Behaviorist: a guy at an ATM machine. Plug in the right numbers and get the cash. Hierarchical learning.
Cognitivist: a child hanging ornaments on a Christmas tree. The tree is the structure provided for him, the ornaments are blocks of knowledge. Learning is memory and context.
Constructivist: kids building the Eiffel Tower with popsicle sticks. They learn something about towers, but the result is limited by their own experience (the Eiffel Tower may look like it belongs in a Simpsons cartoon) and by the nature of the blocks of information used (the Eiffel tower is not made of wood). Collaborative learning.
Connectivist: a surfer. Riding the knowledge connections, she is constantly dealing with flux. Learning is a kind of tension between all the connections.
Mergel says these philosophies are good for different stages of learning, and I agree.