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 Palostraca, 
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 metasoma 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 requirements 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. 
