Albert Barnes Commentary Numbers 3:1

"Now these are the generations of Aaron and Moses in the day that Jehovah spake with Moses in mount Sinai." — Numbers 3:1 (ASV)

The “generations” now given, though entitled those of Aaron and Moses (Aaron standing first as the older brother), are those of Aaron only. The personal dignity of Moses, though it gave him rank as head of his tribe, was not hereditary. He had no successor in his office, and desired to have none (Numbers 14:12; Exodus 32:10), except the distant prophet like unto himself (Deuteronomy 18:18). Aaron was the ancestor of a regular succession of priests.