THE REGION OF THE SPINAL CORD 387 



the animal into the lithe fish-like form. The consideration of the 

 manner in which this latter change was brought about, takes us 

 out of the cranial into the spinal region. 



If we take Limulus as the only living type of the Palreostraca, 

 we are struck with the fact that the animal consists to all intents 

 and purposes of prosomatic and mesosomatic regions only ; the meta- 

 soma consisting of the segments posterior to the mesosoma is very 

 insignificant, so that the large mass of the animal consists of what 

 has become the head-region in the vertebrate ; the spinal region, 

 which has become in the higher vertebrates by far the largest region 

 of the body, can hardly be said to exist in such an animal as Limulus. 

 As to the Eurypterids and others, similar remarks may be made, 

 though not to the same extent, for in them a distinct raetasoma does 

 exist. 



In this book I have considered up to the present the cranial 

 region as a system of segments, and shown how such segments are 

 comparable, one by one, with the corresponding segments in the 

 prosoma and mesosoma of the presumed arthropod ancestor. 



In the spinal region such direct comparison is not possible, as is 

 evident on the face of it ; for even among vertebrates themselves the 

 spinal segments are not comparable one by one, so great is the varia- 

 tion, so unsettled is the number of segments in this region. This 

 meristic variation, as Bateson calls it, is the great distinctive character 

 of the spinal region, which distinguishes it from the cranial region 

 with its fixed number of nerves, and its substantive rather than 

 meristic variation. At the borderland, between the two regions, we 

 see how the one type merges into the other; how difficult it is 

 to fix the segmental position of the spino-occipital nerves ; how much 

 more variable in number are the segments supplied by the vagus 

 nerves than those anterior to them. 



This meristic variation is a sign of instability, of want of fixedness 

 in the type, and is evidence, as already pointed out, that the spinal 

 region is newer than the cranial. This instability in the number of 

 spinal segments does not necessarily imply a variability in the 

 number of segments of the metasoma of the invertebrate ancestor; 

 it may simply be an expression of adaptability in the vertebrate 

 phylum itself, according to the recmirements necessitated by the con- 

 version of a crawling into a swimming animal, and the subsequent 

 conversion of the swimming into a terrestrial or flying animal. 



