221 MOKPHOLOGICAL DEVELOPMENT. 



Leaving out all that is hypothetical, the general argument 

 may be briefly presented thus : The evolution from the 

 simplest known vertebrate animal of a powerful and active 

 vertebrate animal, implies the development of a stronger 

 internal fulcrum. The internal fulcrum cannot be made 

 stronger without becoming more dense. And it cannot be 

 come more dense while retaining its lateral flexibility, with 

 out becoming divided into segments. Further, in conformity 

 with the general principles thus far traced, these segments 

 must be alike in proportion as the forces to which they are 

 exposed are alike, and unlike in proportion as these forces 

 are unlike; and so there necessarily results that unity in 

 variety by which the vertebral column is from the beginning 

 characterized. Once more, we see that the explanation ex 

 tends to those innumerable and more marked divergences 

 from homogeneity, which vertebras undergo in the various 

 higher animals. Thus, the production of vertebra?, the pro 

 duction of likenesses among vertebras, the production of un- 

 likenesses among vertebrae, and the production of unlike- 

 nesses among vertebral columns, are interpretable as parts of 

 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 mystery, offer another mystery of which no solution is 

 to be asked. It does not allege a Platonic iSe o, 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 cer 

 tain traits in vertebras and other bones, it extends to all 

 other traits of vertebras; and at the same time assimilates 

 the morphological phenomena they present to much wider 

 classes of morphological phenomena. 



