原来长子继承是这个道理

from Wealth of nations:

That the power, and consequently the security of the monarchy, may not be weakened by division, it must descend entire to one of the children. To which of them so important a preference shall be given must be determined by some general rule, founded not upon the doubtful distinctions of personal merit, but upon some plain and evident difference which can admit of no dispute. Among the children of the same family, there can be no indisputable difference but that of sex, and that of age. The male sex is universally preferred to the female; and when all other things are equal, the elder everywhere takes place of the younger. Hence the origin of the right of primogeniture, and of what is called lineal succession.