PRESIDENTIAL ADDRESS. 637 
in a position to be affected by changes going on in distant parts, otherwise 
co-operation could not be effected. This co-operation in the lowest metazoon 
seems to be carried out by utilisation of the sensibility to chemical stimuli already 
possessed by the unicellular organism. We have thus co-ordination by means 
of chemical substances (‘ hormones ’) produced in certain cells and carried thence 
by the tissue fluids to other cells of the body, a mechanism of communication 
which we find even in the highest animals, including man himself. To such 
chemical stimuli we may probably ascribe the accumulation of wandering meso- 
derm cells—.e., phagocytes—in an organism such as a sponge, around a seat of 
injury or any foreign substance that has been introduced. By this mechanism it 
is possible for distant parts of the body to react to stimulation of any one part 
of the surface. Communication by this means is, however, slow, and may be 
compared to the state of affairs in civilised countries before the invention of the 
telegraph, when messengers had to ride to different parts of the kingdom in order 
to arouse the whole nation for defence or attack. 
Foresight and Control. 
Increased speed of reaction and therefore increased powers in the struggle 
for existence were obtained when a nervous system was formed, by a modification 
of the cells forming the outer surface of the organism. By the growth of long 
processes from these cells aconducting network was provided, running through all 
parts of the body and affording a channel for the rapid propagation of excitation 
from the surface to the deeper parts, as well as from one part of the surface to 
another. From this same layer were produced the cells which, as muscle fibres, 
would actasthe motive mechanism of the organism. Thus, from the beginning, 
the chief means of attack or escape were laid down in close connection with the 
surface from which the stimuli were received. A further step in the evolution 
of the nervous system consisted in the withdrawal of certain of the sensory or 
receptor cells from the surface, so that a specially irritable organ, the central 
nervous system, was evolved, which could serve as a distributing centre for the 
messages or calls to action initiated by changes occurring at the surface of the 
body. At its first appearance this central nervous system would hardly deserve 
the epithet of ‘central,’ since it formed a layer lying some distance below the 
surface, and extending over a considerable area ; though we find that very soon. 
there is an aggregation of the special cells to form ganglia, each of which 
might be regarded as presiding over the reactions of that part of the animal in 
which it is situated. Thus in the segmental wormlike animals a pair of ganglia 
is present in each body segment, and the chain of ganglia are united by longi- 
tudinal strands of nerve fibres to form the ganglionated cord, or central nervous 
system. 
Such a diffused nervous system, in which all ganglia were of equal value, could, 
however, only act for the common weal of the whole body when a reaction 
initiated by stimulation at one part was not counteracted by an opposing 
reaction excited from another part of the surface. For survival it is necessary 
that in the presence of danger, 7.c., an environment threatening the life of the 
individual or race, the whole activities of the organism should be concentrated 
on the one common purpose, whether of escape or defence. This could be 
effected only by making one part of the central nervous system predominant 
over al] other parts, and the part which was chosen for this predominance was 
the part situated in the neighbourhood of the mouth. This, in animals which 
move about, is the part which always precedes the rest of the body, and therefore 
the part which first experiences the sense impressions, favourable or dangerous, 
arising from the environment. It is this end that has to appreciate the presence 
or approach of food material, as well as the nature of the medium into which the 
animal is being driven by the movements of its body. Thus a predominance 
of the front end of the nervous system was determined by the special develop- 
ment at this end of those sense organs or sensory cells which are projicient— 
t.e., are stimulated by changes in the environment proceeding from disturbances 
at a distance from the animal. The sensory organs of vision, and the organs 
which correspond to our olfactory sense organs and are aroused by minute 
