No, I see that, its to the LEFT of Caste system, its an incomplete link, that continues into Caste system box, from the left to the centre, and heads towards priesthood.
That's a link going from Priesthood to Caste System. We're apparently not allowed to do vertical arrows. The way I have techs laid out on the tree is that each column is supposed to be one "level" of technology, although if there are more than 9 or 10 techs at a particular level, then I spread them out over two columns to avoid having techs right on top of each other.
Caste System's requirements are Priesthood OR Slavery, plus Mysticism. Mysticism is level 2 (Ritualism), Slavery is level 2 (Hunting OR Ritualism), and Priesthood is level 3 (Mysticism + Ceremonial Burial), so Caste System is level 3 (lowest-level combination).
The only line I could run to Caste System is from Priesthood. I don't want to move Caste System forwards or Priesthood backwards; I don't want Caste System to just hang with no lines going to it; and I can't run the line from Slavery because it would have to run underneath Sacrifice, which we really don't want. This was the unfortunate least of all evils option.