It may well be that the length of time required before an artificially-selected race will exhibit, when interbred with the parent species, phenomena of hybridism similar to those which are witnessed when distinct natural species are interbred--will be fatal to the production of this important line of evidence. But there is nothing to hinder us from holding the reasonable belief that such evidence might be obtained if we had command of the necessary conditions; and in the meantime other evidence of the most satisfactory kind is accumulating, and on a vast scale. Whenever a naturalist approaches a problem in the light of the theory of natural selection, and is able, by its aid, to predict a conclusion which subsequent investigation proves to be correct, he is helping in the production of evidence in favour of the theory. When a naturalist has found the formula “if natural selection be true so-and-so ought to happen” the safest of all guides into the unknown, when it has brought him success many times and in very different directions, when he knows that many other workers in other fields of biological inquiry have had a similarly happy experience, he gradually comes to feel a profound confidence in the permanent truth and the far-reaching importance of the great theory which has served him so well. CHAPTER XIX. THE DIFFICULTY WITH WHICH THE “ORIGIN” WAS UNDERSTOOD. Even earlier than Huxley, H. C. Watson wrote warmly accepting natural selection. In his letter, which is dated November 21st, 1859, he said:-- “Your leading idea will surely become recognised as an established truth in science--_i.e._ ‘Natural Selection.’ It has the characteristics of all great natural truths, clarifying what was obscure, simplifying what was intricate, adding greatly to previous knowledge. You are the greatest revolutionist in natural history of this century, if not of all centuries.” [Sidenote: MISUNDERSTANDING OF THE THEORY.] For some years to come, however, such views as these were the exception, as will soon be shown. The Duke of Argyll has argued (_Nineteenth Century_, December, 1887) that the success of “Natural Selection” has followed from the convincing character of the words used, scientific men (“the populace of science” he calls them) being so easily led by the power of loose analogies that they have been convinced of the truth of the principle because they are familiar with Nature on the one hand, and selection as a process on the other!