34 MIND IN EVOLUTION CHAP. 



exclusive property of organisms, for lifeless substances are likewise 

 irritable and respond to external influences by definite changes, 

 e.g. y by the production ot definite substances or of energy, in 

 which process the extent of the production by no means corre- 

 sponds always to the extent of the external impulse. The clearest 

 examples of such cases are afforded by explosive substances. By 

 a slight shock nitro-glycerine is decomposed into water, carbonic 

 acid, oxygen and nitrogen, the process being accompanied by a 

 powerful evolution of energy ; in other words, nitro-glycerine 

 responds to an external influence by an enormous production of 

 energy and a change of material. Hence irritability is not an 

 absolute sign of difference between organisms and inorganic 

 bodies, and it is seen that a fundamental contrast between the 

 two is afforded no more by their dynamical than by their structural 

 and genetic relations." x 



Here again then an analogy can be found, but here again 

 the analogy does not carry us the whole way. After the 

 explosion the nitro-glycerine is gone once for all. Its 

 identity is lost. It is dissipated into component parts. 

 But the muscle which has violently contracted in response 

 to a slight tickling of the skin remains as good a muscle as 

 before. Doubtless it has lost a certain amount of potential 

 energy, and its cells have undergone a chemical change 

 parallel to that experienced by the nitro-glycerine. But 

 there is the muscle sound as ever, and already actively 

 engaged in assimilating new substances from the blood 

 which will restore the proteids oxidised in the contraction. 

 The organism in short in letting energy loose still main- 

 tains itself, and the letting loose of the energy is ordinarily 

 a means of maintaining it. The organism is apparently 

 the only thing that maintains itself by the liberation in 

 response to a stimulus of its own energy. 2 In other words, 

 the liberation of energy is controlled by a principle ot 

 organisation or correlation of changes whereby fresh sub- 

 stance or fresh energy is stored up which will bring the 

 organism back to the very same condition in which it was 

 found before the liberation of energy took place. The 

 example of the muscle will recur to the reader as illus- 

 trating what I mean. 



1 Op. cit. p. 124. 



2 I owe this point to some remarks made to me some years ago by Sir 

 J. Burdon Sanderson. 





