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. For 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, 



