The spine of a vertebrate animal is subjected to certain general strains-- the weight of the body, together with the reactions involved by all considerablemuscular efforts; and under these conditions it has become segregated asa whole. At the same time being exposed to different forces during thoselateral bendings which the movements necessitate, its parts retain a certainseparateness. If we trace up the development of the vertebral column fromits primitive form of a cartilaginous cord in the lowest fishes, we see that,throughout, it maintains an integration corresponding to the unity of theincident forces, joined with a division into segments corresponding to thevariety of the incident forces. Each segment, considered apart, exemplifiesthe truth more simply. A vertebra is not a single bone, but consists of acentral mass with sundry appendages or processes, and in unfinished typesof vertebra these appendages are separate from the central mass, and, indeed,exist before it makes its appearance. But these several independent bonesconstituting a primitive spinal segment, are subjected to a certain aggregateof forces which agree more than they differ: as the fulcrum to a group ofmuscles habitually acting together, they perpetually undergo certain reactionsin common. And accordingly, in the course of development, they graduallycoalesce. Still clearer is the illustration furnished by spinal segmentsthat become fused together where they are together exposed to some predominantstrain. The sacrum consists of a group of vertebra firmly united. In theostrich and its congeners there are from seventeen to twenty sacral vertebra;and, besides being confluent with one another, these are confluent withthe iliac bones, which run on each side of them. If, now, we assume thesevertebra to have been originally separate, as they still are in the embryobird, and if we consider the forces to which they must in such case havebeen exposed, we shall see that their union results in the alleged way. Forthrough these vertebra the entire weight of the body is transferred to thelegs: the legs support the pelvic arch; the pelvic arch supports the sacrum;and to the sacrum is articulated the rest of the spine, with all the organsattached to it and upheld by it. Hence, if separate, the sacral vertebramust be held firmly together by strongly-contracted muscles, and must, byimplication, be prevented from partaking in those lateral movements whichthe other vertebra undergo -- they must be subjected to a common strain,while they are preserved from strains which would affect them differently;and so they fulfil the conditions under which segregation occurs. But thecases in which cause and effect are brought into the most obvious relation,are supplied by the limbs. The metacarpal bones (those which in man supportthe palm of the hand) are separate from one another in most mammals: theseparate actions of the toes entailing on them slight amounts of separatemovements. This is not so however in the ox-tribe and the horse-tribe. Inthe ox-tribe, only the middle metacarpals (third and fourth) are developed;and these, attaining massive proportions, coalesce to form the cannon bone.
In the horse-tribe, the segregation is what we may distinguish as indirect: the second and fourth metacarpals are present only as rudiments united tothe sides of the third, while the third is immensely developed; thus forminga cannon bone which differs from that of the ox in being a single cylinder,instead of two cylinders fused together. The metatarsus in these quadrupedsexhibits parallel changes. Now each of these metamorphoses occurs where thedifferent bones grouped together have no longer any different functions,but retain only a common function. The feet of oxen and horses are used solelyfor locomotion -- are not put, like those of unguiculate mammals, to purposeswhich involve some relative movements of the metacarpals. Thus there directlyor indirectly results a single mass of bone where the incident force is single.
And for the inference that these facts have a causal connexion, we find confirmationthroughout the entire class of birds, in the wings and legs of which, likesegregations are found under like conditions. While this sheet is passingthrough the press (1862), a fact illustrating this general truth in a yetmore remarkable manner, has been mentioned to me by Prof. Huxley who kindlyallows me to make use of it while still unpublished by him. The Glyptodon,an extinct mammal found fossilized in South America, has long been knownas a large uncouth creature allied to the Armadillo, but having a massivedermal armour consisting of polygonal plates closely fitted together so asto make a vast box, inclosing the body in such way as effectually to preventit from being bent, laterally or vertically, in the slightest degree. Thisbox, which must have weighed several hundredweight, was supported on thespinous processes of the vertebrae, and on the adjacent bones of the pelvicand thoracic arches. And the significant fact is that here, where the trunkvertebrae were together exposed to the pressure of this heavy dermal armour,at the same time that, by its rigidity, they were preserved from all relativemovements, they were united into one solid, continuous bone.
The formation and maintenance of a species, considered as an assemblageof similar organisms, is interpretable in an analogous way. Already we haveseen that in so far as the members of a species are subject to differentsets of incident forces, they are differentiated, or divided into varieties.
