The timeline proposed for supposed whale evolution most recently was:
Pakicetids (fully terrestrial): ~50 mya
Ambulocetids (semi-aquatic): 49 mya
Remingtonocetids (semi-aquatic): 49 mya
Rodhocetus (a Protocetid, semi-aquatic): 47 mya
Basilosaurids (fully aquatic): 40 mya
But then, a fully aquatic whale bone was found in antartica, dated 49 mya:
http://www.nbcnews.com/id/44867222/n.../#.WRJoBtLytPY
So now we're looking at:
Pakicetids (fully terrestrial): ~50 mya
New Fossil Jawbone (fully aquatic whale): 49 mya
Ambulocetids (semi-aquatic): 49 mya
Remingtonocetids (semi-aquatic): 49 mya
Rodhocetus (a Protocetid, semi-aquatic): 47 mya
Basilosaurids (fully aquatic): 40 mya
Now we’re looking at much less than 200,000 generations, much too little time to allow the origin and fixation of all the multitude of traits necessary to convert a land-mammal into a whale. It didn't happen.