[NOTE.--The theory set forth in the foregoing chapter, is an elaboration of one suggested at the close of a criticism of Prof. Owen’s _Archetype and Homologies of the Vertebrate Skeleton_, already referred to in § 210 as having been published in the _Medico-Chirurgical Review_ for October, 1858. It is now reproduced in Appendix B. Since the issue of this elaborated exposition, in No. 15 of my serial in December, 1865, verifications of it have from time to time been published. In his work _The Primary Factors of Organic Evolution_, Prof. Cope of Philadelphia writes:-- “Mr. Herbert Spencer has endeavoured to account for the origin of the segmentation of muscles into myotomes, and the division of the sheath of the notochord into vertebræ, by supposing it to be due to the lateral swimming movements of the fishes, which first exhibit these structures. With this view various later authors have agreed, and I have offered some additional evidence of the soundness of this position with respect to the vertebral axis of Batrachia, and the origin of limb articulations. It is true that the origin of segmentation in the vertebral column of the true fishes and the Batrachia turns out to have been less simple in its process than was suggested by Mr. Spencer, but his general principle holds good, now that paleontology has cleared up the subject” (pp. 367–8). An allusion in the foregoing extract is made by Prof. Cope to certain observations set forth in his work entitled _The Origin of the Fittest_. On pp. 305–6 of it will be found the following sentences:--