Where, according to cell theory, did new cells in animals come from?

New cells, according to cell theory, are formed from other cells. The cells in the body perform certain functions. But sometimes there may be a need to provide new functions. So there is a smooth evolution of cells and new types appear. This assumption appeared in the middle of the nineteenth century. Cell theory does more than explain the origin of cells. It connects all organisms and proves their common origin due to the similar structure of cells. The seven kingdoms of living nature have a cellular structure.