24 Mr. H. G. Seeley 07i the Origin 



cation, the force which is potential and persistent, and in each 

 individual is renewed, will, as the opportunities for it to take 

 the kinetic form successively arrive, be manifested as fully as 

 it would originally have been in one individual if the organic 

 machinery had been capable of maintaining the nutrition ne- 

 cessary to elaborate growth. I shall thus be justified in 

 reasoning about the species as though it were an individual, 

 and to conclude that the force which has been shown, both 

 theoretically and experimentally, to be competent to produce 

 an elongation of the spinal cord toward the part called the 

 head, actually does produce the effects which it ought to pro- 

 duce. And the way in which this is done depends upon the 

 means to do it : first, the forcing of the nutritive fluid forward 

 necessarily produces an enlargement of the nervous system at 

 the anterior end ; and, secondly, the growth forward of the 

 nervous system must cause a pressure which will stimulate 

 special growth in that region ; and the parts of the brain 

 which were originally arranged one before each other may 

 come to be forced one over the other by growth forward of the 

 neural tissue pressing into the brain-case. 



And when a brain is examined, in it are found large cavi- 

 ties called ventricles, which are the receptacles of fluid, such 

 as we might theoretically expect. And when the brains of 

 the lower Vertebrata are compared with those of the higher 

 Vertebrata, there will be remarked a gradual increase, as we 

 ascend in organization, in the size of the cerebral lobes, which 

 first push the optic lobes on each side so that the cerebrum 

 abuts against the cerebellum, and finally overrides it. There- 

 fore it must be anticipated that the longer the time for which 

 a vertebrate type of animal has persisted upon the earth's 

 surface, the higher Avill be its nervous organization ; and 

 hence that extinct animals which seem to be the direct repre- 

 sentatives, so far as their bones go, of existing animals, will, 

 so far as they approach nearer to the common vertebrate 

 plan, have had a lower grade of vital organs. Having seen 

 that the movement of the body would be competent, by 

 governing the direction of growth and the distribution of 

 nutriment, to generate the brain from a pre-existing spinal 

 cord, it is probable that the nerves are in the same way 

 affluents to and sustainers of the spinal column, and that their 

 presence preserves its division into segments. 



Having advanced this hypothesis of the vertebrate plan of 

 the central nem-al system, we will endeavour to see how the 

 nerve-matter becomes coated with the investing skeleton. 

 And to do this, it will be requisite to consider the entire body 

 as a machine capable of manifesting the forces of pressure and 



