Genesis 15:10

And he, i.e. Abram, who by Divine instinct and precept did all this which here follows, divided them in the midst, into two equal parts. This was done for two reasons.

1. To represent the torn and distracted condition in which his seed was to lie for a season.

2. To ratify God's covenant with Abram and his seed; for this was a rite used in making covenants, as appears both from Scripture, Jeremiah 34:18, and other authors. Laid each piece one against another, partly to encourage hope, that God would in his time put those parts together, and unite those dry bones, (to which the Israelites are compared, Ezekiel 37:1), and clothe them with flesh; and partly that the persons entering into covenant might pass between those parts, and so testify their union and conjunction in one and the same sacrifice. The birds divided he not, either because there were two birds, and the one was laid against the other, which answered to the division of the larger creatures; or because they belonged not to the ceremony of the covenant, but were for the use of sacrifice, wherein they were to be offered whole, as afterwards was prescribed, Leviticus 1:15,17.