The fundamental thesis of the present work is that the three contemporaneous philosophic systems, those of Criticism, Positivism, and Evolutionism, are merely varieties of a single species, as strictly parallel manifestations of a common stock of beliefs and general hypotheses. The basic identity of the thought of this century is shown by the ever increasing convergence of the great leading ideas, as exhibited in the prevailing theories of knowledge, by the preponderance of relativism, and of agnosticism. It reveals itself, especially in the similar conceptions formed by the most varied systems, not only of the essential characters of philosophy, its method, and the ends it ought to pursue, but also of the scientific laws which govern its evolution. We cannot follow the author through his discussion of all these points and we must therefore restrict ourselves to the most salient features of his argument. Modern philosophy is represented by three principal schools: Criticism which originated with Kant, Positivism founded by Comte, and Evolutionism introduced by Spencer. These three systems had a common ancestry, that of sensualism. The critical philosophy is the legitimate heir of sensuous idealism, and the positive philosophy the immediate descendant of sensuous materialism. The evolution philosophy is itself rooted in sensualism, but it is really a conciliator of the two great philosophies which preceded it, Criticism and Positivism. This conclusion, which appears to us just, is supported by various considerations to which reference here is not necessary. M. de Roberty bears testimony to the influence of the philosophy of Kant over the development of the evolutionist conception, which could be applied to society only by giving an apparent universality to the mechanical hypothesis. This was accomplished by Spencer, as it had been done to some extent by Comte. The popularity of the evolution philosophy is explained by the author as due to its admixture of agnosticism with a monism which captivates the masses “by the audacious assertion that it has raised all veils and resolved all enigmas.” Kant, Comte, and Spencer have equally seized this characteristic trait of the genius of our century. They each treat, says M. de Roberty, of the most transcendent problems of metaphysics, and place them carefully under the cover of the experimental method. Let us add that they are each different expressions of that genius, which marks the progress of the mental evolution of mankind.