Assurance was given to Abram of the land of Canaan for an inheritance. God never promises more than he is able to perform, as men often do. Abram did as God commanded him. He divided the beasts in the midst, according to the ceremony used in confirming covenants, Jer 34:18, 19.
Having prepared according to God's appointment, he set himself to wait for the sign God might give him. A watch must be kept upon our spiritual sacrifices. When vain thoughts, like these fowls, come down upon our sacrifices, we must drive them away, and seek to attend on God without distraction.