When sixteen years ago I first inquired into the actual and ultimate significance of sexual reproduction, I thought I had found it in this ceaseless production of new individualities. This seemed to me a sufficient reason for the introduction of amphimixis into nature, since the difference between individuals is the basis of the process of selection, and thus the basis of all the transformations of organisms, which we may refer to natural or sexual selection. Now these differences of selection-value are--as I believed then, and do still--not only by far the most frequent organic changes, but also the most important, since they not only initiate, but control new lines of evolution. Therefore I still regard amphimixis as the means by which a continual new combination of variations is effected a process without which the evolution of this world of organisms so endlessly diverse in form and so inconceivably complex, could not have taken place. But I do not regard this amphimixis as the real root of variation itself, for that must depend not on a mere exchange of ids, but rather upon a variation of the ids. The ids of a worm of the primitive world could not without variation now make up the germ-plasm of an elephant, even if it be true that mammals are descended from worms. The ids must have been meanwhile transformed times without number by the modification, degeneration, and new formation of determinants. Amphimixis, that is, the union of two germ-plasms, does not of itself cause variation of the determinants, it only arranges the ids (the ancestral plasms) in ever-new combinations. If the origin of variation were limited to that alone, a transmutation of species and genera would only be possible on a very limited scale; there could at most be a narrow circle of variations, just as in the example already given of the packs of cards; even if the taking away and mixing up of the halves were repeated a thousand times, a definite though undoubtedly large number of card-combinations would in the long run recur again and again. But the case is different with the germ-plasm and amphimixis, where there is an infinitely more varied series of results, because the individual cards--the ids--are variable, even between one time of sifting and shuffling and another, and therefore infinitely productive of variety in the course of numerous repetitions of the shuffling.