So much for the different lines followed by the mechanical theories of to-day. An idea of their general tenor can be gained from a series of much quoted general treatises, of which we must mention at least the “classics.” In Wagner’s “Handwörterbuch der Physiologie,” 1842, Vol. I., Lotze wrote a long introductory article to the whole work, on “Life and Vital Force.” It was the challenge of the newer views to the previously vitalistic standpoint, and at the same time it was based on Lotze’s general principles and interspersed with philosophical criticism of the concepts of force, cause, effect, law, &c.(70) A similar train of ideas to Lotze’s is followed to-day by O. Hertwig, especially in his “Mechanismus und Biologie.”(71) Lighter and more elegant was the polemic against vital force, and the outline of a mechanical theory which Du Bois-Reymond prefaced to his great work, “Untersuchungen über die tierische Electricität” (1849). It did not go nearly so deep as Lotze’s essay, but perhaps for that very reason its phrases and epigrams soon became common property. We may recall how he speaks of vital force as a “general servant for everybody,” of the iron atom which remains the same whether it be in the meteorite in cosmic space, in the wheel of the railway carriage, or in the blood of the thinker, and of analytic mechanics which may be applied even to the problem of personal freedom. The most comprehensive and detailed elaboration of the mechanical theory of life is to be found in Herbert Spencer’s “Principles of Biology.”(72) Friedrich Albert Lange’s “History of Materialism” is a brilliant plea for mechanical theories,(73) which he afterwards surpassed and neutralised by his Kantian Criticism. Verworn, too, in his “Physiology”(74) gives a clear example of the way in which the mechanical theory in its most consistent form is sublimed, apparently in the idealism of Kant and Fichte, but in reality in its opposite—the Berkeleyan psychology. A similar outcome is in various ways indicated in the modern trend of things. CHAPTER IX. CRITICISM OF MECHANICAL THEORIES.