While thus the ideas of Lamarck gradually fell into partial oblivion, yet contemporaneous with and following them arose several other series of thoughts, views, and investigations, which, although they only indirectly prepared for the revival of the evolution theory, yet exercised a deeper and more lasting influence on the minds of scientists. We refer to the ideas in regard to natural phenomena held during the first decades of our century; further, to the principles of comparative anatomy which, up to the present time, partly dependent {34} and partly independent of natural philosophy, have been expressed, valued, and admired as leading thoughts; and, lastly, to the empiric results of comparative anatomical and biological investigations in palaeontology and geology, as attained by the help of those very principles. And even physics and astronomy had to cooeperate in preparing the way for the idea of evolution. The philosophical ideas referred to, together with the points of view and results of comparative anatomy, led more and more decisively to the idea of an _original form_, or _type_, which retains its identity in all the modifications of form in plants and animals; and of a _ground-plan_, which is realized in the systems of the plant and animal world in higher and higher differentiations and in more and more developed modifications, diverging farther and farther from the prototype until it reaches its highest form, still reducible to the prototypes, in the most highly organized dicotyledons in plants, and in the animal world in the mammalia, and lastly in man.