292 



NATURE 



\Aiigust 9, 1877 



I described last year, whereby individuals of this species 

 were shown to have the power of following a moving 

 beam of light round and round the vessel in which they 

 were contained. I may also remark that individuals of 

 this species present much more nervous energy than 

 those of any other species of Medusae which I have had 

 the opportunity of observing. 



1 have now, ladies and gentlemen, communicated some 

 of the points wherein my work has tended to elucidate 

 the early stages in the evolution of nerves and nervous 

 systems. And these are just the stages concerning which 

 elucidation is most required. When once nerve-fibres 

 and nerve-cells have been fully evolved and arranged in 

 the form of simple reflex mechanisms, the subsequent 

 history of their evolution into compound nervous systems 

 is readily intelligible. The principles on which this higher 

 evolution is effected are throughout the same, and result 

 essentially in establishing ever more and more advanced 



degrees of integration. Compare, for instance, the ner- 

 vous systems of an earth-worm, a centipede, an insect, and 

 a spider ; and observe the progressive fusion of ganglia 

 which has taken place. The progressive centralisation 

 which is thus effected is no doubt ultimately due to natu- 

 ral selection, if not exclusively, at any rate in large part; 

 for this increasing consolidation of the reflex mechanisms 

 must be of great benefit to the organisms which present 

 it — serving as it does to render possible muscular move- 

 ments ever more and more varied and combined. In 

 the vertebrated series of animals the evolution of cen- 

 tral nervous matter consists chiefly in adding to the size 

 of ganglia by increasing the number of their ultimate 

 nervous elements, nerve-cells and nerve-fibres. This 

 progressive increase in the size of ganglia is especially 

 remarkable in the case of the cerebral hemispheres. Now 

 the cerebral hemispheres are the ganglia which we know 

 to be the exclusive seat of the intellectual faculties ; and 

 their progressive increase in bulk as we ascend through 



the animal series, is undoubtedly to be regarded as the 

 structural correlative of that progressive advance of the 

 intellectual powers which is so conspicuously apparent as 

 we ascend from the lower animals to Man. 



And now, in conclusion, I should like to observe, that 

 even in this the highest product of nervous evolution — 

 the supreme ganglia or cerebral hemispheres of Man — 

 not only do we still encounter the same fundamental con- 

 stituents of structure as we observe in all other ganglia ; 

 but the cells and fibres in the brain of a man do not 

 differ in any marked degree from the cells and fibres in the 

 ganglion of an Aurclia. Theie is, however, a prodigious 

 difference in the product of their operation. When ordi- 

 nary ganglion cells discharge their influence, the result is, 

 as we have seen, a muscular contraction ; but when 

 cerebral cells discharge their influence, we of to- day can 

 have no doubt that the result is a mental change. And 

 although we freely acknowledge that we are here standing 

 on the border-land of insoluble mystery, we are not 

 afraid to assert with confidence, that in the amazing com- 

 pliexty of the brain's structure — amid those millions on 

 millions of interlacing cells and fibres — we have the 

 physical aspect of all those relations, which in their 

 psychical aspect we know as thoughts and feelings. Do 

 you think that this sounds like materialism .' I am not 

 here to-night to discuss that point ; but I may observe in 

 passing, that even were I able to tell you the particular 

 cerebral elements which I now use in expressing this state- 

 ment to you, I should be just as much or just as little on the 

 way towards proving materialism, as I am when I tell you 

 that a blow on the head produces insensibility. Science 

 can never go further than common sense in proving 

 any necessary connection to subsist between mind and 

 matter ; for all that science can ever do is to ascertain 

 numerous details with regard to such connection as un- 

 doubtedly does exist, and which, as a matter of daily 

 experience, common-sense has already and completely 

 recognised. However, materialism or no materialism, it 

 is manifest that the facts being what they are, Mr. 

 Spencer's theory as to the genesis of nerves must not be 

 allowed to stop short just where its presence is most 

 required. As we have seen that the cerebral hemispheres 

 of man resemble all other ganglia in structure, we cannot 

 hesitate in concluding that if Mr. Spencer's theory is valid 

 in explaining the genesis of nerves in general, it can be 

 no less valid in explaining the genesis of these supreme 

 ganglia in particular. And as we have every reason to 

 believe that the functional operations of these supreme 

 ganglia are inseparably associated with our thoughts and 

 feelings, we are driven to the yet further conclusion, 

 that if Mr. Spencer's theory is of any validity at 

 all, our possible as well as our actual thoughts 

 and feelings are determined by the strictly physical con- 

 ditions under which molecular waves of stimulation 

 course through the structure of the brain. So that in 

 this Spencerian hypothesis of lines of discharge becom- 

 ing more and more definite by use, we have a physical 

 explanation, which is perhaps as full and as complete as 

 such an explanation can ever be, of the genesis of mind. 

 From the time that intelligence first dawned upon the scene 

 of life, whenever a new relation had to be established in 

 the region of mind, it could only be so established in virtue 

 of some new line of discharge being excavated through 

 the substance of the brain. The more often this relation 

 had to be repeated in the mind, the more often would 

 this discharge require to take place in the brain, and so 

 the more easy would every repetition of the process 

 become ; until at last the line of discharge grows into a 

 nerve-fibre, and becomes the inherited property of the 

 race. Thus it is, according to the theory, that there is 

 always a precise proportion between the constancy with 

 which any relations have been joined together during the 

 history of intelligence, and the difficulty which intelligence 

 now experiences in trying to conceive of such relations as 



