Just to pick up on the Darlington comment, your transistors are not connected as a Darlington pair. With a Darlington, the emitter of one feeds into the base of the other so you get 1.3V drop across the total base emitter junction. Yours are connected in parallel, so the drop across R1 will only be one VBE.
Regarding your comment , there is a comparator connected between these 2 pins, so nothing happens until this voltage reaches 0.45V, then the comparator kicks in and limits the output current. Increasing this resistor reduces the trip current, thus providing variability in the output current