202 MORPHOLOGICAL DEVELOPMENT. 



257. After these explanations the process of eventual 

 segmentation in the spinal axis itself, will be readily under- 

 stood. The original cartilaginous rod has to maintain longi- 

 tudinal rigidity while permitting lateral flexion. As fast as 

 it becomes definitely marked out, it will begin to concentrate 

 within itself a great part of those pressures and tensions 

 caused by transverse strains. As already said, it must be 

 acted upon much in the same manner as a bow, though it is 

 bent by forces acting in a more indirect way ; and like a bow, 

 it must, at each bend, have the substance of its convex side 

 extended and the substance of its concave side compressed. 

 So long as the vertebrate animal is small or inert, such a 

 cartilaginous rod may have sufficient strength to withstand 

 the muscular strains ; but, other things equal, the evolution 

 of an animal that is large, or active, or both, implies mus- 

 cular strains that must tend to cause modification in such a 

 cartilaginous rod. The results of greater bulk and of greater 

 vivacity may be best dealt with separately. As the 



animal increases in size, the rod will grow both longer and 

 thicker. On looking back at the diagrams of forces caused 

 by transverse strains, it will be seen that as the rod grows 

 thicker, its outer parts must be exposed to more severe ten- 

 sions and pressures, if the degree of bend is the same. It is 

 doubtless true that when the fish or reptile, advancing by 

 lateral undulations, becomes longer, the curvature assumed 

 by the body at each movement becomes less ; and that from 

 this cause the outer parts of the notochord are, other things 

 equal, less strained the two changes thus partially neutral- 

 izing one another. But other things are not equal. For 

 while, supposing the shape of the body to remain con- 

 stant, the force exerted in moving the body increases as the 

 cubes of its dimensions, the sectional area of the notochord, 

 on which fall the reactions of this exerted force, increases 

 only as the squares of the dimensions : whence results an 

 intenser stress upon its substance. Merely noting that the 

 other varying factor the resistance of the water may here 



