IT IS IMPORTANT TO NOTE THAT THE POSTERITY OF CAIN COMES IN THE FIRST TÔKEDÔTH, viz., that of the generations of the heavens and the earth; and not in the book of the generations of Adam.
The posterity of Seth commences with the generations of Adam: showing that the two accounts are distinct, and deal with two different subjects.
There were 130 years before Seth was born and substituted for Abel in the line of the promised seed.
In those 130 years after Cain, Adam must have begotten sons and daughters, as in the 800 years after Seth.
If Abel died in a.m. 125, and Abel and Cain had children before that year, even supposing they had no descendants till they reached the age of sixty-five, Adam could have had 130 children. And if each of those could have a child at sixty-five years of age, one in each successive year, there would have been 1,219 in a.m. 130. If we suppose Adam’s earlier sons and daughters to have had children at the age of twenty one instead of at sixty-five, there would have been over half a million in the 130 years , without reckoning the old or young, and this at a very moderate of increase.
It is generally assumed that Adam and Eve had no children beyond those named. But, as in the line of Seth, it is very clear form Genesis 5:4 that they had, we may well conclude that the same was the case in the line of Cain. It is a gratuitous assumption that Abel had no posterity.