PRESIDENTIAL ADDRESS. 639 
of the moth. The central nervous system, however, is so constituted that every 
passage of an impulse along any given channel makes it easier for subsequent 
impulses to follow the same path. In the new nerve centre, which presents a 
derived circuit for all impulses traversing the lower centres, the response to the 
attractive impulse of the flame is succeeded immediately by the strong inhibitory 
impulses set up by the pain of the burn. Painful impressions are always pre- 
dominant. Since they are harmful, the continued existence of the animal 
depends on the reaction caused by such impressions taking the precedence of and 
inhibiting all others. The effect therefore of such a painful experience on the 
new upper brain must far outweigh that of the previous impulse of attraction. 
The next time that a similar attractive impression is experienced the derived 
impulse traversing the upper brain arouses not the previous primary reaction, 
but the secondary one, viz., that determined by the painful impressions attending 
contact with the flame. As a result, the whole of the lower tracts, along which 
the primary reaction would have travelled, are blocked, and the reaction—now 
an educated one—consists in withdrawal from or avoidance of the formerly 
attractive object. The burnt child has learnt to dread the fire. 
The upper brain represents a nerve mechanism without distinct paths, er 
rather with numberless paths presenting at first equal resistance in the various 
directions. As a result of experience, definite tracts are laid down in this system, 
so that the individual has the advantage not only of his lower reflex machinery 
for reaction, but also of a machinery which with advance in life is adapted 
more and more to the environment in which he happens to be. This educable 
part of the nervous system—#.e., the one in which the direction of impulses 
depends on past experience and on habit—is represented in vertebrates by the 
cerebral hemispheres. From their first appearance they increase steadily in 
size as we ascend the animal scale, until in man they exceed by many times in 
bulk the whole of the rest of the nervous system. 
We have thus, laid down automatically, increased power of foresight, founded 
on the Law of Uniformity. The candle flame injures the skin once when the 
finger is brought in contact with it. We assume that the same result will follow 
each time that this operation is repeated. This uniformity is also assumed in 
the growth of the central nervous system and furnishes the basis on which the 
nerve paths in the brain are laid down. ‘The one act of injury which has fol- 
lowed the first trial of contact suffices in most cases to inhibit and to prevent 
any subsequent repetition of the act. 
The Faculty of Speech. 
If we consider for a moment the vastness and complexity of the stream of 
impressions which must be constantly pouring into the central nervous system 
from all the sense organs of the body, and the fact that, at any rate in the growing 
animal, every one of these impulses is, so to speak, stored in the upper brain, 
and affects the whole future behaviour of the animal, even the millions of nerve 
cells and fibres which are to be found in the human nervous system would seem 
to be insufficient to carry out the task thrown upon them. Further development 
of the adaptive powers of the animal would probably have been rendered im- 
possible by the very exigencies of space and nutrition, had it not been for the 
development of the power of speech. A word is a fairly simple motor act and 
produces a correspondingly simple sensory impression. Every word, however, 
is a shorthand expression of a vast sum of experience, and by using words as 
counters it becomes possible to increase enormously the power of the nervous 
system to deal with its own experience. Education now involves the learning 
of these counters and of their significance in sense experience ; and the reactions 
of the highest animal, man, are for the most part carried out in response to words 
ane are governed by past education of the experience-content involved in each 
word. 
The power of speech was probably developed in the first place as a means 
of communication among primitive man living in groups or societies ; as a means, 
that is to say, of procuring co-operation of different individuals in a task in which 
the survival of the whole race was involved, But it has attained still further 
