§ 259. Of course the foregoing synthesis is to be taken simply as an adumbration of the process by which the vertebrate structure may have arisen through the continued actions of known agencies. The motive for attempting it has been two-fold. Having, as before said, given reasons for concluding that the segments of a vertebrate animal are not homologous in the same sense as are those of an annulose animal, it seemed needful to do something towards showing how they are otherwise to be accounted for; and having here, for our general subject, the likenesses and differences among the parts of organisms, as determined by incident forces, it seemed out of the question to pass by the problem presented by the vertebrate skeleton. Leaving out all that is hypothetical, the general argument may be briefly presented thus:--The evolution from the simplest known vertebrate animal of a powerful and active vertebrate animal, implies the development of a stronger internal fulcrum. The internal fulcrum cannot be made stronger without becoming more dense. And it cannot become more dense while retaining its lateral flexibility, without becoming divided into segments. Further, in conformity with the general principles thus far traced, these segments must be alike in proportion as the forces to which they are exposed are alike, and unlike in proportion as these forces are unlike; and so there necessarily results that unity in variety by which the vertebral column is from the beginning characterized. Once more, we see that the explanation extends to those innumerable and more marked divergences from homogeneity, which vertebræ undergo in the various higher animals. Thus, the production of vertebræ, the production of likenesses among vertebræ, the production of unlikenesses among vertebræ, and the production of unlikenesses among vertebral columns, are interpretable as parts of one general process, and as harmonizing with one general principle. Whether sufficient or insufficient, the explanation here given assigns causes of known kinds producing effects such as they are known to produce. It does not, as a solution of one mystery, offer another mystery of which no solution is to be asked. It does not allege a Platonic ἰδέα, or fictitious entity, which explains the vertebrate skeleton by absorbing into itself all the inexplicability. On the contrary, it assumes nothing beyond agencies by which structures in general are moulded--agencies by which these particular structures are, indeed, notoriously modifiable. An ascertained cause of certain traits in vertebræ and other bones, it extends to all other traits of vertebræ; and at the same time assimilates the morphological phenomena they present to much wider classes of morphological phenomena. * * * * *