THE AKGtMENTS FROM 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 



