The theory of cell division, or cell theory, states that all living cells arise from pre-existing cells. This means that new cells are not spontaneously generated; they are formed through the division of existing cells.

There are two main types of cell division:

* Mitosis: This is the process of cell division that creates two identical daughter cells from a single parent cell. It's used for growth, repair, and development.

* Meiosis: This is the process of cell division that produces gametes (sex cells) with half the number of chromosomes as the parent cell. This is essential for sexual reproduction.

Therefore, the idea that cells arise from other cells through cell division is a fundamental principle in biology, and it's a core concept in understanding how life on Earth grows and replicates.