164 THE MEANING OF EVOLUTION 
with protoplasm. This apparently foamy, jellylike, 
transparent material is the only living substance in 
all the world. Animals and plants are larger or 
smaller collections of the little masses of protoplasm 
which we know as cells. The lowest animals are each 
made up of but a single cell. This consists of a 
small mass of protoplasm surrounded almost always 
by a thicker skin or covering, known as the cell wall 
and enclosing a complicated kernel known as the nu- 
cleus. The protoplasm seems to be the living sub- 
stance itself. The cell wall is not a simple dead scum 
on the outside of the protoplasm, but is itself able to 
do certain things which can only, so far as we know, 
be done by living substances. or instance, of two 
materials dissolved in the water in which the cell 
floats, the wall may permit one to soak into the ani- 
mal and keep the other out. The one allowed to 
enter will usually be found good to be used for food 
by the cell. The nucleus seems to store within itself 
the record of its past history and thus enable the cell 
to do in the future what its ancestors did in the past. 
Such simple cells can exhibit in very low form all 
the activities the higher animals show in much more 
elaborate development. <A one-celled animal can move 
about, can recognize the proximity of food, can en- 
gulf its food and digest it, can build up its own sub- 
stance out of the digested food, can absorb oxygen, 
