extruding the debris 
at any convenient part of the body. I refer to these crea- 
tures because I shall have to mention them again in con- 
nection with the defence of the structures of the higher 
animals from foreign bodies and other harmful particles. 
The animal kingdom is divided into two sub-kingdoms, 
