The first comprehensive attempt to reduce to order and unity the ample biological material which systematic research had accumulated in the eighteenth century was made by what we call "the older natural philosophy" at the beginning of the nineteenth century. Reinhold Treviranus (of Bremen) had made a suggestive effort to accomplish this difficult task on monistic principles in his _Biology, or Philosophy of Living Nature_ (1802). Special importance attaches to the year 1809, in which Jean Lamarck (of Paris) published his _Philosophie Zoologique_, and Lorentz Oken (of Jena) his _Manual of Natural Philosophy_. I have fully appreciated the service of Lamarck, the founder of the theory of descent, in my earlier writings. I have also recognized the great merit of Lorentz Oken, who not only aroused a very wide interest in this science by his _General Natural History_, but also put forward some general observations of great value. His "infamous" theory of a primitive slime, and the development of infusoria out of it, is merely the fundamental idea of the theory of protoplasm and the cell which was long afterwards fully recognized. These and other services of the older natural philosophy were partly ignored and partly overlooked, because they went far beyond the scientific horizon of the time, and their authors to an extent lost themselves in airy and fantastic speculations. The more scientists confined themselves in the following half-century to empirical work and the observation and description of separate facts, the more it became the fashion to look down on all "natural philosophy." The most paradoxical feature of the situation was that purely speculative philosophy and idealist metaphysics had a great run at the same time, and their castles in the air, utterly destitute of biological foundation, were much admired.