When countries industrialise, why do jobs from agriculture shift to manufacturing?

Question

This is a historical trend whether it may 18th century Britain or 19th century America or what's currently happening in China. What I am confused is by the fact that manufacturing jobs such as *working on an assembly line* are not that well paying. Still the trend holds true. I want to know what are the causes for the shift.

Answer 1

Even though one might consider unskilled labor in manufacturing as being poorly paid, it is all relative.  Agrarian productivity gains drives down the demand for unskilled farm labor, small farms give way to large commercial and mechanized farms due to economies of scale, and wages for unskilled farm labor are driven down as the need for farm labor drops. Factory jobs are more plentiful, i.e., there is higher demand for labor, and thus relatively higher paid.