The Book of generations is an hypothesized text that the modern documentary hypothesis claims was used by the redactor of the torah to connect up parts of the priestly source and the JE source. The text is no longer extant, but in the hypothesis, portions of the text survive as part of the torah. In particular, the text is believed to be fragmented across several portions of the text, since it was used as a filler for several joins. It was Frank Moore Cross who first demonstrated that the text was separate to the other sources, which are substantially larger in comparison.
The standard reconstructed version of the text, formed by collecting together the various fragments, indicates that its presence is usually announced in the torah by the words "and these are the generations of [insert name of biblical figure here]". The text itself appears to be a basic genealogy of the main ancestors of the Israelites, putting emphasis on the main line that leads from Adam, to Noah, then to Abraham, Jacob, and to Moses. However, there also appear to be a few asides to detail significant lines apart from this, such as that concerning Edom.
Both the priestly source (P), and the Jahwist (J) and Elohist (E) sources, also seem to have included portions of the book of generations, albeit variant editions. Frequently, when portions of the book of generations occur in the torah, they are nearby to similar lists of descendants and family trees which are ascribed to the other sources (i.e. J, E, or P).
Although, due to the nature of its use, many parts of the book of generations do not survive, much of the genealogy that is missing is covered by the other sources. Consequently it is possible that in the future the text might be fully reconstructed, although there is very little agreement on the matter in the present day.
The genealogy for certain individuals appear to have a large number of properties in common with those for others. This may be an indicator that at least one of these genealogies is an accidental corruption of a copy of another, or a deliberate but subtle alteration.
For example, in the Jahwist (J) text (Genesis 4:17–18), the family tree descending from Cain has the following properties (amongst others):
Whereas the descendants of Seth (Genesis 5:3–26) have the following properties: