I'm with Michael on this one, at least as regards the "If all the young girls" verses running straight on to the "Sally my dear" verses.

Possibly the amalgamation happened only once, and the combined song then spread around to various singers. But, even if it did happen that way, the question remains why two songs with only the most tenuous resemblance in subject matter should ever have been stuck together in the first place, and why singer after singer has kept them together. As Michael says, there are plenty of instances of songs sharing the same tune where that hasn't happened.

The other amalgamation seems to me less incongruous. At the end of a working day the singer first thinks about his girlfriend, and then his thoughts move on to reflecting about the various kinds of work that he does throughout the year. When I am relaxing and letting my mind wander, it can easily make bigger jumps than that between subjects.

Richard