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412 of the stigma to be the female principle, for experiment had taught him, that if a stigma exchanges the moisture from another stigma for its own, and is then dusted with its own pollen, hybrid form is produced. In any case Koelreuter had a more correct idea of the nature of sexual fertilisation than any of his predecessors, and it was one specially adapted to enable his contemporaries to understand the results of experiments in hybridisation, while the hybrids themselves supplied most convincing arguments against the prevailing theory of evolution.

We have arrived at Koelreuter's most important performance, the production of hybrids. Here was a case for skilful experimentation, not for microscopic observation, and here he obtained results in which nothing afterwards required to be changed, but which when combined with later observations have been used for the discovery of general laws in hybridisation. The first hybrid which he obtained by placing the pollen of Nicotiana paniculata on the stigmas of N. rustica, produced pollen that was impotent; but he soon after obtained hybrids from the two species which produced seeds capable of germination, and in 1763 he described a considerable number of hybrids in the genera Nicotiana, Kedmia, Dianthus, Matthiola, Hyoscyamus, and others. In the last portion of his great work (1766) he speaks of eighteen attempts to obtain hybrids with five native species of Verbascum, and submits Linnaeus' views on hybrid plants, which we have already described, to a withering criticism. He shows at the same time from experiment, that if the stigma of a plant receives its own pollen and pollen from another plant at the same time, the former only is effectual, and that this is one reason why hybrids which can be raised artificially are not found in nature. We must not attempt to give a detailed account of his famous hybrids of the third, fourth, and fifth degrees, nor of his experiments on other points, such as the reverting of