Charles Ellicott Commentary Genesis 6:1

"And it came to pass, when men began to multiply on the face of the ground, and daughters were born unto them," — Genesis 6:1 (ASV)

When men (the adam) began to multiply.—The multiplication of Adam's race was probably comparatively slow. This was because each patriarch reached a great age before his first-born son was born. However, since the name recorded is not necessarily that of the eldest, but of the son who held the birthright, it does not automatically mean that in every case the son named was absolutely the eldest. There may have been other substitutions besides that of Seth for Cain; Noah, born when his father was 182 years old, seems to be a case in point.

He was selected to be the restorer of mankind because of his piety, and may have had many brothers and sisters older than himself. Each patriarch, however, fathered sons and daughters, and since we find Cain building a city, he must have recognized, in any case, the possibility of a considerable population settling around him. It was probably, as we noted earlier, around the time of Enoch that the corruption of Adam’s family began to become widespread.