He asked himself, therefore, how he could now explain certain things in nature. He had learned from the great Johannes Müller that species were unchangeable, and it was impossible to conceive the spontaneous generation of the living from the dead. The essence, the predominant element of the living thing was the mysterious, purposive “vital force.” The first of these three ideas of the master’s to be surrendered entirely by him was the vital force. Even in Müller’s lifetime, and in his own laboratory, so to say, his pupil, Du Bois-Reymond, made the first great breach in the doctrine with his famous study of animal electricity, a really pioneer piece of work, especially as regards method, at that time. It was now more than ever probable that there was no more a special vital force besides the simple natural forces than there was a God distinct from nature. The animal or the plant was a wonderful outcome of the same laws that had built the crystal or the globe. The sharp distinction between living and dead matter fell into the waste-basket, where so many other Dualistic tags lay, cut off by the shears of science. But if one of Müller’s theses was abandoned, another was retained as a real blessing with all the more tenacity by his pupils—the thesis that even the scientific investigator shall always “think”—nay, even “philosophise.” Müller called it “using one’s imagination,” in his desire to emphasise it. Now it was certainly a fair philosophic deduction from Du Bois-Reymond’s discoveries that one ought no longer to be so rigid as regards the possibility of spontaneous generation. If the same natural forces are at work in the organic and the inorganic, the living and the dead, it is no longer inconceivable theoretically that life and inorganic matter only differ in degree, not in kind. The distinction might become so slender—either now, or at least in past times—that an apparent “spontaneous generation” might really take place. Here again, it is plain, Haeckel had a greater freedom than Darwin. Working gradually from above, Darwin desisted when he came to spontaneous generation, and left room for God. Haeckel came into an open field, believing that there was no eternal Deity and that spontaneous generation itself was by no means a forbidding conception. The problem for him was merely, how he could work upward through the plants and animals of all geological periods until he reached man. He was bound to seek to dispense even here with the historical vital force, and explain everything by the great natural laws of the cosmos. It was in this frame of mind that he received Darwin’s book. Can it be in the least surprising that it “profoundly moved” him. It opened out to him the whole way, just as he desired it. Müller’s third thesis, the immutability of species, broke down. But what did it matter? It was now possible for the first time to construct a philosophical zoology and botany in Müller’s sense, without any vital force and without God.