ORGANIC EVOLUTION— THE FACTORS 115 



mony with a complex and heterogeneous environment, 

 but that still further to increase this harmony with 

 changeful surroundings, it has developed in the struc- 

 tures an immense power of varying in the individual 

 under appropriate stimulation,^ the amount of the 

 power of varying as regards each structure being in 

 proportion to the importance of its presence there as a 

 factor in survival ; so that some structures, such as 

 teeth, have little or no power of varying under stimula- 

 tion, since their function is such that in relation to it 

 week by week and year by year the environment varies 

 little ; whereas other structures, such as skin or muscle, 

 have greater powers of varying, since their functions are 

 such that in relation to them week by week or year by 

 year the environment varies more. By this power of 

 varying along certain lines under appropriate stimula- 

 tion, all the multitudinous structures of the high 

 animal body are brought into exquisite harmony, each 

 one with all the others ; and therefore, should an animal 

 vary in such a manner as to have a structure — say a 

 certain muscle — larger than the corresponding struc- 

 ture in its progenitors, or rather, to speak more pre- 

 cisely, should an animal vary in such a manner that 

 the structure develops under a " normal " amount of 

 stimulation more than with the same amount of stimu- 

 lation it developed in the progenitors, then, since this 

 extra development puts an extra strain on all co-opera- 

 tive structures, there results a proportionate increase of 

 size in them all. On the other hand, should an animal 

 vary in such a manner that the structure develops 

 under a normal amount of stimulation less than with 

 the same amount of stimulation the corresponding 

 structure developed in the progenitors, then, since the 

 lesser development puts a smaller strain on all co- 

 operative structures, there results a proportionate and 

 * Vide page 89, et sec^. 



