226 MORPHOLOGICAL DEVELOPMENT. 



about it; and in the Permian batrachians, equally related to 

 salamanders and frogs, these osseous elements are arranged in 

 the sheath or skin of the chorda ; and they are in the form of 

 regular concave segments, very much like such segments as 

 you can take from the skin of an orange — but parts of a 

 cylinder, and having greater or less dimensions according to 

 the group or species. Now, the point of divergence of these 

 segments is on the side of the column. The contacts are 

 placed on the side of the column where the segments separ- 

 ate — the upper segments rising and the lower segments 

 coming downward. To the upper segments are attached the 

 arches and their articulations, and the lower segments are 

 like the segments of a cylinder. If you take a flexible 

 cylinder, and cover it with a more or less inflexible skin or 

 sheath, and bend that cylinder 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 Permian 

 Batrachia. ... In the cylinder bending both ways, of 

 course the shortest line of curve is right at the centre of the 

 side of that cylinder, 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 construction 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. . . . 

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

 Vertebrata of the Permian period." 



In his essay on " The Mechanical Causes of the Develop- 

 ment of the Hard Parts of the Mammalia," published in the 

 American Journal of Morphology (Vol. Ill), Prof. Cope has 

 carried the interpretation further, by showing that in kindred 

 ways the genesis of articulations and limb-bones may be ex- 



