258 THE THEORIES OF EVOLUTION 



other. Seen from the side they 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 verte- 

 bral 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 to- 

 gether. This arrangement results from the neces- 

 sary 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 con- 

 vexity of the cylinder. Here is the closest folding of 

 the sheath, and here, consequently, the lines of fold in 

 soft material, or interruption 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 

 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." . . . 



"From the rhachitomous column two divergent lines 

 have arisen. The inferior segment has been retained 

 in the fish-batrachian line, whence I have termed their 



