Êxodo 11:5

All the firstborn... shall die. — The Heb. word translated firstborn is applied only to males; and thus the announcement was that in every family the eldest son should be cut off. In Egypt, as in most other countries, the law of primogeniture prevailed — the eldest son was the hope, stay, and support of the household, his father’s companion, his mother’s joy, the object of his brothers’ and sisters’ reverence. The firstborn of the Pharaoh bore the title of erpa suten sa, or “hereditary crown prince,” and succeeded his father, unless he died or was formally set aside during his father’s lifetime. Among the nobles, estates were inherited, and sometimes titles descended to the firstborn. No greater affliction can be conceived, short of the general destruction of the people, than the sudden death in every family of him round whom the highest interests and fondest hopes clustered.

The maidservant that is behind the mill marks the lowest grade in the social scale, as the king that sits upon his throne marks the highest. All alike were to suffer. In every family there was to be one dead (Êxodo 12:30).

All the firstborn of beasts. — The aggravation of the calamity by its extension to beasts is very remarkable, and is probably to be connected with the Egyptian animal-worship. At all times there were in Egypt four animals regarded as actual incarnations of deity, and the objects of profound veneration. Three of these were bulls, while one was a white cow. It is not unlikely that all were required to be “firstborns;” in which case the whole of Egypt would have been plunged into a religious mourning on account of their deaths, in addition to the domestic mourning that must have prevailed in each house. The deaths of other sacred animals, and of many pet animals in houses, would have increased the general consternation.