Based on Smith's classic geologists that observations and the of many followed, one of the most important and basic principles in historical geology was formulated: fossil organisms succeed one another in a definite and determinable order, and therefore any time period can be recognized by its fossil content. to be known as the principle of fossil succession. In other words when fossils are arranged according to their age by applying the law of superposition to the rocks in which they are found, they do not present a random or haphazard picture. To the contrary, fossils show changes that document the evolution of life through time.