212 MORPHOLOGICAL DEVELOPMENT. 



and equal ; and from the centre we advance, through a reverse 

 series of changes, to the other side. 



Thus it is demonstrable that any substance in which the 

 power of resisting compression is unequal to the power of 

 resisting tension, cannot be subject to alternating transverse 

 strains, without having a central portion differentiated in its 

 conditions from the outer portions, and consequently dif- 

 ferentiated in its structure. This conclusion may easily be 

 verified by experiment. If something having a certain 

 toughness but not difficult to break, as a thick piece of sheet 

 lead, be bent from side to side till it is broken, the surface of 

 fracture will exhibit an unlikeness of texture between the 

 inner and outer parts. 



§ 255. And now for the application of this seemingly- 

 irrelevant truth. Though it has no obvious connection with 

 the interpretation of vertebral structure, we shall soon see 

 that it fundamentally concerns us. 



The simplest type of vertebrate animal, the fish, has a 

 mode of locomotion which involves alternating transverse 

 strains. It is not, indeed, subjected to alternating transverse 

 strains by some outer agency, as in the case we have been 

 investigating: it subjects itself to them. But though the 

 strains are here internally produced instead of externally 

 produced, the case is not therefore removed into a wholly 



94 different category. For sup- 

 mm ^. posing Fig. 284 to represent 



^-- -""" the outline of a fish when 



bent on one side (the dotted lines representing its outline 

 when the bend is reversed), it is clear that part of the sub- 

 stance forming the convex half must be in a state of tension. 

 This state of tension implies the existence in the other half 

 of some counter-balancing compression. And between the 

 two there must be a neutral axis. The way in which this 

 conclusion is reconcilable with the fact that there is tension 

 somewhere in the concave side of a fish, since the curve is 



