On the return journey from South America, which amounted to a circumnavigation of the globe, the struggle was renewed at the Galapagos islands. Volcanic forces had raised these islands from the bed of the ocean in comparatively recent times. They were, therefore, bound to be a virgin province at the time. Now, however, the walls of the crater were clothed with vegetation, birds flew after insects, and gigantic turtles and lizards lived on the shores. Whence did these plants and animals come? Darwin examines them. They have an unusual appearance, and seem to point to America. Yet not a single species is now wholly American; each has its peculiarities. An historical controversy arises over the islands, and men range themselves in parties once more. Empty islands emerge from the blue waters. How are they to be populated? There are two possibilities. One is that God has created the animals and plants—Galapagos animals and plants. But in that case why has he created them entirely on the American model, while diverging from it in small details? The second possibility is that the animals and plants were brought by the current or the wind from the neighbouring American coast; they are American plants and animals. After landing on the islands, they adapted themselves to their new surroundings, and were altered. Hence both the resemblance and the difference. The theory assumes, of course, that species are mutable. If that is so, we can explain everything—without God. But the greatest and tensest struggle began when Darwin returned home. He approached the most audacious, but most striking fact, for his purpose. Up to this the question had been whether new species were produced by God or by natural necessity. Now a third element was introduced, man himself. He also alters species, as a breeder of pigeons, rabbits, sheep. He has done it with success for ages—only the Linnés and Cuviers had not noticed the fact. How does he accomplish it? A breeder desires to give his sheep finer wool. He examines the wool of a thousand sheep. The difference between them is so slight that it is of no practical consequence. But the farmer selects the male sheep out of the thousand that has the best quality of wool, and the corresponding female. He crosses the two. Their young have wool of a slightly improved quality, and he picks out the best amongst them once more for crossing. He continues this through several generations. At last, with his continuous selection and crossing, the quality of the wool increases so much that any one can recognise it at once, and it has a distinct cultural value. In this way improved races of animals and large numbers of fine flowers have been produced by breeders: by artificial selection of the fittest to reproduce in each generation. This was done by man—not by God, not by nature in remote times, but under our very eyes, by man.