The explanation of the origin of new organs seemed at first to afford an insuperable difficulty to the Darwinian theory, since, as Mivart objected, it was not possible to perceive how natural selection could be able to effect the formation of new organs unless they executed corresponding functions from the very beginning. This difficulty, however, has been completely overcome by the theory of altered functions (_Functionswechsel_) which was first proposed by Dorhn, and particularly in recent years by Kleinenberg. According to this theory, in all these cases we have simply to deal with a gradual change in form of already existing organs, which, originally being used to perform one set of functions, are modified so as to perform another. Thus the later developed organs of mastication and the feelers of insects were originally organs of locomotion, legs; and these in the still earlier stages of creeping motion performed appropriate functions as the crooked appendages of the body-rings. The wings of birds were, in their progenitors, forelegs; the tongue of air-breathing vertebrates originated from the fish-bladder, which before that was chiefly an organ of swimming. The knowledge thus acquired of the natural connection of the processes of evolution also explains, according to Kleinenberg, why organs which are at present completely useless, must yet necessarily appear in the formation of the embryo; for example, the gill-openings in the higher classes of vertebrates, which have no functions to perform at any stage of vertebral development, and which furnished Meckel the first intimation of the fundamental biogenetic law. But as soon as it was explained that the gill-openings furnished the foundation of the development of later-appearing organs with actual functions to perform, it was rendered clear why they should continually recur; namely, because they form the indispensable links of a chain which extends from the dim past of the type in question down to the present time.