4 PHYSIOLOGY 



them at the point where they are built up to form apparently an integral 

 part of the living framework. During activity there is a discharge of the 

 products of oxidation of the food-stuffs from this living matter, which there- 

 fore becomes reduced in mass. This reduction, or disintegration of the 

 living matter, associated with activity, is always followed by a period of 

 increased integration, during which the organism grows by the assimilation 

 of more food. Our conception of life must therefore involve the idea of a 

 constantly recurring cycle of processes, one of building up, repair, or inte- 

 gration, and the other, associated with activity, of destruction or disinte- 

 gration. If the former process predominates, we obtain a steady increase 

 in the mass of the organism, an increase which we speak of as growth, and 

 in many cases, as in that of plants, it is this power of growth which we take 

 as our criterion of the existence of life. In fact, the possession by the green 

 parts of plants of the power of utilising the energies of the sun's rays for the 

 integration of food-stuffs, such as starch, with a high potential energy, is 

 the necessary condition for the existence of all higher forms of life on this 



earth. 



Closely associated with the property of growth is the power possessed 

 by all living organisms of repair, i.e. the replacement by newly formed 

 healthy living material of parts which have been damaged by external 

 events. 



The process of growth does not, in the individual, proceed indefinitely. 

 At a certain stage in its life every organism divides, and a part or parts of 

 its substance are thrown off to form new individuals, each of them endowed 

 with the same properties as the parent organism, and destined to grow until 

 they are indistinguishable from the organism whence they were derived. 

 In the lowest forms of life, the unicellular organisms, these processes of growth 

 and division may go on until brought to an end by some change in the 

 environment which will not allow the necessary conditions of life, viz. 

 assimilation and disintegration, to proceed. In all the higher forms, how- 

 ever, after the process of reproduction has been completed, the parent 

 organism begins to undergo decay, and the processes of assimilation and 

 repair no longer keep pace with those of destruction, however favourable 

 the environment, until finally death of the organism takes place. 



All these phenomena, viz. assimilation, respiration, activity associated 

 with the discharge of energy, growth, reproduction, and death itself, are 

 bound up in our conception of life. All have one feature in common, viz. 

 they are subject to the statement that every living organism is endowed 

 with the power of adaptation. Adaptation may indeed receive the definition 

 which Herbert Spencer has applied to life " the continuous adjustment 

 of internal relations to external relations." A living organism may be 

 regarded as a highly unstable chemical system which tends to increase itself 

 continuously under the average of the conditions to which it is subject, 

 but undergoes disintegration as a result of any variation from this average. 

 It is evident that the sole condition for the survival of the organism is that 

 any such act of disintegration shall result in so modifying the relation of the 



