4o8 



NATURE 



[November 24, 192 1 



Moreover, the early fossil vertebrates had, in 

 fact, lenses neither larger nor smaller on the 

 average than those of the present day. If de- 

 structive anti-lens had been continually produced 

 and had acted, its effect would have been cumula- 

 tive. A constructive substance must, then, have 

 also been continually produced to counteract it. 

 Such a theory might perhaps be defended ; but 

 would it bring us any nearer to the solution of 

 the problem? 



The real weakness of the theory is that it does 

 not escape from the fundamental objections we 

 have already put forward as fatal to Lamarckism. 

 If an effect has been produced, cither the sup- 

 posed constructive substance was present from 

 the first, as an ordinary internal environmental 

 condition necessary for the normal development 

 of the character, or it must have been introduced 

 from without by the application of a new stimulus. 

 The same objection does not apply to the destruc- 

 tive effect. No one doubts that if a factor could 

 be destroyed by a hot needle or picked out with 

 fine forceps the effects of the operation would 

 persist throughout subsequent generations. 

 Nevertheless, these results are of the greatest 

 interest and importance. 



There remains another question we must try 

 to answer before we close, namely, "What share 

 has the mind taken in evolution?" From the 

 point of view of the biologist, describing and 

 generalising on what he can observe, evolution 

 may be represented as a series of metabolic 

 changes in living matter moulded by the environ- 

 ment. It will naturally be objected that such a 

 description of life and its manifestations as a 

 physico-chemical mechanism takes no account of 

 mind. Surely, it will be said, mind must have 

 affected the course of evolution, and may indeed 

 be considered as the most important factor in the 

 process. Now, without in the least wishing to 

 deny the importance of the mind, I would main- 

 tain that there is no justification for the belief 

 that it has acted or could act as something guid- 

 ing or interfering with the course of metabolism. 

 This is not the place to enter into a philosophical 

 discussion on the ultimate nature of our experi- 

 ence and its contents, nor would I be competent 

 to do so ; nevertheless, a scientific explanation of 

 evolution cannot ignore the problem of mind if 

 it is to satisfy the average man. 



Let me put the matter as briefly as possible at 

 the risk of seeming somewhat dogmatic. It will 

 be admitted that all the manifestations of living 

 organisms depend, as mentioned above, on series 

 of physico-chemical changes continuing without 

 break, each step determining that which follows ; 

 also that the so-called general laws of physics 

 and of chemistry hold good in living processes. 

 Since, so far as living processes are known and 

 understood, they can be fully explained in accord- 

 ance with these laws, there is no need and no 

 justification for calling in the help of any special 

 vital force or other directive influence to account 

 for them. Such crude vitalistic theories are now^ 



NO. 2717, VOL. 108] 



discredited, but tend to return in a more subtle 

 form as the doctrine of the interaction of body 

 and mind, of the influence of the mind on the 

 activities of the body. But, try as we may, we 

 cannot conceive how a physical process can be 

 interrupted or supplemented by non-physical 

 agencies. Rather do we believe that to the con- 

 tinuous 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 controlling the 

 other, since they are not independent of each 

 other. Indissolubly connected, any change in the 

 one is necessarily accompanied by a correspond- 

 ing change in the other. The mind is not a pro- 

 duct 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 

 philosophers, nevertheless it seems to me to be 

 the best and indeed the only working hypothesis 

 the biologist can use in the present state of know- 

 ledge. 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 happen with- 

 out 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 knowledge of the one 

 from the other. We thus complete our descrip- 

 tion 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 in- 

 stance, the physico-chemical chain of events may 

 appear to be so completely known that the corre- 

 sponding mental events are usually not mentioned 



