SEGREGATION. 483 



variety of the incident forces. Each segment, con- 



sidered apart, exemplifies the truth more simply. A verte- 

 bra is not a single bone, but consists of a central mass with 

 sundry appendages or processes; and in rudimentary types 

 of vertebrae, these appendages are quite separate from the 

 central mass, and, indeed, exist before it makes its appear- 

 ance. But these several independent bones, constituting a 

 primitive spinal segment, are subject to a certain aggregate 

 of forces which agree more than they differ : as the fulcrum 

 to a group of muscles habitually acting together, they per- 

 petually undergo certain reactions in common. And ac- 

 cordingly, we see that in the course of development they 

 gradually coalesce. Still clearer is the illustration 



furnished by spinal segments that become fused together 

 where they are together exposed to some predominant strain. 

 The sacrum consists of a group of vertebrae firmly united. 

 Ir the ostrich and its congeners there are from seventeen to 

 twenty sacral vertebrae; and besides being confluent with 

 each other, these are confluent with the iliac bones, which 

 run on each side of them. If now we assume these vertebrae 

 to have been originally separate, as they still are in the em- 

 bryo bird; and if we consider the mechanical conditions to 

 which, they must in such case have been exposed; we shall 

 see that their union results in the alleged way. For through 

 these vertebrae the entire weight of the body is transferred 

 to the legs : the legs support the pelvic arch ; the pelvic arch 

 supports the sacrum; and to the sacrum is articulated the 

 rest of the spine, with all the limbs and organs attached to 

 it. Hence, if separate, the sacral vertebrae must be held 

 firmly together by strongly-contracted muscles; and must, 

 by implication, be prevented from partaking in those lateral 

 movements which the other vertebrae undergo they must 

 be subject to a common strain, while they are preserved 

 from strains which would affect them differently; and 

 so they fulfil the conditions under which segregation 

 occurs. But the case's in which cause and effect 



