Neither should we omit to pay them full homage for the eminent services rendered to the science, and among these not the least important was the antagonism which their theories aroused in the minds of the working classes. Pessimists unwittingly often do more for progress than optimists. To these two writers fell the task of criticising economic doctrines and institutions, a task that has been taken up by other writers in the course of the century, but which seems as far from completion as ever. Karl Marx, another critic, is intellectually a scion of the Ricardian family. It would be a mistake to imagine that all their theories savour of pessimism, but their reputation has always been more or less closely linked with the gloomier aspect of their teaching. I: MALTHUS[290] Malthus is best known for his “law of population.” That he was a great economist, even apart from his study of that question, might easily be proved by reference to his treatise on political economy, or by a perusal of the many miscellaneous articles which he wrote on various economic questions. A consideration of many of these theories, notably the theory of rent, must be postponed until we come to study them in connection with the name of Ricardo. THE LAW OF POPULATION Twenty years had elapsed since the publication of Smith’s immortal work, without economics making any advance, when the appearance of a small anonymous volume, known to be the work of a country clergyman, caused a great sensation. Even after the lapse of a century the echo of the controversy which it aroused has not altogether passed away. At first sight one might be led to think that the book touches only the fringe of economics, seeing that it is chiefly a statistical study of population, or demography, as the science is called to-day. But this new science, of which Malthus must be regarded as the founder, was separated from the main trunk of economics at a much later date. Furthermore, we shall find that the influence of his book upon all economic theories, both of production and distribution, was enormous. The essay might even be considered a reply to that of Adam Smith. The same title with slight modification would have served well enough, and James Bonar wittily remarks that Malthus might have headed it _An Essay on the Causes of the Poverty of Nations_. The attempt to explain the persistence of certain economic phenomena by connecting them with the presence of a new factor, biological in its character and differing in its origin both from personal interest and the mere desire for profit, considerably expanded the economic horizon and announced the advent of sociology. We know that Darwin himself acknowledged his indebtedness to the work of Malthus for the first suggestion of what eventually became the most celebrated scientific doctrine of the nineteenth century, namely, the conception of the struggle for existence and the survival of the fittest as one of the mainsprings of progress.