This theory of descent was advocated by Lamarck and by Geoffrey St. Hilaire. It did not, however, meet with general approval. These naturalists could not prove the correctness of this theory and, therefore, it remained only a hypothesis, a mere assumption. When Darwin came, however, with his main book, The Origin of Species, it struck like a thunderbolt; his theory of evolution was immediately accepted as a strongly proved truth. Since then the theory of evolution has become inseparable from Darwin’s name. Why so? This was partly due to the fact that through experience ever more material was accumulated which went to support this theory. Animals were found which could not very well be placed into the classification such as oviparous mammals (that is, animals which lay eggs and nourish their offspring from their breast.――Translator), fishes having lungs, and invertebrate animals. The theory of descent claimed that these are simply the remnants of the transition between the main groups. Excavations have revealed fossil remains which looked different from animals living now. These remains have partly proved to be the primitive forms of our animals, and that the primitive animals have gradually developed to existing ones. Then the theory of cells was formed; every plant, every animal, consists of millions of cells and has been developed by incessant division and differentiation of single cells. Having gone so far, the thought that the highest organisms have descended from primitive beings having but a single cell, could not appear as strange. All these new experiences could not, however, raise the theory to a strongly proved truth. The best proof for the correctness of this theory would have been to have an actual transformation from one animal kind to another take place before our eyes, so that we could observe it. But this is impossible. How then is it at all possible to prove that animal forms are really changing into new forms? This can be done by showing the cause, the propelling force of such development. This Darwin did. Darwin discovered the mechanism of animal development, and in doing so he showed that under certain conditions some animal-kinds will necessarily develop into other animal-kinds. We will now make clear this mechanism.