It's a tough challenge. The textual sources are really limited and he seems to have drawn on most of them. I, of course, would want to look into actual Daoist writings, but I'm not optimistic that relevant late Ming or early Ching Dynasty Daoist sources are going to surface anytime soon, if they exist. I would also look in theatrical sources. He does deal with Shaolin Buddhist origins, and that's great. Aye...maties, but here is the money quote:
"Qi Jiguang's boxing, the major source of Taijiquan techniques, and the internal School Boxing of Wang Zhennan are both traceable to maritime Zhejiang in the early sixteenth century. Its city of Ningbo had been the official port for Japanese missions. After their forced termination in 1549, its off-shore Zhoushan Island became a base for Japanese and local pirates. It was there that Qi Jiguang describes learning the practical art of boxing in Major Liu's thatched hall. Manuals by generals Qi Jiguang, and his mentor Yu Dayou, leaders against Japanese pirate attacks, provide us with the first detailed knowledge of Chinese (internal) fencing and boxing." [Page 7.]
Dude, you hear that! Taijiquan comes from fighting pirates on the sea! Johnny Depp look out! After Pirates of the Caribbean 3, we can make Tai Chi Pirates of Zhejiang!
I always felt like all those dantian circles had something to do with the sea. When you are fighting on boats for weeks at a time all your organs learn to move with the natural pulsation of your "sea-legs." I know my Chen style Taijiquan improved a lot after a month of working 20 hours a day standing in a modified horse stance on fishing boats in Alaska.