In popular speech speech the word theory means a "guess". But a scienticfic theory is more then a guess. Its a logical tested, well supported explanation for a set of facts. Great evidence to support the theory are:
Radiometric Dating
Stratigraphy
Molecular Clocks
The Fossil Records
Transitional Forms
Theory of Natural Selection
Divergent Evolution Paths
Convergent Evolution Paths
Parrelle Evoultion Paths
Genetic Drift







