INTRODUCTION 5 



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 neu- 

 tralisation, 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 compound 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 environment. Out of the many 

 such compounds which might have come into being, only such would survive 

 in which the process of exothermic disintegration tended towards a condition 

 of greater stability, so that the process would' come to an end spontaneously, 

 and the organism or compound be enabled to await the more favourable 

 conditions necessary for the continuance of its growth. With the con- 

 tinued cooling of the earth, the new production of endothermic compounds 

 would become rarer and rarer ; and in all probability the beginning of life, 

 as we know it, was the formation of some complex substance, analogous 

 to the present chlorophyll corpuscles, with the power of absorbing the 

 newly penetrating sun's rays and utilising them for the endothermic forma^ 

 tion of further unstable compounds. Once given an unstable system, 

 such as we have imagined, the great principle laid down by Darwin, viz. 

 survival of the fittest, will suffice to account for the production from it by 

 evolution of the ever-increasing variety of living beings which have appeared 

 in the later history of this globe. The ' adaptation,' i.e. the reactions of 

 the primitive living material to changes in its environment, must become 

 ever more and more complex, since only by means of increasing variety of 

 reaction is it possible to provide for the stability of the system within greater 

 and greater range of external conditions. The difference between higher 

 and lower forms is therefore one of complexity of reaction, or of range 

 of adaptation. 



In all the physiological processes which we shall study in the course of 

 this work, adaptation will be found the constant and guiding quality. The 



