THE SHAPES OF VERTEBRATE SKELETONS. 221 



What must its modification be ? Considering the compli- 

 cation of the influences at work, conspiring, as above indi- 

 cated, in various ways and degrees, we cannot expect to do 

 more than form an idea of its average character. The nature 

 of the changes which the notochord is likely to undergo, where 

 greater bulk is accompanied by higher activity, is rudely 

 indicated by Figs. 291, 292, and 293. The successively 



*91 



thicker lines represent the successively greater strains to 

 which the outer layers of tissue are exposed; and the widen- 

 ing inter-spaces represent the greater extensions which they 

 have to bear when they become convex, or else the greater 

 gaps that must be formed in them. Had these outer layers 

 to undergo extension only, as on the convex side, continued 

 natural selection might result in the formation of a tissue 

 elastic enough to admit of the requisite stretching. But at 

 each alternate bend these outer layers, becoming concave, 

 are subject to increased compression a compression which 

 they cannot withstand if they have become simply more 

 extensible. To withstand this greater compression they must 

 become harder as well as more extensible. How are these 

 two requirements to be reconciled? If, as facts warrant 

 us in supposing, a formation of denser substance occurs at 

 those parts of the notochord where the strain is greatest; 

 it is clear that this formation cannot so go on as to produce 

 a continuous mass : the perpetual flexions must prevent this. 

 If matter that will not yield at each bend, is deposited while 

 the bendings are continually taking place, the bendings will 

 maintain certain places of discontinuity in the deposit 

 places at which the whole of the stretching consequent on 

 each bend will be concentrated. And thus the tendency will 

 be to form segments of hard tissue capable of great resistance 



