THE SHAl'ES OF VERTEBRATE SICEI.ETONS. 209 



one general process, and as harmonizing with one general 

 principle. 



Whether sufficient or insufficient, the explanation here 

 given assigns causes of known kinds producing effects such 

 as they are known to produce. It does not, as a solution of 

 one mj'stery, offer another mystery of which no solution is 

 to be asked. It does not allege a Platonic Ibia, or fictitious 

 entity, which explains the vertebrate skeleton by absorbing 

 into itself all the inexplicability. On the contrary, it assumes 

 nothing beyond agencies by which structures in general are 

 moulded — agencies by which these particular structures are, 

 indeed, notoriously modifiable. An ascertained cause of 

 certain traits in vertebrae and other bones, it extends to all 

 other traits of vertebrae ; and at the time assimilates the 

 morphological phenomena they present to much wider classes 

 of morphological phenomena. 



