The hypothesis that stands the test of time (often tested and never rejected) are called theories. A theory is supported by a great deal of evidence.

Productive theory keeps on suggesting new hypotheses and so testing goes on. Many biologists take it as a challenge and exert greater efforts to disapprove of the theory.

Law and Principle:

If a theory survives such a doubtful approach and continues to be supported by experimental evidence, it becomes a law or principle. A scientific law is a uniform or constant fact of nature. It is an irrefutable theory. Examples of the biological theory are Hardy-Weinberg's law and Mendel’s law of inheritance.