OF THE EARLIER VERTEBRATA. 37 



or a sinking below this medial line of beauty. And it may- 

 be of use, as it is unquestionably of interest, to conceive, after 

 this manner, of a certain type of skeleton, embodying, as it 

 were, the central or primary type of all vertebral skeletons, 

 and consisting of a double range of rings, united by the bodies 

 of the vertebrae, as the two rings of a figure 8 are united at 

 their point of junction ; the upper ring forming the enclosure 

 of the brain, — spinal and cephalic ; the lower that of the 

 viscera, — respiratory, circulatory, and digestive. Such is the 

 idea embodied in Professor Owen's archetypal skeleton. It 

 is a series of vertebrae composing double rings, — their brain- 

 rings comparatively small in the vertebrae of the trunk, but 

 of much greater size in the vertebrae of the head. But it 

 must not be forgotten, that central ideas, however necessary 

 to the classification of the naturalist, are not historic facts. 

 We may safely hold with the philosophic painter, that the 

 outline of the typical human nose is a straight line ; but it 

 would be very unsafe to hold, as a consequence, that the first 

 men had all straight noses. And when we find it urged by 

 at least one eminent assertor of the development hypothesis, 

 — Professor Oken, — that light was the main agent in develop- 

 ing the substance of nerve, — that the nerves, ranged in pairs, 

 in turn developed the vertebrae, each vertebra being but " the 

 periphery or envelope of a pair of nerves," — and that the 

 nerves of those four senses of smell, sight, taste, and hearing, 

 which, according to the Professor, " make up the head," ori- 

 ginated the four cranial vertebrae which constitute the skull, 

 — it becomes us to test the central idea, thus converted into 

 a sort of historic myth, by the realities of actual history. "What, 

 then, let us inquire, is the real history of the cerebral develop- 

 ment of the vertebrata, as recorded in the rocks of the earliei 

 geologic periods ? 



Though the vertebrata existed in the ichthyic form through- 

 out a part of the vastly extended Silurian period, we find in 



