It is important to note that the posterity of Cain comes in the First Toledoth, vis., 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. See the Structures on pp. 3 and 5 (Gen. 2:4--4:26; 5:1--6:8).
The generations of the heavens and the earth (2:4--4:26).
2:4-25. Before the Fall.
J2 3:1-34. The Fall.
J3 4:1-26. After the Fall.
The expansion of J3. "After the Fall" (4:1-26), p. 8.
L 1-16. Adam's sons: Cain and Abel.
M 17-24. Cain's son: Enoch.
L 25. Adam's son: Seth.
M 26. Seth's son: Enos.
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 dies in A.M. 125, and Abel and Cain had children before that year, even supposing they had no descendants till they reach the age of sixty-five, Adam could have had 130 children. And if each of these 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 rate 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 clear from Gen. 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.
It is manifest that the history assumes a considerable population; and the fact that there is no attempt to explain it proves its genuineness, and shows that we are left to explain it for ourselves in the only natural way by which it can be explained.