This is best to be studied, perhaps, by taking to hand the writings of a man who may be characterised pre-eminently as a spokesman of the materialistic movement in Germany,—I mean JAKOB MOLESCHOTT. His well known work _Der Kreislauf des Lebens_ has become in its last, the eighth edition, something quite different from what it was in its first; and the rich collection of his lesser writings (_Kleinere Schriften_, 2 Vols., 1879-87) also offers the philosopher, especially from a methodological point of view, much that is worthy of especial attention. Moreover, this reciprocal influence of mind upon mind is manifested in the case of many of the most distinguished investigators of the last thirty years, in the most remarkable and gratifying manner. It is impossible to study the discourses and treatises of physiologists like DU BOIS-REYMOND and WILHELM PREYER, of physicists like HELMHOLTZ and ERNST MACH, and the discussions occasioned by their works, without being surprised at the extent to which the points of view of psychology and of the theory of cognition have penetrated into the problems and inquiries of the physical sciences. And _vice versa_ philosophical works, like FR. A. LANGE'S History of Materialism (_Geschichte des Materialismus und Kritik seiner Bedeutung in der Gegenwart_), UEBERWEG'S Collected Essays (_Gesammelte Abhandlungen_, just recently edited in a commendable manner by Moritz Brasch), the numerous works of LUDWIG NOIRÉ, and, last but not least, the entire scientific activity of WILHELM WUNDT,—all show an intimate familiarity with the methods of the physical sciences and an assimilation of materials from these branches of knowledge such as the speculative period can furnish no example of. * * * * *