THE SHAPES OF VERTEBRATE SKELETONS. 227 



plained. On p. 163 he enunciates the general principle of 

 his interpretation as follows : — 



"It cannot have been otherwise than that, since the 

 motions of animals continued during the evolution of their 

 hard parts, these hard parts grew in exact adaptation to 

 these movements. Thus at the points of greatest flexure 

 joints would be formed, and between these joints the deposit 

 would be continuous." 



Evidently if osseous structures are produced by deposits 

 of calcareous matters in pre-existing cartilaginous structures, 

 or other structures of flexible materials, the deposits must be 

 so carried on that while dense resistant masses are produced 

 these must admit of such free movements as the creature's 

 life necessitates, and must so form adapted joints. 



Let it be understood, however, that the hypothesis set 

 forth in the foregoing chapter and extended by Prof. Cope, 

 which serves to interpret a large part of the phenomena 

 of osseous structures in the Vertebrata, does not serve to 

 interpret them all. While the formation of hard parts has 

 been in large measure initiated and regulated by tensions and 

 pressures, there are hard parts the formation of which cannot 

 be thus explained. The bones of the skull are the most 

 obvious instances. These are apparently referable to no 

 other cause than the survival of the fittest — the survival of 

 individual animals in which greater density of the brain- 

 covering yielded better protection against external injuries. 

 Without enumerating other instances which might be given, 

 it will suffice to recognize the truth that natural selection of 

 favourable variations and the inheritance of functionally- 

 produced changes have all along co-operated: each of them 

 in some cases acting alone, but in other cases both acting 

 together.] 



