They had to chase after their prey. Faster cheetahs survived and produced more offspring. They moved from a high density forest to flat lands and had to run faster. They had to compete with bigger and stronger animals for their prey. Common vernacular: an untested idea or opinion; speculation. Scientific definition: an explanation of a set of natural phenomena, based upon proven or testable hypotheses and observations; theory tries to explain what we observe in nature based on testable concepts. Examples: theory of continental drift pangaea, kinetic molecular theory, theory of relativity, the big bang theory , theory of panspermia. Verifiability true verification would require a positive result in every instance; has to be good, testable, and tests must be able to prove theory wrong (theory you can"t prove wrong isn"t as strongly supported by evidence) Fallibility only when a theory could be wrong is it impressive that it survives testing and criticism karl popper.