less entire; and the ossification of its sheath and substance is laid down in the three segments already described.
The two segments visible from one side of the column form two wedges with their apices together, and their bases one up and the other down. Now, if a person who wears a coat of rather thick material will examine the folds of his sleeve as they are produced on the inner side of his arm, he will see a figure nearly like that of the segments of the vertebral column described. The folds will correspond to the sutures, and the interspaces to the bony segments. He will find that the spaces are lens-shaped, or, when viewed in profile, wedge-shaped, with the apices together. This arrangement results from the necessary mechanics of flexure to one side. In flexure of a cylinder like the sleeve, or like a vertebral column, the shortest curve is along the line of the greatest convexity of the cylinder. Here is the closest folding of the sheath, and here, consequently, the lines of fold in soft material, or fracture in hard material, will converge and come together. That is just what they do in both the sleeve and the rhachitomous vertebral column, the only difference being that in the animal
Fig. 6.—The Folds on the Inner Side of a Coat-sleeve, which correspond with the lines separating the segments of the vertebræ of the Eryops. The letter i is the basal segment or intercentrum; the p corresponds with the lateral segment, the pleurocentrum; and n represents the basal part at the upper or neural arch, which rests on p and i.
it is exhibited on both sides, and on the sleeve on only one side. This difference is, of course, due to the fact that the animal can bend himself in both directions, while the arm only bends in one direction.
It results from the above observations that the structure of the rhachitomous vertebral column has been produced by the movements of the body from side to side, as in swimming, during the process of the deposit of mineral material in and around the chorda dorsalis.[1] Here we have another convincing proof that use and effort have produced animal structure.
Instances like the above can be cited from many departments of zoölogy wherever paleontology has pointed out the lines of descent. I will not cite them further, but will draw some conclusions which are necessary and which are of general. interest.
It is evident that use and effort imply some kind of movement on the part of the animal which puts them forth. Hence I have called
- ↑ This subject is more fully treated of in the "American Naturalist" for January 1884.
