22 Mr. H. G. Seeley on the Origin 



the chapter on classification, must be regarded as forming a 

 group of equal zoological value with the Vertebrata. 



The division of the nervous system and of the skeleton into 

 a long posterior part and a wide anterior part is the essential 

 vertebrate character. And if we are to understand what cha- 

 racters are essential, and why they undergo change, an attempt 

 must be made to state clearly what they are, and why they 

 exist. It will be sufficient, with regard to the spinal column, 

 to know that it is a central, somewhat cylindrical mass, ex- 

 tending the length of the vertebral column generally, giving 

 off at intervals pairs of nerves, and tapering towards the tail. 

 While the brain is posteriorly continuous with the spinal cord, 

 it is much larger, and consists of parts which are sometimes 

 arranged one before the other, and sometimes one over the 

 other ; it usually gives off nerves to the eyes, the ears, the 

 nose, &c. 



The vertebrae have a common basis, on which the neural 

 column rests, and which is a subcylindrical column, called the 

 notochord. When segmented and ossified, it forms the part 

 of each vertebra named the centrum ; and this centrum gives 

 attachment to a pair of bones which arch over the spinal cord 

 and are separated from others by the intervertebral nerves ; 

 they may become inseparably united to the centrum or always 

 remain distinct. The skull is made by a number of small 

 bones which suturally unite, or simply overlap each other, so 

 as to enclose the brain, which case usually may be separated 

 vertically down the sutures into three more or less well-defined 

 segments, each consisting of a bone at the base, a bone on 

 each side for the sides of the arch, and one or two bones above 

 vaulting it over. A necessary and separate part of the skull 

 is comprised generally under the terms upper and lower jaws. 



Now we have to inquire why these parts exist — in other 

 words, how they come to grow. And all growth has been 

 seen to be organic dialysis, which takes place under the in- 

 fluence of alternating pressure and tension and rest. How, 

 then, does this law apply to the formation of the vertebrate 

 skeleton, and account for the formation of bones so deeply 

 seated and well protected, and for the formation and com- 

 plexity of brains and crania ? I will endeavour to explain. 



All vertebrate animals are locomotive, and all fish and all 

 immature Amphibia live in water. These animals progress 

 backward, though we usually name tlie motion forward ; that 

 is, each uses its tail to obtain a leverage by which it retreats, 

 the animal's head necessarily going where the tail sends it. 

 It is therefore evident that the head, in piercing the water, 

 experiences some pressure alternating with rest, while the 



