INTRODUCTION 5 



allow the necessary conditions of life, viz. assimilation and disintegra- 

 tion, to proceed. In all the higher forms, however, 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 environ- 

 ment, until finally death of the organism takes place. 



All these phenomena, viz. assimilation, respiration, activity asso- 

 ciated 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 under- 

 goes 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 system to the environment that it is once more 

 restored to the average in which assimilation can be resumed. Every 

 phase of activity in a living being must be not only a necessary sequence 

 of some antecedent change in its environment, but must be so adapted 

 to this change as to tend to its neutralisation, and so to the survival 

 of the organism. This is what is meant by ' adaptation.' Not 

 only does it involve the teleological conception that every normal 

 activity must be for the good of the organism, but it must also apply 

 to all the relations of living beings. It must therefore be the guiding 

 principle, not only in physiology with its special preoccupation with 

 the internal relations of the parts of the organism, but also in the other 

 branches of biology, which treat of the relations of the living animal 

 to its environment, and of the factors which determine its survival in 

 the struggle for existence. The origin of new species and the succession 

 of the different forms of life upon this earth depend on the varying 

 perfection of the mechanisms of adaptation. 



We may imagine that the first step in the evolution of life was taken 

 during the chaotic chemical interchanges which accompanied the 

 cooling down of the molten mass forming the earth, when some com- 

 pound was formed, probably with absorption of heat, endowed with 

 the property of continuous polymerisation and growth at the expense 

 of surrounding material. Such a substance could continue to exist 

 only at the expense of the energy derived from the surrounding medium, 

 and would undergo destruction with any stormy change in its environ- 

 ment. Out of the many such compounds which might have come into 



