APPENDIX 379 



He claims that a primitive backbone * was formed 

 in this way, composed of wedges of bone, with their 

 bases placed alternately above and below. The folds 

 made in the thick cloth of a coat sleeve, by bending 

 the arm, illustrate, he says, the mechanical principles 

 involved in making this backbone. From this double 

 row of wedges, the author imagines that two distinct 

 kinds of vertebral columns have arisen; from the 

 lower row, the backbone of the fish, and, from the 

 upper, that of the higher vertebrates. 



Having derived these two kinds of vertebral col- 

 umns in this way, it becomes easy, by a still more lib- 

 eral use of the imagination, to account for the various 

 styles of vertebrae; viz., concave in front, concave 

 behind, concave at both ends, and plane at both ends, 

 with various modifications of these forms. In a word, 

 it may be said that all the joints of the body, and the 

 shapes and sizes of all the bones, have- been deter- 

 mined by impact, strain, friction, and torsion. 



The same is true of teeth: " The history of the in- 

 cisor teeth of the Mammalia exhibits three processes, "f 

 hypertrophy, specialization and atrophy. The " more 

 severe, direct irritation from use than any others in 

 the jaws," % will account for the increased diameter 

 of the molars. " The origin of the canine " teeth " is 

 due to the strains sustained by them." || "The pro- 

 gressive lengthening of the incisors " § has been 

 through use. As to the Artiodactyla, "Why the 

 superior incisors should have disappeared in this 

 group, is not yet clear to my mind." If It is, indeed, 

 refreshing, to find even one case which the author can 

 not explain readily by impact, strain, and friction, nor 

 by disuse. 



It is claimed that the crowns of teeth, which differ 

 widely in structure, have been produced by wear, and 



* Page 370. t Page 32S. % Page 331. fPage 331. 



§ Page 346. 1 Page 357. 



