Possible Clues:

• Constantly-changing one-celled creatures
• A doctor backing Lincoln's simple form of life
• Shape-shifting animal
• Pseudopod organism
• Creature on a microscope slide
• A crowd crossing middle of field by a very small animal
• A graduate outside old English type of cell
• Possible parasite upset Lincoln after a short time
• One multiplying by division
• Certain protozoan