84 SECTIONAL ADDRESSES 
Rather do we believe that to the continuous physico-chemical series 
of events there corresponds a continuous series of mental events 
inevitably connected with it; that the two series are but partial views 
or abstractions, two aspects of some more complete whole, the one 
seen from without, the other from within, the one observed, the other 
felt. One is capable of being described in scientific language as a 
consistent series of events in an outside world, the other is ascertained 
by introspection, and is describable as a series of mental events in 
psychical terms. There is no possibility of the one affecting or con- 
trolling the other, since they are not independent of each other. 
Indissolubly connected, any change in the one is necessarily accom- 
panied by a corresponding change in the other. The mind is not a 
product of metabolism as materialism would imply, still less an 
epiphenomenon or meaningless by-product as some have held. I am 
well aware that the view just put forward is rejected by many philoso- 
phers, nevertheless it seems to me to be the best and indeed the only 
working hypothesis the biologist can use in the present state of 
knowledge. The student of biology, however, is not concerned with 
the building up of systems of philosophy, though he should realise that 
the mental series of events lies outside the sphere of natural science. 
The question, then, which is the more important in evolution, the 
mental or the physical series, has no meaning, since one cannot happeu 
without the other. The two have evolved together pari passu. We 
know of no mind apart from body, and have no right to assume that 
metabolic processes can occur without corresponding mental processes, 
however simple they may be. 
Simple response to stimulus is the basis of all behaviour. Responses 
may be linked together in chains, each acting as a stimulus to start the 
next; they can be modified by other simultaneous responses, or by the 
effects left behind by previous responses, and so may be built up into 
the most complicated behaviour. But, owing to our very incomplete 
knowledge of the physico-chemical events concerned, we constantly, 
when describing the behaviour of living organisms, pass, so to speak, 
from the physical to the mental series, filling up the gaps in our know- 
ledge of the one from the other. We thus complete our description 
of behaviour in terms of mental processes we know only in ourselves 
(such as feeling, emotion, will) but infer from external evidence to take 
place in other animals. 
In describing a simple reflex action, for instance, the physico- 
chemical chain of events may appear to be so completely known that 
the corresponding mental events are usually not mentioned at all, their 
existence may even be denied. On the contrary, when describing com- 
plex behaviour when impulses from external or internal stimuli modify 
each other before the final result is translated into action, it is the 
intervening physico-chemical processes which are unknown and perhaps 
ignored, and the action is said to be voluntary or prompted by emotion 
or the will. 
The point I wish to make, however, is that the actions and 
behaviour of organisms are responses, are characters in the sense 
described in the earlier part of this address. They are inherited, they 
