762 The Relations of Mind and Matter. (August, 
For this reason the loss of the head section in certain insects 
seems very slightly to decrease their motor powers. 
As we ascend through the Vertebrata, however, nervous cen- 
tralization rapidly proceeds. The cephalic ganglion, or mass of 
cephalic ganglia, grows more and more predominant over the in- 
ferior ganglia, and the sensory currents are diverted more and 
more fully towards this central region. Throughout the whole 
upward range the phenomena of nerve development indicates 
that reflex action grows less and less, and conscious action more 
and more declared. In certain fossil vertebrates of huge form, 
the mass of the inferior surpasses that of the cephalic ganglia, 
indicating a great degree of reflex action. Throughout the fossil 
series cephalization seems to steadily increase, indicating a grow- 
ing superiority of conscious over reflex action. 
Yet this change was only made by slow stages, and an inter- 
esting intermediate stage seems to have intervened between reflex 
and habitually intelligent action. This is the stage of instinct, in 
which reflex action and consciousness seem to be combined. In 
this stage the sensory currents, on reaching the inferior ganglia, 
seem to partly follow an habitual channel to the muscles, partly 
_ to pursue a channel to the central ganglion, where they awaken 
consciousness. It is a usual stage in the change from fully con- 
scious to reflex action. In every such case intelligence may 
exercise some controlling function, with a possible modification 
of the reflex action. Finally, in the highest animals, the upward 
channel has become the habitual one for all but the most deep- 
seated or incessantly continued sensory impressions, and intelli- 
- gent has grown more and more dominant over reflex and instinc- 
tive action, until in man it has become the controlling agent in 
the great sum of actions, . 
- There is a principle, already stated, to which this increasing 
cephalization seems due. The nervous system is not readily 
conductive to unusual sensory impressions. It has become so to 
ordinary impressions, each of which has established for itself a 
habitual channel. But every unusual impression is checked at 
the ganglion, and can only make its way onward if of consider- 
able strength, and then over the general motor system, not over 
_ any particular motor nerves. If this sensation becomes an ordi- 
_ fary one, natural selection decides whether it shall gain some 
- Settled led channel of outflow to the muscles, or shall be finally 
