Percy writes:
The fossil record is one of change over time. It records species change from Indoyus and Pakeicetus through intermediate species up to modern whales.
A nice relatively short yet detailed summary of the state of whale fossil lineage from about ten years ago can be found
here.
Also the Smithsonian has another relatively clear and well laid out description of the learning process the scientists went through including the advent of genetic evidence that can be found
here.
Both articles show that the evidence developed over a long period of time as new discoveries were made yet every new discovery confirmed the hypothesis that whales descended from land dwelling mammals.
Edited by jar, : fix sub-title