208 MORPHOLOGICAL DEVELOPMENT. 



exposed them, have involved variations in the process of 

 solidification. 



259. Of course the foregoing synthesis is to be taken 

 simply as an adumbration of the process by which the verte- 

 brate structure may have arisen through the continued actions 

 of known agencies. The motive for attempting it has been 

 two-fold. Having, as before said, given reasons for con- 

 cluding that the segments of a vertebrate animal are not 

 homologous in the same sense as those of an annulose animal 

 or a phaenogamic axis, it seemed needful to do something 

 towards showing how they are otherwise to be accounted for ; 

 and having here, for our general subject, the likenesses and 

 differences among the parts of organisms, as determined by 

 incident forces, it seemed out of the question to pass by the 

 problem presented by the vertebrate skeleton. 



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 vertebrae undergo in the various 

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

 duction of likenesses among vertebrae, and the production of 

 unlikenesses among vertebrae, are interpretable as parts of 



