204 MORPHOLOGICAL DEVELOPMENT. 



rate it will be clear that the efforts made by fish in rushing 

 upon prey or escaping enemies (and it is these extreme efforts 

 which here concern us) must, as fish become more active, 

 rapidly exalt the strains to be borne by their motor organs ; 

 and that of these strains, those which fall upon the noto- 

 chord must be exalted in proportion to the rest. Thus the 

 development of locomotive power, which survival of the 

 fittest must tend in most cases to favour, involves such in 

 crease of stress on the primitive cartilaginous rod as will 

 tend, other things equal, to cause its modification. 



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 



4& 



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 war 

 rant us in supposing, a formation of denser substance occurs 



