They can shew that in cultivated plants, domesticated animals, and in the several races of men, such alterations have taken place. They can show that the degrees of difference so produced are often, as in dogs, greater than those on which distinctions of species have been founded. They can shew, too, that the changes daily taking place in ourselves--the facility that attends long practice, and the loss of aptitude that begins when practice ceases,--the strengthening of the passions habitually gratified, and the weakening of those habitually curbed,--the development of every faculty, bodily, moral, intellectual, according to the use made of it--are all explicable on this principle. And thus they can shew that throughout all organic nature there is at work a modifying influence of the kind they assign as the cause of these specific differences; an influence which, though slow in its action, does, in time, if the circumstances demand it, produce marked changes--an influence which, to all appearance, would produce in the millions of years, and under the great varieties of condition which geological records imply, any amount of change." These and many other instances which might be brought together from the published writings of the half-century before the publication of the _Origin_, show conclusively that the idea of evolution was far from new, and that all through the first part of this century dissatisfaction with the doctrine of the fixity of species and of their miraculous creation was growing. The great contribution of Darwin was this: First, by his theory of natural selection, he brought together the known facts of variation, of struggle for existence, and of adaptation to varying conditions, in such a way that they provided men with a rational and known cause, a cause the operation of which could be seen, for the origin of species by means of preservation of favoured races. Next, as to the origin of species, he brought together not only proofs of the actual operation of natural selection, but a body of evidence in favour of the fact of evolution that was, beyond all comparison, more striking than had been adduced by any earlier philosophical or biological writer. He convinced naturalists that evolution was by far the most probable way in which the living world had come to be what it is, and he made them turn to examination of the animal and vegetable kingdoms with a lively hope that the past history of the living world was not an insoluble problem. Darwin's doctrine brought a new life into biological study, and the result of the incomparably greater bulk of investigation that followed the year 1859 was a continual increase of evidence in favour of the probability of evolution, until now the whole scientific world, and the majority of those who are unscientific, are content to accept evolution as the only reasonable explanation of the living world.