THE ARGUMENTS FKOM MORPHOLOGY. 383 



segment or vertebra, but of several segments fused to- 

 gether. In man there are five of these confluent sacral 

 vertebrae ; 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 vertebras like those 

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

 one simple 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 develop- 

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

 forming its separate constituent vertebrae, and then de- 

 stroying their separateness ? In the embryo of a mammal 

 or bird, the substance of the vertebral column is, at the out- 

 set, continuous. The segments that are to become vertebrae, 

 arise gradually in the midst of this originally-homogeneous 

 axis. 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 passed out of its ori- 

 ginal unity into disunity, by separating itself into segments, 

 passes again into unity by the coalescence of these segments. 

 To what end is this construction and re-construction ? If, 

 originally, the spine in vertebrate animals consisted from 

 head to tail of separate moveable segments, as it does still in 

 fishes and some reptiles — if, in the evolution of the higher 

 Vertebrata, certain of these moveable segments were ren- 

 dered less moveable with respect to each other, by the 

 mechanical conditions to which they are exposed, and at 

 length became relatively immoveable ; it is comprehensible 

 why the sacrum formed out of them, should continue ever 

 after to show more or less clearly 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 crusta- 

 ceans, " says Mr Darwin ; referring to the well-known fact 



