Throughout the 1800s, scientists viewing organisms under microscopes made a series of observations.
A scientist viewing plant structures observed that plants are made up of very small repeating units. Each of these was surrounded by a wall and contained a nucleus.
A scientist examining tissue from tadpoles observed that animal structures are made up of repeating units. Each of these contained a nucleus.
A scientist viewing samples of pond water observed small, freely moving organisms. Each of these contained a single nucleus.
These observations led the scientists to develop a theory about living organisms. Which of the following conclusions about living things is best supported by these observations?
