306 THE STRUCTURAL EVIDENCE OF EVOLUTION. 



sidewise, you of course will find that the wrinkles or fractures of 

 that part of the surface will take place along the line of the 

 shortest curve, which is on the side ; and, as a matter of fact, you 

 have breaks of very much the character of the segments of the Per- 

 mian Batrachia. It may not be so symmetrical as in the actual 

 animal, for organic growth is symmetrical so far as not interfered 

 with ; for, when we have two forces, the one of hereditary growth, 

 and the other of change or alteration, and they contend, you will 

 find in the organic being a quite symmetrical result. That is the 

 universal rule. In the cylinder bending both ways, of course the 

 shortest line of curve is right at the center of the side of that cyl- 

 inder, and the longest curve is of course at the summit and base, 

 and the shortest curve will be the point of fracture. And that is 

 exactly what I presume has happened in the case of the construc- 

 tion of the segments of the sheath of the vertebral column, by the 

 lateral motion of the animal in swimming, and which has been the 

 actual cause of the disposition of the osseous material in its form. 

 I have gone beyond the state of the discussion in calling attention 

 to one of the forces which have probably produced this kind of 

 result. That is the state of the vertebral column of many of the 

 Vertebrata of the Permian period.* 



I go back to the Mammalia and call attention to the teeth. 

 The ordinary tooth of the higher type of the Mammalia, whether 

 hoofed or not, with some exceptions, is complex with crests or 

 cusps. By cutting the complex grinding surfaces, we find they 

 have been derived by the infolding of extensions of four original 

 cusps or tubercles. They have been flattened, have been rendered 

 oblique, have run together, have folded up, have become acute, 

 have descended deeply, or have lifted themselves, so that we have 

 teeth of all sorts and kinds, sometimes very elegant, and often- 



* Note (Ed. 1886). — Some further elucidation of this point is necessary, since the 

 fishes have not produced this kind of segmentation by the lateral motions of the 

 vertebral column in swimming ; at least, such segmentation is not yet known among 

 them, but rather simple discoidal segmentation. The rhachitomous segmentation 

 above described would result from a greater flexure than that required for the 

 propulsion of a fish through the water. This increased flexure was no doubt the 

 concomitant of the acquisition of a terrestrial mode of life by the early batrachians. 

 Progression on land by an animal with weak limbs requires much greater flexure of 

 the column than the act of swimming by the use of the caudal part of the body as 

 in fishes. One can easily convince himself of this by comparing the movements of 

 a fish in the water with those of a salamander on land. The snakes, where limbs 

 are wanting, show an even stronger flexure in progression. 



