Horn of Plenty (S1E17)

The Enterprise crew is excited to meet the mythical Aldeans until they propose a classic "children for information" exchange, but when Commander Riker balks at the deal they take some kids anyway. This is a real problem for Captain Picard, because Wesley (the boy?) is among their victims. Why'd the Aldeans take only the white kids? How long can Picard go without Wesley? Didn't we already fix the ozone layer? It's the episode where there's no calculus allowed!