THE ARGUMENTS FROM MORPHOLOGY. 471 



this rigid portion of the vertebral column is not made of 

 one long segment or vertebra, but of several segments fused 

 together. In man there are five of these confluent sacral 

 vertebra?; and in the ostrich tribe they number from seven- 

 teen to twenty. Why is this? Why, if the skeleton of each 

 species was separately contrived, was this bony mass made 

 by soldering together a number of vertebra like those forming 

 the rest of the column, instead of being made out of one 

 single piece? And why, if typical uniformity was to be 

 maintained, does the number of sacral vertebrae vary within 

 the same order of birds? Why, too, should the development 

 of the sacrum be by the round-about process of first forming 

 its separate constituent vertebra, and then destroying their 

 separateness ? In the embryo of a mammal or bird, the 

 central element of the vertebral column is, at the outset, 

 continuous. The segments that are to become vertebrae, 

 arise gradually in the adjacent mesoderm, and enwrap this 

 originally-homogeneous axis or notochord. Equally in those 

 parts of the spine which are to remain flexible, and in those 

 parts which are to grow rigid, these segments are formed; 

 and that part of the spine which is to compose the sacrum, 

 having acquired this segmental structure, loses it again by 

 coalescence of the segments. To what end is this construc- 

 tion and re-construction? If, originally, the spine in verte- 

 brate animals consisted from head to tail of separate move- 

 able segments, as it does still in fishes and some reptiles if, 

 in the evolution of the higher Vertebrata, certain of these 

 moveable segments were rendered less moveable with respect 

 to one another, by the mechanical conditions they were 

 exposed to, and at length became relatively immovable; it 

 is comprehensible why the sacrum formed out of them, 

 should continue ever after to show its originally-segmented 

 structure. But on any other hypothesis this segmented 

 structure is inexplicable. " We see the same law 



in comparing the wonderfully complex jaws and legs in crus- 

 taceans," says Mr. Darwin: referring to the fact that those 



