And he said unto him,
Take me a heifer three years old, and a female goat three years old, and a ram three years old, and a turtle dove, and a young pigeon.
All Commentaries on Genesis 15:9 Go To Genesis 15
Augustine of Hippo
AD 430
Here also, in fine, a symbol was given, consisting of these animals: a heifer, a shegoat, a ram and two birds, a turtledove and pigeon, that he might know that the things which he had not doubted should come to pass were to happen in accordance with this symbol. The heifer may be a sign that the people should be put under the law, the shegoat that the same people were to become sinful, the ram that they should reign. Perhaps these animals are said to be of three years old for this reason: that there are three remarkable divisions of time, from Adam to Noah, and from him to Abraham, and from him to David. David, on the rejection of Saul, was first established by the will of the Lord in the kingdom of the Israelite nation. In this third division, which extends from Abraham to David, people grew up as if passing through the third age of life. Or perhaps it may be that they had some other more suitable meaning. Still I have no doubt whatever that spiritual things were prefigured by them as well as by the turtledove and pigeon.
