Bill Tilman and Dan Bryant had not acclimatized well at altitude, so Gavin was brought in as "new blood" meanwhile Frank Smythe was de facto climbing leader.
Bill Tilman and Dan Bryant had not acclimatised well at altitude and so were not included in the party  both men agreed this was the right decision, Tilman even to the extent of providing financial support. Gavin were brought in as "new blood" having impressed Smythe in the Alps. Frank Smythe (Everest 1933) was de facto climbing leader and Shipton (1933, 1935), Percy Wyn Harris (1933), Edwin Kempson (1935), Charles Warren (1935), Bill Wager (1933) and Edmund Wigram (1935) all had previous Everest experience. 
