542 MANUAL OF ELEMENTARY ZOOLOGY 



by the action of external forces, the existence of the 

 organism is maintained by a reaction in which energy is 

 liberated and directed from within. The question suggests 

 itself whether this behaviour on the part of living beings 

 be due to their possessing any property that is not found 

 in the rest of nature — whether, that is, life differs fundament- 

 ally from the processes of the lifeless world. This is the 

 ultimate problem of Biology, and it brings us back to the 

 consideration of those primary features of life with which 

 we began our study of the animal organism. 



i. Irritability is not peculiar to living beings. Any 

 substance may be said to be irritable in which a change 

 can be caused to take place by a stimulus whose magnitude 

 bears no relation to that of the change it starts, and there 

 is no lack of such substances among lifeless things. An 

 instance that we have already used shows this clearly. 

 Explosives disintegrate on the receipt of stimuli, and the 

 firing of a gun by a slight pressure on the trigger is 

 analogous to the performance of the movements which 

 precede the firing, by a man on the receipt of such a 

 stimulus as the sight of a game-bird. 



2. In regard to automatism the position is less clear, 

 and here no decision is possible until we know in what 

 the automatism of living beings consists. A clock and 

 a mass of radium are both automatic in the sense that 

 each has an activity which is not directly dependent on 

 external stimuli, but it does not follow that the automa- 

 tism of a living being is of the same kind as that of either 

 of these. 



3. On the other hand, little reflection is needed to 

 show that disintegration with evolution of energy is a 

 common process in lifeless things. We have already seen, 

 for instance, that explosives respond to stimuli by disin- 

 tegration. There remains, however, the possibility that, 

 though the mere fact of disintegration with evolution of 

 energy is not peculiar to living beings, the details of the 

 way in which the freed energy appears may be so. The 

 substance of the human body and that of a fire or a 

 cartridge alike disintegrate with evolution of energy, but 

 the way in which the energy appears is very different in 

 the human body and in the other cases. Can we find in 



