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, must be taken into consideration 
in studying the change of substance which 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 differently 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, ete. 
“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 
