482 SEGREGATION. 



feldspar, and mica, being kept for a long time in a fluid 

 and semi-fluid state — a state of comparative mobility — un- 

 dergo tliose changes of position which the forces impressed 

 on them by their fellow units necessitate. Having time in 

 which to generate the requisite motions of the atoms, the 

 differential forces arising from mutual polarity, segregate 

 the quartz, feldspar, and mica, into crystals. How com- 

 pletely this is dependent on the long-continued agitation of 

 the mixed particles, and consequent long-continued mobil- 

 ity by small differential forces, is proved by the fact that in 

 granite dykes, the crystals in the centre of the mass, where 

 the fluidity or semi-fluidity continued for a longer time, are 

 much larger than those at the sides, where contact with the 

 neighbouring rock caused more rapid cooling and solidifica- 

 tion. 



§ 166. The actions going on throughout an organism 

 are so involved and subtle, that we cannot expect to identify 

 the particular forces by which particular segregations are 

 effected. Among the few instances admitting of tolerably 

 definite interpretation, the best are those in which mechani- 

 cal pressures and tensions are the agencies at work. We 

 shall discover several on studying the bony frame of the 

 higher animals. 



The vertebral column of a man, is subject, as a whole, to 

 certain general strains — the weight of the body, together 

 with the reactions involved by all considerable muscular 

 efforts; and in conformity with this, it has become segre- 

 gated as a whole. At the same time, being exposed to differ- 

 ent forces in the course of those lateral bendings which the 

 movements necessitate, its parts retain a certain separate- 

 ness. And if we trace up the development of the vertebral 

 column from its primitive form of a cartilaginous cord in the 

 lowest fishes, we see that, throughout, it maintains an inte- 

 gration corresponding to the unity of the incident forces, 

 joined with a division into segments corresponding to the 



