The science of biology has historically been one of observation of events which occur within the natural world.

Molecular biology borrows from the physical sciences the use of model systems which limit variables and control conditions for more precise interpretation of experimental results.

Since the early 1900's, experimental systems have progressed from whole, complex multicellular organisms such as corn plants and fruit flies to simple single-celled organisms such as bacteria and individual eukaryotic cells and purified cellular components studied  in-vitro (outside the body).