FACTS AND FACTORS OF DEVELOPMENT 71 



In these three grades the advance is from extreme 

 blind uniformity of action, with little or no choice, to 

 a stage of less rigid uniformity. . . . Under con- 

 ditions of domestication the action of natural selec- 

 tion has been relaxed, with the result that the rigor 

 of instinctive co-ordination, which bars alternative 

 action, is more or less reduced. Not only is the door 

 to choice thus unlocked, but more varied opportuni- 

 ties and provocations arise, and thus the internal 

 mechanism and the external conditions and stimuli 

 work both in the same direction to favor greater 

 freedom of action. When choice thus enters no new 

 factor is introduced. There is greater plasticity 

 within and more provocation without, and hence the 

 same bird, without the addition or loss of a single 

 nerve cell, becomes capable of higher action and is 

 encouraged and even constrained by circumstances 

 to learn to use its privileges of choice. Choice, as I 

 conceive it, is not introduced as a little deity en- 

 capsuled in the brain. . . . But increased plasticity 

 invites greater interaction of stimuli and gives more 

 even chances for conflicting impulses. 



(d) Finally in all animals behavior is modi- 

 fied through previous experience, just as 

 structure is also. Where several responses to a 

 stimulus are possible and where experience has 

 taught that one response is more satisfactory 

 than another, action may be limited to this 

 particular response, not by external compul- 



