Let us now advert to another of Mr. Spencer's illustrations of the futility of the "supernatural" and of the rationality of the "natural" interpretation.[82] This illustration is derived from what are called "homologous" organs; and the particular instance selected is the vertebral column.[83] There are creatures, such as snakes, a low order of the vertebrate kingdom, in which the bony axis is divided into segments of about the same dimensions from end to end, for the obvious advantage of flexibility throughout the whole length of the animal. But in most of the higher vertebrata, some parts of this axis are flexible and others are inflexible; and this is especially the case in that part of the vertebral column called the sacrum, which is the fulcrum that has to bear the greatest strain to which the skeleton is exposed, and which is yet made not of one long segment or vertebra, but of several segments "fused together." Mr. Spencer says: "In man there are five of these confluent sacral vertebræ; and in the ostrich tribe they number from seventeen to twenty. Why is this? Why, if the skeleton of each species was separately contrived, was this bony mass made by soldering together a number of vertebræ like those forming the rest of the column, instead of being made out of one single piece? And why, if typical uniformity was to be maintained, does the number of sacral vertebræ vary within the same order of birds? Why, too, should the development of the sacrum be the roundabout process of first forming its separate constituent vertebræ, and then destroying their separativeness? In the embryo of a mammal or bird, the substance of the vertebral column is, at the outset, continuous. The segments that are to become vertebræ, arise gradually in the midst of this originally homogeneous axis. Equally in those parts of the spine which are to remain flexible, and in those which are to grow rigid, these segments are formed, and that part of the spine which is to compose the sacrum, having passed out of its original unity into disunity by separating itself into segments, passes again into unity by the coalescence of these segments. To what end is this construction and reconstruction? If, originally, the spine in vertebrate animals consisted from head to tail of separate movable segments, as it does still in fishes and some reptiles--if, in the evolution of the higher vertebrata, certain of these movable segments were rendered less movable with respect to each other, by the mechanical conditions to which they were exposed, and at length became relatively immovable--it is comprehensible why the sacrum formed out of them should continue ever after to show more or less clearly its originally segmented structure. But on any other hypothesis this segmented structure is inexplicable."