He indicates the dispositions of each, calling the one kids, the other sheep, that He might indicate the unfruitfulness of the one, for no fruit will come from kids; and the great profit from the other, for indeed from sheep great is the profit, as well from the milk, as from the wool, and from the young, of all which things the kid is destitute.
-St. John Chrysostom, Homily on Matt. XXV, 4th Century