The dividing landmark in the history of economic thought in the nineteenth century is usually placed in the, seventies with the arrival of the new utility theories of Jevons and the Austrian School. Interest was focused on what was peculiar to a particular system of economic relations, even at the expense of a wider, but perhaps more barren, generality. In the realm of economic thought it is not difficult to see a parallel tendency at work. Abstraction was to be made of human activity, its characteristics and its relationships, and only the reflection of them in the mind to be taken as the data for economic interpretation. Karl Marx's retort to Senior remained unchallenged until, towards the end of the century, the concept of marginal increments, a concept borrowed from the differential calculus, was introduced as an attempt to give greater precision to economic notions.