DEPENDENCE ON ASSOCIATION. 97 



mands a sufficient cause in the form of force, so 

 every activity of an organism demands a sufficient 

 stimulus. If, now, we look for the stimuli which 

 produce these activities, we are obliged to include 

 every force that affects the organism ; and such 

 forces we must I'ecognise as the causes of the 

 activity, growth, and development of each individual 

 organism, and also of the changes occurring in indi- 

 viduals during the evolution of the species. 



Among the countless forces affecting organisms, 

 we find a number which operate alike upon all. 

 These are gravity, light, heat, electrical conditions, 

 and the various combinations of energy comprised 

 under the head of nourishment. To these therefore 

 we may ascribe those characteristics common to all 

 organisms; and to the nourishment especially may 

 we ascribe that amount of energy which is expended 

 in the motion, heat, light, and electricity, produced 

 by organisms. But in so far as these forces are 

 universal, they give us no clue to the origin of the 

 diversity among organisms. The nourishment, it is 

 true, differs in different organisms, but this seems 

 a secondary rather than a primary cause of diversity. 

 Evidently, then, it is to the varying conditions and 

 quantities of the above-named forces acting as 

 stimuli, and also to the great number of those par- 

 ticular stimuli which are always peculiarly affepting 



