530 TRANSACTIONS OF SECTION I. 



of a short cuticle but an extending cell-body, possessed of no more than a short 

 nerve-fibre and an extensive set of dendritic processes. As each new dendritic 

 process makes contact with a new branch of the excited afferent neurone ils 

 growth will be more and more limited. We have here, then, encountered the 

 limits of growth of the nerve-cell. 



There is no difficulty other than that due to the short time at my disposal 

 in compounding these statements so as to cover the whole scale of differential 

 cell-growth, and within each cell of the relative growth of its several parts, 

 that is observed within the nervous system. I may perhaps be permitted this 

 abrupt closure to a development of the probabilities underlying the following 

 expression of opinion. 



I hold it as probable that all the individual structures of the nervous sys- 

 tem, and so in the brain, have just so much difference from one another in size. 

 in shape, and in function, as is the outcome of that measure of purely physical 

 experience to which each one of them has been subjected; and that the physio- 

 logical function of each one of them is of the simplest kind. The magnificent 

 utility of the whole system, where the individual units have such simplicity, 

 is due to the physically developed peculiarities of their arrangement in relation 

 to one another, and to the receptive surfaces and motor-organs of the body. 



To relieve the monotony of this discussion, let us turn away for a moment 

 to the consideration of certain physical mechanisms found in the body, external 

 to the central nervous system; mechanisms that are placed, so to speak, upon 

 the front of that system so that they are capable rather of affecting it than of 

 being affected by it, and this to such a degree that we must suppose them as 

 rather assisting in the development of the central nervous system than as being 

 assisted to their development by the central nervous system. There are, for 

 example, the lens systems of the eyeball and the sound-conducting and resonant 

 systems of the ear. Now, in dealing with the central nervous system, the sug- 

 gestion was made that it was developed by just such physical conditions as 

 are transmitted through it in its adult form. In dealing with the eyeball, it 

 is clear that an admission of this sort is not easy. During the evolution asso- 

 ciated with natural selection the eyeball is formed by light. It must be so. 

 The eye is as perfect an optical instrument as could be made with a full know- 

 ledge of the part played by matter and special arrangements of matter in 

 reflecting, refracting, and absorbing light. Long prior to the development of 

 man, who at a later date acquired sufficient knowledge of these properties to aid 

 him in the formation of crude lenses, there was to be found upon the general 

 surface of the animal world lenses of very great perfection, in fact, complete 

 cameras. Had the first optician then known what was in him he would have 

 been saved infinite pains. Had he indeed known even the lens systems formed 

 on the leaves of plants. Surely there is no escape from the statement that 

 either external agency cognisant of light, or light itself, has formed and developed 

 to such a state of perfection this purely optical mechanism, and that natural 

 selection can have done no more than assist in this process. The influence of 

 natural selection depends upon the frequency of variations, and it is important 

 that there is no variation that has not behind it some cause. In this special 

 case of variation in physical arrangements, it is indeed probable that the most 

 frequent cause of variation would be exerted by physical conditions, since in this 

 case the factors that are thus introduced by variation are not distinguished by 

 any chemical peculiarity. Thinking of the few possible physical causes of 

 variation, there can be little doubt that light itself would produce some variation 

 in this optical instrument, and that the variations produced by light would be 

 just those more likely to be adapted to the subsequent traverse of light than 

 such as were accidentally produced by some other physical cause. Accepting 

 such a statement, we may say that in the course of development light formed 

 the eye by its action upon such tissues as those of which the general surface 

 of the body is composed. Now in just the same way there can be little doubt 

 but that sound formed the sound-conducting and resonant portions of the ear. 

 We may perhaps go further than this statement, and say that not only has 

 this mechanism placed in front of the central nervous system been formed in 

 this fashion, but that the parts of the central nervous system behind it have 

 been formed by physical effects transmitted from the ear through this keyboard 

 where sound is transformed into nervous impulses. Thus also, when thinking 



