The growth of a many-celled organism is due to an increase in the number of cells.

Here's why:

* Cell division: Multicellular organisms grow by a process called cell division, where existing cells divide to create new cells.

* Increase in size: While cells themselves can grow slightly, the primary way multicellular organisms grow is through the creation of more cells.

* Differentiation: These new cells often differentiate, meaning they develop specialized functions within the organism.

Let me know if you'd like to know more about cell division or the process of growth in multicellular organisms!