222 THE HISTORY OF CREATION, 



direct influence of the nature of the soil and of the 

 dwelling-place also belong to it ; and further, the extremely 

 important and varied influence which is exercised upon 

 every animal and every plant by the surrounding organ- 

 isms, friends and neighbours, enemies and robbers, para- 

 sites, etc. All these and many other very important 

 influences, all of which more or less modify the organism in 

 its material composition, mnst be taken into consideration 

 in studying the change of substance wliich goes on in living 

 things. Adaptation, accordingly, is the consequence of all 

 those material variations which are produced in the change 

 of substance of the organism by the external conditions of 

 existence, or by the influences of the surrounding external 

 world. 



How very much every organism is dependent upon the 

 whole of its external surroundings, and changed by their 

 alteration, is, in a general way, well known to every one. 

 Only think how much the human power of action is de- 

 pendent upon the temperature of the air, or how much the 

 disposition of our minds depends upon the colour of the sky. 

 Accordingly as the sky is cloudless and sunny, or covered 

 with large heavy clouds, our state of mind is cheerful or dull. 

 How diflerently do we feel and think in a forest during a 

 stormy winter night and during a bright summer day ! 

 All the different moods of our soul depend upon purely 

 material changes of our brain, upon movements of molecular 

 plasma, which are started through the medium of the senses 

 by the different influences of light, warmth, moisture, etc. 

 " We are a plaything to every pressure of the air." No less 

 important and deeply influential are the effects produced 

 upon our mind and body by the different quality and 



