Darwin is talking about the guachos using "lazos" and "bolas". The general idea of the bola is two stones encase in leather, each hitched to the end of an 8 foot rawhide braided thong. They are whirled overhead, and the cast. When the thoong strikes leg of a steer, the balls wrap aound in opposite directions, and grip very firmly.
He also tells about boys catch smaller game with similar devicers, including hares "on their forms". The form is the place where the hare spends his nights and hides during the day. I suppose it gets its name from the fact that it is a snug fit.That is definition 13 in my dictionary.