Synopsis of the Books of the Bible, by John Nelson Darby, [1857-62], at sacred-texts.com
Abram seeking, at Sarah's instigation, to anticipate the will of God and the accomplishment of the promise in its time, we have the covenant of the law in Hagar, the source of distress and disquietude. God, however, takes care of the seed according to the flesh. The application of this as a figure is clear from Galatians 4. The pride of man under the law is marked in Hagar's spirit, yet her son cannot be heir. The haste of man, who will not wait God's time, will not wait on Him as to means of accomplishment (so was it with Jacob for the blessing) is full of moral warning to us; it is ever the source of disquietude and sorrow. Hagar, too, was an Egyptian a remembrance, also, of the want of faith in Abram. The law and flesh, and indeed sin, ever go together (see Joh 8:34-36); and in connection with the unbelief of nature, that is, Egypt.
As regards the order of these chapters, I may add, 12, 13, 14 go together, and are dependent on the double manifestation of God to Abram; first, to call him, and then in Canaan. We have power, failure, return, and enduring heavenly faith contrasted with worldliness, and thereto the display of earthly power attached, to that faith, closing with victory; God possessor of heaven and earth, and Melchisedec.
Though chapter 15 stands alone as a whole, chapter 16 is so far connected with it, that it is the fleshly attempt on Sarah's part to have the seed which was assured by the word of the Lord to Abram in the beginning of chapter 15. Here all is failure; but the purposes of God will be accomplished according to promise, and not of the flesh and man's will.