For example, Dan writes on his blog, Migration, about the evolution of flight:
There have been a lot of variations in theories on the origin of flight since the 1860’s, but most people nowadays group them generally into two categories. The first, the Cursorial model, proposed that flight evolved in running bipeds through a series of short jumps. Meanwhile the second, the Arboreal Model, proposed that those pre-avian dinosaurs (theropods) where gliders originally, much like “flying” squirrels today.He then present the intriguing hypothesis that flight evolved as the birds ran up steeper and steeper inclines. He has a nice video, too. Check it out.