"In North America, the black hear was seen by Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the county, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous a a whale." 

C. Darwin (1859) "Origin of Species"  p. 184. On the advice of friends, Darwin removed the story from subsequent editions.
 

The reconstructions show fossil species with various degrees of aquatic adaptation, as models for the evolution of whales. The series is not intended to represent an evolutionary lineage. (from Futuyma 1997)