The evolutionary process in organic nature is much more difficult to grasp. Here we must distinguish two different series of biological development, which have only been brought into proper causal connection by means of our biogenetic law (1866); one series is found in embryology (or ontogeny), the other in phylogeny (or race-development). In Germany "evolution" always meant embryology, or a part of the whole, until forty years ago. It stood for a microscopic examination of the wonderful processes by means of which the elaborate structure of the plant or animal body is formed from the simple seed of the plant or the egg of the bird. Until the beginning of the nineteenth century the erroneous view was generally received that this marvellously complicated structure existed, completely formed, in the simple ovum, and that the various organs had merely to grow and to shape themselves independently by a process of "evolution" (or unfolding), before they entered into activity. An able German scientist, Caspar Friedrich Wolff (son of a Berlin tailor), had already shown the error of this "pre-formation theory" in 1759. He had proved, in his dissertation for the doctorate, that no trace of the later body, of its bones, muscles, nerves, and feathers, can be found in the hen's egg (the commonest and most convenient object for study), but merely a small round disk, consisting of two thin superimposed layers. He had further showed that the various organs are only built up gradually out of these simple elements, and that we can trace, step by step, a series of real new growths. However, these momentous discoveries, and the sound "theory of epigenesis" that he based on them, were wholly ignored for fifty years, and even rejected by the leading authorities. It was not until Oken had re-discovered these important facts at Jena (1806), Pander had more carefully distinguished the germinal layers (1817), and finally Carl Ernst von Baer had happily combined observation and reflection in his classical _Animal Embryology_ (1828), that embryology attained the rank of an independent science with a sound empirical base.